MANPOWER DEVELOPMENT / TRAINING DEPARTMENT

AREA 1 : HANDOUT

This HANDOUT is intended to help operators for a better understanding of the process principles, process
flows and operating variables. It is in form of Questions and Answers.

Q. 1 WHAT IS THE PURPOSE OF CDU UNIT ?

The purpose of CDU unit is to separate crude oil into various fractions with different characteristics for
further use.

Q. 2 WHAT IS CRUDE OIL ?

Crude oil is a mixture of hydrocarbons classified as Paraffinic, Naphthenic & Aromatic hydrocarbons.

Q. 3 DOES CRUDE OIL CONTAIN IMPURITIES ?

Yes it does, mainly salts, sediments, then oxygen, sulphur, nitrogen, metals in form of organic
compounds etc.

Q. 4 WHAT DOES SPECIFIC GRAVITY MEAN ?

It is the weight per unit volume measured at 60 deg F or 15 deg C.

Q. 5 WHAT IS API GRAVITY ?

Internationally, crude oils are expressed in terms of API (American Petroleum Institute) Gravity which is
given by this formula:
OAPI = (141.5 / Sp. Gr.) – 131.5

Q. 6 WHAT IS TBP DISTILLATION CURVE ?

It is the curve of crude oils that gives the yield of each distilled product.

Q. 7 WHAT ARE THE FRACTIONS OBTAINED IN DISTILLATION UNITS?

Whole Naphtha – Straight Run Kerosene – Light Gas Oil – Heavy Gas Oil – Reduced Crude Oil (or Long
Residue)

Q. 8 WHAT IS WHOLE NAPHTHA ?

It is the overhead product which is composed of Gas – LPG, L. Naphtha and H. Naphtha fractions with
an EBP not exceeding 175 deg C.

Q. 9 WHAT IS STRAIGHT RUN KEROSENE ?

Is a fraction used for duel purpose, thus aviation turbine and household Kerosene.

Q. 10 DO WHOLE NAPHTHA AND KEROSENE CONTAIN IMPURITIES?

Yes, mainly Sulfur, but also Nitrogen, Organic acids etc.

Q. 11 WHAT IS LIGHT GAS OIL ?

It is the fraction which is used in Diesel Fuel and for its characteristics doesn’t need any further
treatment.

Q. 12 WHAT IS HEAVY GAS OIL ?

It is a fraction which is used in Diesel fuel and for its characteristics is suitable as FCC feed stock.

Q. 13 WHAT IS REDUCED CRUDE OIL ?

It is what is left at the bottom of the main fractionator after separating lighter fractions.

Q. 14 IS REDUCED CRUDE OIL A FINISHED PRODUCT ?

No, it is not. It is further processed in Vacuum Units where other valuable fractions are extracted.

Q. 15 WHAT ARE THE PROCESSES IN CDU ?

There are four processes, which are: Desalting, Crude fractionation, Off Gas compression and LGO
Dehydration.

Q. 16 WHAT IS DESALTING?
Desalting is a process by which salts, sediments and other impurities are removed from the crude oil.

Q. 17 WHY IS DESALTING NECESSARY ?

Desalting is necessary because impurities are the main causes of corrosion and fouling problem in CDU
equipment.

Q. 18 WHAT IS CORROSION ?
Corrosion is a chemical attack on the equipment caused by acid solutions originating from the
impurities contained in crude oil.

Q. 19 WHAT IS THE DIELECTRIC STRENGTH OF OIL ?

The distance between two electrodes is kept @ 2.5 mm & voltage of around 60 KV is applied for one
minute in oil bath. (it has to withstand this much voltage without giving a spark). If spark (Arc) is
generated that means oil is not healthy. Oil is an insulator having insulating media with very high
resistance.

Q. 20 WHAT IS FOULING ?
Fouling is plugging of equipment such as heat exchangers, caused by impurities contained in crude
oil and favoured by increase in temperature.

Q. 21 WHAT IS THE RESULT OF CORROSION ?

Equipment is designed to work under specified conditions of pressure and temperature. A chemical
attack would undermine its resistance to these conditions.

Q. 22 ARE THERE MATERIALS TO RESIST CORROSION ?

Yes, there are, but as they are very expensive and very often not 100% effective, it is preferable
whenever possible to remove the impurities which cause corrosion.

Q. 23 WHAT IS THE RESULT OF FOULING ?

It reduces the efficiency of equipment in terms of heat exchange, thus affecting the heater load, flow
rate capacity as the passage section is reduced and fractionation quality as trays in the columns are
not completely free.
Q. 24 IS IT POSSIBLE TO REMOVE ALL IMPURITIES FROM CRUDE OIL ?
An ideal 100% efficient system does not exist but certain conditions can be obtained in order to
minimize impurities content and therefore to minimize side effects and problems which can be easily
controlled.

Q. 25 WHAT IS DESALTING PROCESS ?

Desalting is a process whereby water is injected into crude oil (normally 2 to 6% wt) and finely dispersed
by means of a special mixing valve in order to dissolve and dilute the various impurities. This water –
crude oil mixture enters a vessel (normally a Petroce low velocity type electric desalter is used) where
under certain conditions of temperature and pressure the salty water is separating the settling and a
high voltage electric field generated by special transformers which causes the coalescence of the
salty water droplets which, eventually, collect at the bottom of the vessel and hence discharged.

Q. 26 WHAT IS THE WORKING PRINCIPLE OF DESALTING WITH PETRECO ?

It is based on four main factors, temperature (and its relation with pressure), dielectric nature of crude
oil, high voltage electric field effect and setting time. A high voltage electric field applied to the crude
oil / water emulsion under certain conditions of temperature and pressure and in a given time (setting
time) will affect only the salty water droplets which will become electrically charged. These droplets
are in this way attracted to one another thus forming bigger droplets which by gravity collect at the
bottom of the vessel & hence discharged. Dielectric strength of oil is expressed in kilo volts.

Q. 27 WHAT IS THE EFFECT OF TEMPERATURE IN DESALTING ?

The specific gravity of water up to boiling point is almost constant, whereas the specific gravity of crude
oil reduces as temperature increases. This means that at the highest (relatively) temperature possible
the difference in specific gravity between water and crude oil is also the highest possible and this
favours separation.

Q. 28 WHAT IS THE UPPER TEMPERATURE LIMIT IN DESALTING ?

It depends on the design of the system and the economics of it. Temperature goes along with pressure
but its limit is that vaporization is to be avoided as desalting would become inefficient. Normally such
equipment design pressure ranges between 8 to 12 kg/cm2 and operating temperature between 100
to 130 deg C with an efficiency of about 95%.

Q. 29 WHAT IS THE RELATION BETWEEN CRUDE OILS AND TEMPERATURE IN DESALTING PROCESS ?
In a system where pressure is fixed by design, temperature varies directly according to crude oils
specific gravity. Low specific gravity crude oils require low operating temperature.

Q. 30 WHAT IS EFFICIENCY IN DESALTING PROCESS ?

It is referred to as the amount of salts removed over the total salts contend expressed in percentage
points.

Q. 31 WHAT DOES “DIELECTRIC NATURE OF CRUDE OIL” MEAN ?

If we immerse two electrodes connected with a current generating circulating glass containing distilled
water, the result would be that no noticeable current would be measured through the distilled water
between the two electrodes. As soon as some salts are dissolved in this water, there will be a current
whose intensity is proportional to the amount of salt dissolved. Crude oils act like distilled water, no
current or electrical field would affect them. Crude oils contain however, some impurities such as
organic acids which like salts dissolve in water would diminish their dielectric nature, thus affecting
desalting.
Q. 32 HOW DO ORGANIC ACIDS AFFECT DESALTING ?
As both salty water and crude oil containing organic acids are charged by an electrical field, the
breaking of oil / water emulsion and then separation becomes more difficult, thus affecting desalting
efficiency.

Q. 33 WHAT IS NECESSARY TO DO TO REDUCE THE EFFECTS OF ORGANIC ACIDS IN CRUDE OIL DESALTING
?
There are basically two ways : desalting is more effective at a pH ranging between 7 to 7.5. Lower pH
indicates presence of acidity. PH can be controlled by injecting a 5 to 10 % caustic solution. The second
way is to inject a demulsifying agent, whose action is to help organic acids.

Q. 34 WHAT IS PH ?
PH or “Potential of Hydrogen” is a measure of the acidity or alkalinity of a solution, numerically equal
to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity.

Q. 35 HOW MUCH CAUSTIC SOLUTION CAN BE INJECTED IN DESALTING PROCESS ?

As much as required to have a pH in the 7 – 7.5 range. Normally 5 – 10 ppm (part per million) of caustic
on crude oil flow rate would do the job.

Q. 36 IS CAUSTIC SOLUTION INJECTED DOWNSTREAM OF THE DESALTER ?

The desalter has been designed to remove 95% of salts with a max 3 ptb (1 pound per thousand barrels
– 2.85 ppm) residual salt. Under normal conditions soda would be injected to neutralize the chloridic
acid originating from the residual salt left in the crude, but as the coils in CDU I, heater and the transfer
line are in a stainless steel material, any soda injected into the system would affect it by what is called
“caustic embitterment”. Stainless steels are very sensitive to caustic action. At heater operating
temperature they become very fragile.

Q. 37 WHY IN THE CDU I, HEATER COILS AND TRANSFER LINE ARE IN STAINLESS STEEL MATERIALS ?
Nigerian crude oils such as UQCC and gulf crudes contain about 0.5% wt of Naphthenic acids which
attack equipment. This attack is favoured by high temperature and high velocity and these conditions
are found in the heater coils, transfer lines and the hottest parts of columns. The only material suitable
to resist Naphthenic acids action is stainless steel of the ASA 312 – ASA 316 brand.

Q. 38 WHAT IS HIGH VOLTAGE ELECTRIC FIELD IN DESALTING PROCESS ?

It is a field created between the gap of two special electrodes positioned at a certain level in the
desalter by a high voltage generating transformers. A desalter is a cylindrical vessel horizontally placed
of about 15 m length and about 3 m diameter. Inside, in the upper part along the length, two series of
special grill form electrodes are mounted. The lower electrodes are connected to the transformers
while the upper electrodes are earthed. The voltage is in the range of 15000 volts and above because
this is the requirement to obtain a substantial electrical field in the gap between the electrodes. The
gap is 152 mm.

Q. 39 WHAT ARE THE NECESSARY CONDITIONS TO OPERATE A DESALTER WITH HIGH VOLTAGE ELECTRIC
FIELD ?
The first condition is that the vessel be full of liquid as the system has been designed to operate in a
total liquid atmosphere. In fact a low level switch interlock prevents feeding the electrodes. Second
condition is that a certain temperature be reached before the electric field becomes effective. The
consequence of not attaining these conditions is bad separation and water carry over. This means that
before switching on the electric field the desalter has to be completely filled up and secondly, water
injection can be started only when the temperature is appropriate.

Q 40 WHAT ARE THE ELECTRICAL INSTRUMENTS TO MONITOR THE ELECTRICAL FIELD IN DESALTING
OPERATIONS ?
The Petreco desalter is a three phase system with three groups of electrodes connected to three
transformers. One voltmeter and one ammeter are provided for each transformer unit. The current
measured by the ammeter is the function of water injection rate, interface level and emulsion stability.
In fact the current will increase if even one of the three above mentioned factors increase.

Q. 41 WHAT ARE THE BASIC INSTRUMENTS NECESSARY IN THE DESALTER OPERATION?

They are (a) The Mixing Valve (b) Interface Level Controller (c) Effluent Draw-off Valve

Q. 42 WHAT IS THE FUNCTION OF THE MIXING VALVE ?

The basic function of the mixing valve is to create a crude oil / water emulsion which permits the finely
dispersed water to dissolve and dilute impurities. Emulsification is achieved by a controlled pressure
drop condition through the valve. In normal operation pressure drop ranges between 1 to 2 kg/cm 2.

Q. 43 WHAT ARE THE FUNCTIONS OF THE INTERFACE LEVEL CONTROLLER AND EFFLUENT DRAW OFF VALVE
?
The interface level controller controls the Effluent Draw-off valve in order to maintain a constant
interface level. Any upward variation of such interface level affects the electric field, so for good
operations the interface level should be kept at the minimum possible. The controller is located at the
top of the desalter. The oil / water interface level should be kept at 395 mm below the vessel centre
line.

Q. 44 WHAT DOES “SETTLING TIME” MEAN IN DESALTER OPERATION?

It is the minimum required time to allow a good separation between crude oil and salty water. It is
linked with the volume of the desalter and the throughput design of the unit. Any variation of the
settling time depends on two variables the first being the flow rate adjustment and the second the
changing of operating temperature in the desalter. An increase of the operating temperature results
in a reduction of the settling time. An increase of the throughput will also reduce the settling time.

Q. 45 WHAT DOES “DISTILLATION” MEAN ?

Distillation means the basic process for crude oil. The word “distillation” is however improper as such in
crude distillation unit. Distillation means that a compound is brought to its Boiling Point so that its vapours
can be separated from the rest. It takes advantage of one of the physical characteristics of each
hydrocarbon, that is its own boiling point. This technique of separating fractions by distillation was in
use a long time ago. Now the process is that of separating fractions by “Fractioned Condensation:
which basically consists of heating the crude up to a certain controlled temperature so that most of
the components turn into vapours and then condensating them at designed points to form various
fractions.

Q. 46 HOW ARE THE CHARACTERISTICS OF THESE FRACTIONS IDENTIFIED ?

First of all the characteristics of these fractions are linked to the specifications of the process plants for
the fractions destined to undergo a subsequent treatment. Secondly these characteristics are linked
to the market needs for commercial products. The main characteristic of the whole Naphtha, i.e. the
EBP which cannot exceed 175 deg C. The reason is that whole Naphtha is not a commercial product,
thus it needs further treatment and splitting and blending before becoming a commercial product like
Gasoline. The units designed to treat the whole Naphtha are designed to treat up to a maximum of
175 deg C as EBP. Gasoline main characteristic is the Octane Number (ON). So the 175 deg C EBP
characteristic is linked to the specification of the process plant while the ON of the Gasoline is linked
to the market needs.

Q. 47 WHAT IS MEANT BY YIELD OF CRUDE OIL FRACTIONS ?

yield is the percentage amount of a given fraction in a crude oil and depends solely on the type of
crude being processed. Crudes can be grouped into light, medium and heavy crude oils. This
classification does not consider the type and quality of hydrocarbons composing the crude oils. A light
crude oil is a crude that contains more light fractions than heavier ones so its yield in light fractions will
be higher.

Q. 48 HOW IS FRACTIONATION ACHIEVED IN CDU ?

A total of fifty two (52) trays are installed in the main column for establishing equilibrium contact
between the ascending vapours and the descending liquids. The crude, preheated to an established
temperature, enters tangentially the low part of the column in a zone called “flash zone”. In this zone
due to flash effect vapours separate from liquids and start ascending through the trays. Liquids collect
at the bottom of the column. As the temperature in the column decreases gradually due to a number
of downflowing refluxes, vapours condense and collect on trays according to their characteristics. As
a result, the lightest fraction (whole Naphtha) accumulates at the top while the heaviest fraction
(reduced crude oil) collects at the bottom and intermediate fractions (Kero – LGO – HGO) in between.

Q. 49 WHAT IS EQUILIBRIUM CONTACT ?

It is the function of the trays installed in the column. A fractionation tray is a round plate filled with holes
that allows the passage of the ascending vapours. In the middle of the tray there is a slot where the
forming liquids collect in a sort of gutter connected to a draw off piping system. Part of the ascending
vapours passing through the holes will condense on the tray due to the action of the descending
liquids. A level of liquid is continuously formed and maintained on the tray while the ascending vapours
bubble through it. The condition of tray temperature determines the equilibrium between vapours and
liquid, thus a variation of reflux determines a variation of tray temperature which determines in turn a
variation of vapours / liquid equilibrium.

Q. 50 WHAT IS FLASH EFFECT ?

It is an effect that due to the rapid increase in volume from the transfer line to the flash zone allows not
only the separation of vapours from liquids but also produces some additional vaporization. To
vaporize, liquids absorb heat and the extension of vaporization is linked with the drop in temperature
between transfer and flash zone. This additional vaporization or over flash is necessary in order to ensure
that adequate reflux be available in the trays between the flash zone and the HGO draw off tray.
These extra vapours will condense and collect on the tray below the HGO draw off pan. When a
certain level is reached there is an overflow of liquid that through a piping system is sent to the bottom
of the column above the stripping trays. This system is called run back and its flow rate 91.5 to 2% wt on
crude) is proportional to the overflash.

Q. 51 WHAT IS THE FUNCTION OF REFLUX ?

Vapours ascend until they find a zone in the fractionator where the temperature is low enough for
them to condense. The drop in temperature through the column can be achieved in two ways, one
being the size of the column and the second the use of a cooling liquid. During their ascent, vapours
cool down naturally but slowly and it would take a very tall column to achieve condensation. Obviously
a column of that size would be out of the question for economic reasons. This problem has been
overcome by pumping liquids to cool the vapours, thus subtracting heat, until they condense. In this
way a tall column is not needed (however the size of the column depends on the volume of
hydrocarbons to be processed) because the cooling liquids do the job. These cooling liquids are called
either refluxes or pump arounds or side refluxes. Their composition reflects that of the fractions to be
extracted from the fractionator. The number of refluxes is a function of the number of fraction to be
extracted. The main fractionator is equipped with a top reflux and three pumparounds or side refluxes.
Other columns such as stabilizers, strippers etc are provided with one top reflux only, as only one
fraction is extracted.

Q. 52 WHAT ARE THE HEAT TRANSFER TRAYS ?

In the column there are basically two groups of trays: Fractionation Trays (described in Q No. 49) and
Heat Transfer trays. They are so called because their function is to implement the action of the side
refluxes. They are situated in between the fractionation tray groups. A heat transfer tray is a plate also
filled with holes but instead of having a central collecting gutter it has two lateral collecting pans. From
the two pans a draw off piping system conveys the liquid accumulating on the pans to a pump (side
reflux pump) which recirculates then (first through heat exchangers in order to reduce temperature)
and sprinkle them above the tray. The ascending vapours meet the sprinkled liquid and by this contact
some heat will be exchanged between the vapours and the liquids causing some condensation. A
variation of the ______________ of the liquids causes a variation of the condensation rate of the vapours,
thus changing vapours / liquids equilibrium.

Q. 53 WHAT ARE THE EFFECTS OF TEMPERATURE IN DISTILLATION ?

Distillation is achieved by means of temperature. It should be the maximum possible in order to obtain
a good fractionation, but on the other hand, it has a cracking effects on hydrocarbons, so it is limited
to reduce at the minimum cracking effect. Years ago the transfer temperature used to be in the 300
to 320 deg C range. Nowadays this range is up to 360 to 380 deg C, the reason being the improved
distillation technology. Higher temperatures demands, however, equipment made out of material with
higher stress resistance degree, which is more expensive.

Q. 54 WHAT ARE THE EFFECTS OF PRESSURE IN A DISTILLATION TOWER ?

The operating pressure of a fractionator depends on the nature and the RVP of the feedstock.
Distillation units normally operate at a relatively low pressure (1 to 1.3 kg/cm 2 in the flash zone). By
increasing the operating pressure, fractionation is affected as fractions become lighter, but also, after
increasing the transfer temperature as well, fractionation will improve, but this will also lead to higher
cracking rate, higher material stress, low efficiency. It is clearly evident that the improved fractionation
is overcome by the negative effects. So pressure is designed to be kept at the minimum compatible
with the nature and RVP of the feedstock.

Q. 55 WHAT IS RVP ?
RVP is a laboratory test which measures the vapour pressure of hydrocarbons at a given temperature
(100 deg F or 37.8 deg C)

Q. 56 WHAT ARE THE CUTS RANGES IN CDU ?

TBP CLEAR CUT

RANGE deg C CDU 1 CDU 2
FOR DESIGN
CUTS OR DESIGN OPERATING DESIGN OPERATING
FRACTIONS IBP EBP IBP EBP IBP EBP IBP EBP
 WHOLE NAPHTHA --- 170 35 165 ---- 150 35 160
175 170
KEROSENE 170 140 155 235 150 245 150 230
175 250 165 240
LIGHT GAS OIL 245 350 235 350 245 325 225 325
250 330 235 335
HEAVY GAS OIL 350 370 250 400 325 370 300 370
330 310 385
REDUCED CRUDE 370 -- -- -- 370 -- 260 30%
OIL 300 500

Q. 57 WHY ARE THE CUTS OBTAINED IN OPERATING CONDITIONS NOT CORRESPONDING TO DESIGN
VALUES ?
100% fractionation efficiency is impossible as there are either overlaps and gaps among the various
fractions. However specifications are given to establish limits for overlaps and gaps. To __________ cuts
are adjusted in accordance with an established program to meet certain yields.

Q. 58 WHAT ARE THE OVERLAPS AND GAPS ?

Overlap means that two consecutive fractions have a range of hydrocarbons whose boiling points are
in common, while gap is an interval between two consecutive fractions boiling points, which means
that the EBP of a fraction is lower than the IBP of the next heavier fraction. In the ASTM distillation curves
gaps and overlaps re not, however, referred to the IBPs or EBPs, but to the 5% and 95% points of each
fraction, the reason being the difficulty in determining such points. As a matter of fact 5% and 95%
points are much more reliable.

Q. 59 WHAT ARE THE SPECIFICATIONS OF LIMIT GAPS AND OVERLAPS ?

The ASTM curves corrected for loss will show a minimum gap of 15 deg C between the 95% point of the
whole Naphtha and 5% point of the Kerosene, a minimum gap of 12 deg C between the 95% point of
the kerosene and 5% point of the Light Gas oil, a minimum gap of 5 deg C between the 95% point of
the Light Gas Oil and the 3% point of the Heavy Gas Oil. The 90% point of Heavy Gas Oil shall not be
greater than 380 deg C, while the atmospheric residue shall not contain more than 3% on crude
distilling below 370 deg C.

Q. 60 ARE THERE OTHER SPECIFICATIONS FOR CDUs FRACTIONS ?

Yes, there are flash point minimum values for Kerosene, Light Gas oil, Heavy Gas Oil and Reduced
Crude Oil which are 40 deg C, 74 deg C, 90 deg C and 150 deg C respectively. Then the water content
in the Light Gas Oil fraction which shall not be greater than 100 ppm.

Q. 61 WHAT IS THE FLASH POINT AND ITS IMPLICATION IN PRODUCT HANDLING ?

Flash point is a temperature at which a product must be heated so that its vapours ignite with the
presence of a flame or spark. Kerosene for instance has a minimum flash point specification of 40 deg
C while the operating one is put at 50 deg C. it is obvious that handling kerosene at 35 deg C is safer
because vapours will not ignite if any flame or spark occurs, but they will if they are allowed to reach
the flash point.

Q. 62 WHAT DETERMINES A LOW FLASH POINT AND HOW CAN IT BE ADJUSTED ?

A low flash point is determined by the presence of light ends in heavier fractions. At every point of the
fractionation there is always an equilibrium between vapours and liquid phases. A mixture of various
hydrocarbons brought up to a given temperature will produce a pressure which is the sum of the partial
pressure produced by each hydrocarbon. Light hydrocarbons will contribute most of this pressure as
they evaporate most but still some of them will remain liquid for the equilibrium law. A variation of the
total pressure will cause the conditions to adjust the Flash point of a given fraction.

Q. 63 WHAT METHOD IS USED TO ADJUST THE FLASH POINT OF A GIVEN FRACTION ?

Side fractions drawn off the fractionator enter special strippers where some superheated steam is
injected. The function of the steam is that of lowering the overall pressure of vapours thus facilitating
the evaporation of light ends still tapped in the liquid.

Q. 64 IS THERE ANY OTHER SYSTEM TO ADJUST FLASH POINT ?

Yes. Flash point apart from being conditioned by the presence of light ends, is also conditioned by the
IBP of a given fraction. So a variation of the IBP will determine the variation of the flash point.

Q. 65 STRIPPING STEAM IS ALSO INJECTED INTO THE BOTTOM OF THE FRACTIONATOR. WHY ?
Stripping steam injected into the bottom of the fractionator has two functions: One is to strip the liquid
residue and the second is to help additional vaporization of crude oil in the flash zone thus avoiding
exceeding the maximum heater temperature outlet.

Q. 66 WHAT ARE THE CHARACTERISTICS OF STRIPPING STEAM ?

Stripping steam is a low pressure steam which is superheated in the convection zone of the crude oil
main heater. Its temperature is in the range of crude oil transfer temperature but his is not really relevant
for the function it has. However steam temperature upper limit depends on the quality of equipment
while it is important that it be always dry.

Q. 67 WHY IS STRIPPING STEAM SUPERHEATED ?

A volume of water when evaporating, expands at least about 1600 times, so if wet steam is allowed
into the fractionator can cause damage because its expansion cannot be controlled, hence the
necessity to have it superheated at all times.

Q. 68 HOW MUCH STRIPPING STEAM IS NORMALLY INJECTED ?

Average injection of stripping steam is in the range of 2 to 2.5% of the cut to be stripped. Higher values
are not effective and would increase the overhead condensers load. In addition the overhead vapour
temperature would approach the steam dew point which is undesirable for corrosion problems caused
by acids solutions formed by the condensation of steam.

Q. 69 WHAT IS LGO DEHYDRATION ?

It is a process whereby water contained in LGO is eliminated or reduced within limits by evaporation
at vacuum conditions (50 mm Hg absolute pressure and 120 deg C of temperature)

Q. 70 WHAT IS THE PURPOSE OF LGO DEHYDRATION ?

Among LGO specifications there is one Cloud Point which is affected by the presence of water, thus
by eliminating water, LGO could point is improved.

Q. 71 WHAT IS CLOUD POINT ?

Cloud point is a standard Laboratory test which measures the temperature at which an oil sample
develops a haze or opacity or cloud from solid waxy particles. It is a rough indicator of wax content.
For Gas oil the limit is 40 deg F (4.4 deg C) max. Gas oil extracted from Nigerian crude oil has a cloud
point of – 4 to + 4 deg C, while gas oil extracted from Venezuelan crude oil has a cloud point of – 14
to – 18 deg C.
Q. 72 HOW IS DEHYDRATION ACHIEVED ?
The stripped light gas oil is charged to a column called vacuum dryer. Since its temperature, from the
stripper is too high for dehydrating, light gas oil is cooled down to a convenient temperature, that is
about 120 deg C. An ejector system creates vacuum in the dryer and water contained in the gas oil is
rapidly evaporated and evacuated by the ejectors.

Q. 73 WHY IS IT NECESSARY TO REDUCE LIGHT GAS OIL TEMPERATURE ?

Under standard conditions gas oil boiling points are 230 to 360 deg C. At vacuum conditions these
boiling points are greatly reduced. Therefore if gas oil is fed into the dryer at stripper temperature that
is above 250 deg C, a great part of it would evaporise together with the water making meaningless
use of the dryer. However, at operating conditions, a small amount of gas oil evaporates but is
reasonably low.

Q. 74 HOW IS THE GAS OIL TEMPERATURE CONTROLLED ?

The liquid temperature of LGO is controlled by a temperature control valve which bypasses an LGO
cooler. The variation of the bypass opening determines the variation of the LGO liquid temperature.

Q. 75 HOW ARE THE VAPOURS, EVACUATED BY THE EJECTORS, DISPOSED OF ?

Evacuated vapours and exhaust steam are condensed in the ejector condensers. The condensate is
collected in a slop oil separator from where slop oil is sent into the slop header and the water (foul
water) is sent to the sour water stripper unit for disposal.

Q. 76 WHAT IS OFF GAS RECOVERY SYSTEM ?

Light hydrocarbons that accumulate in the fractionator overhead vessel are mostly in gas phase
because the operating pressure of the overhead vessel is not high enough to allow their condensation.
These hydrocarbons include C1, C2, C3, C4 and some C5. To maintain the pressure of the system these
gases have to be discharged, but since this pressure is low they would be disposed of to the Blow Down
system, thus losing valuable products if an Off Gas Recovery System was not installed with the purpose
of recovering them.

Q. 77 WHAT DOES OFF GAS RECOVERY PROCESS CONSISTS OF ?

The non condensed off gas from the main fractionator overhead receiver is compressed by a
compressor and absorbed by the liquid from the receiver. The mixed stream is cooled to obtain good
absorption and condensation of the off gas.

Q. 78 HOW IS THIS PROCESS ACHIEVED ?

In the main fractionator overhead receiver, vapour, oil and water phases are formed. Vapour, namely
off gas is sent into a K.O. drum from where is sucked and compressed by an off gas compressor. The
oil from the overhead receiver is pumped to the Naphtha Hydrotreater Unit (NHU). Part of it is however
sent to the delivery line of the gas compressor to contact compressed gas. The mixed streams pass
through a water condenser and are then introduced into an off gas condensate drum. In this drum
gases heavier than C2 are condensed while and remain mostly in gas phase and are sent into the Fuel
Gas header for disposal or to be spilled back to the main fractionator overhead receiver for pressure
control. The liquid from the off gas condensate drum is pumped to rejoin the liquid that is sent to the
Naphtha Hydrotreater Unit.

Q. 79 WHY IS THE OFF GAS SENT INTO A KNOCK OUT DRUM BEFORE BEING COMPRESSED ?
The off gas can entrain liquid that is dangerous for the smooth running of the gas compressor. The K.O.
drum prevents such liquid from going into the gas compressor by knocking it down. In addition the KO
drum is fitted with alarms that can stop the compressor automatically if the liquid level goes beyond
the maximum permitted. A steam coil is fitted to the bottom of the KO drum. Low pressure steam is
used to vaporize entrained liquid so that the compressor receives gaseous products only.

Q. 80 WHAT IS THE EFFECT OF PRESSURE ON THE OFF GAS RECOVERY ?

Higher pressure will ease the recovery of lighter gas but the maximum pressure depends, however, on
the characteristics of the off gas compressor.

Q. 81 WHAT IS THE EFFECT OF TEMPERATURE ON THE OFF GAS RECOVERY ?

Higher temperature will adversely affect the recovery of the off gas as it reduces its condensation rate
and this is why the compressed gas as it has gained temperature from the compressor, is introduced
into a water cooler.

Q. 82 WHAT IS THE ABSORPTION CAPACITY OF THE LIQUID THAT CONTACTS COMPRESSED OFF GAS ?
The absorption capacity of the liquid depend on its flow rate, therefore the higher the flow rate the
higher the off gas absorption by the liquid. Sufficient flow rate of the liquid should be kept to maintain
the desired degree of condensation.

Q. 83 WHAT IS A FURNACE OR HEATER ?

A furnace is a piece of equipment designed to generate heat and transfer it to the charge.

Q. 84 HOW IS HEAT GENERATED IN A FURNACE ?

Heat is generated by using the basic process of combustion which is produced with burners that
combine fuel and air and the mixture is burnt at the burner tip.

Q. 85 WHAT DOES A FURNACE CONSIST OF ?

A furnace consists of a fire box or combustion chamber, a convection section and a stack, fitted with
a damper, to discharge the product of the combustion or Flue gas into atmosphere at a convenient
height. The internal walls of all these parts are covered with special refractory lining. Tubes in which
charge is passed are fitted on the walls and ceilings of the furnace. On the floor of the combustion
chamber there is number of holes through which a number of burners are fitted to produce combustion
to generate heat. Devices such as temperature and pressure indicators, temperature, flow and
pressure controllers, oxygen analysers and others are installed to control the combustion, the transfer
of heat and to monitor the working condition of this piece of equipment.

Q. 86 WHAT IS COMBUSTION ?
Combustion is a chemical reaction by which fuel brought to ignition temperature combines with
oxygen to produce flue gas, heat and light.

Q. 87 WHAT IS FUEL ?
Fuel is a substance that by the effect of its oxidation is capable of generating heat.

Q. 88 HOW ARE FUELS CLASSIFIED ?

Fuels are classified as follows:
- Solids such as wood, charcoal, coke etc.
- Liquids such as kerosene, diesel, heavy oils etc.
- Gases such as methane, natural gases, artificial gases etc.
Q. 89 WHAT IS THE COMPOSITION OF A FUEL ?
Fuels physically, greatly differ from one another because of their different molecular structure but
basically their chemical composition is similar as they are constituted of carbon and hydrogen
combined together in various proportions.

Q. 90 WHAT IS MEANT BY FUEL HEAT VALUE ?

The heat value of a fuel means the amount of heat expressed in kilocalories, produced by the
complete combustion of one kg of it. Knowing that 1 kg of pure carbon generates 8080 Kcal and 1 kg
of hydrogen generates 34000 Kcal after complete combustion, is possible to determine the heat value
of a fuel by knowing its carbon and hydrogen content. This heat value is determined in laboratories by
means of special instruments called calorimeters.

Q. 91 WHAT IS THE DIFFERENCE BETWEEN GROSS HEAT VALUE AND NET HEAT VALUE ?
Gross heat value is the heat determined in laboratories and takes into account all the heat produced
by the complete combustion of 1 kg of fuel.
Net heat value is the heat that is usable in the heat transfer process. Its value is, of course, lower than
the gross because the heat released by the produced water in vapour phase during condensation is
not considered.

Q. 92 WHAT IS MEANT BY FURNACE EFFICIENCY ?

Furnace efficiency is expressed in percentage points and is the amount of heat effectively used over
the amount of available heat. The easiest way of determining the efficiency of a furnace is that of
checking the ratio between the amount of heat acquired by the charge and the amount of net heat
produced by the fuel in a given time.

Q. 93 WHAT IS THE CHEMISTRY INVOLVED IN COMBUSTION ?

Fuel components are carbon and hydrogen and the reactions will be as follows:
1 kg of C consumes 2.66 kg of oxygen to form 3.66 kg of carbon dioxide or CO 2 plus 8080 Kcal for
complete combustion.
1 kg of C consumes 1.33 kg of oxygen to form 2.33 kg of carbon monoxide of CO plus 2170 Kcal for
incomplete combustion.
1 kg of H2 consumes 8 kg of oxygen to form 9 kg of water or H2O plus 34000 Kcal.

Q. 94 WHAT IS THE AVERAGE NET HEAT VALUE OF FUELS ?

It is necessary to make a distinction between the liquid and gaseous fuels. Liquid fuel heat values are
referred to by the standard weight of one kg. Gaseous fuels heat values are referred to by the standard
volume of one cubic meter or NCM under standard conditions (at atmospheric pressure and zero deg
C temp). For example 1 kg of hydrogen will develop 29600 Kcal net heat value (being the gross heat
value 34000 Kcal) but 1 m3 of hydrogen will develop 2570 kcal net heat value (being the gross valve
3050 kcal) only. For liquid fuels the net heat value ranges between 9000 and 11000 kcal / kg. This value
depends on the density of the fuel, thus the higher the density the lower the heat value and this is
because of the major concentration of carbon which has a heat value lower than that of hydrogen.
For gaseous fuel the net heat value ranges between the already mentioned 2570 kcal for hydrogen
up to 13580 kcal for acetylene (C2H2)
Refinery fuel oil heat value is almost constant with no appreciable variation, while fuel gas heat valve
can vary greatly. Fuel gas contains H2, CH4, C2H6 as main components but C2H8 and C4H10 are also
present. The emission of gas from the various units and which forms the fuel gas network is not constant
and as consequence the F G composition changes continuously thus affecting its heat value.
Q. 95 WHAT IS TEMPERATURE ?
Temperature is, as a general definition, the state of hotness or coldness of a body, but in more definite
terms, is the measurement of the state of agitation or turbulence of the molecules forming the matter.
Absolute temperature of zero corresponds to molecules in a static condition. Temperature is normally
measured in two scales, thus degrees centigrade ( 0 C) or degrees Fahrenheit ( 0 F) are used.

Q. 96 WHAT IS THE RELATIONSHIP BETWEEN THE TEMPERATURE AND HEAT ?

Whilst temp has been defined in question No. 94, heat is the amount of thermal energy possessed by
a body and is directly related to the temperature. Temperature is measured in ( 0 C) or ( 0 F) whereas
heat is measured in calories.

Q. 97 WHAT IS CALORIE ?
Calorie or small calorie is the amount of heat necessary to increase the temperature of one gram of
water by 1 0 C. K cal or big calorie, is one thousand time the small calorie. As a normal reference Kcal
and 0 C will normally apply on this handout.

Q. 98 WHAT IS SPECIFIC HEAT ?

Specific heat is the amount of heat necessary to increase the temperature of 1 kg of a substance by
1 deg C. water has by far the highest specific heat among the various substances. In fact 1 Kcal is
necessary to raise the temp of 1 kg of water by 1 deg C while only about half Kcal is necessary to raise
the temperature of 1 kg of oil by the same amount. Steel specific heat is 0.15 Kcal/kg/deg C which
means that using a same amount of heat 1 kg of steel raises its temperature almost seven times
compared to that of 1 kg of water. This definition is important because its understanding helps the
understanding of heater operation.
Q. 99 HOW IS HEAT TRANSMITTED TO THE CHARGE ?
Transmission of heat is performed in three ways: by radiation, by convection and by conduction. But it
is by conduction that, as a final step, heat is transmitted to the charge, while radiation is the heat
originating source.

Q. 100 WHAT IS RADIATION ?

Radiation is the propagation of waves or particles such as light or radiant heat. Burners produce a
flame as a result of the combustion process. This flame radiates heat. Tubes exposed to the flame
receive radiant heat directly. In fact this is the reason why the combustion chamber is also called
radiant zone.

Q. 101 WHAT IS CONVECTION ?

Convection is the transfer of heat by massive motion of flue gas which contacts tubes. The product of
combustion is a mass of hot gas which is called flue gas. Tubes contacted by the motion of this flue
gas receive what is called convectional heat. The part of the heater that is not directly exposed to the
action of the flames is called convection zone.

Q. 102 WHAT IS CONDUCTION ?

Conduction is the transfer of heat through a conducting medium without perceptible motion of the
medium itself. In our case the medium is the tubes which receive heat by either radiation or
convection. Heat is conducted through the thickness of the tubes to reach the charge.

Q. 103 WHAT IS REFRACTORY ?

Refractory is a special material that is used to cover the inside of a heater in order to reflect the heat
back into the firebox. A furnace cannot exist without refractory. Material composing refractory must
be heat resistant as temperatures approaching 1000 deg C in the radiant and convection zones are
common features. Alluminium oxides and silica are normally used for refractory making. However,
materials that melt at temperatures above 1600 deg C can be usefully employed for refractory making
being the main condition or characteristics a low thermal conductivity to minimize heat dispersion.

Q. 104 WHAT EQUIPMENT IS NECESSARY TO MONITOR THE PERFORMACNE OF TUBES IN A HEATER

There are two ways to monitor the performance of tubes and both essential because they are
complementary visual inspection to detect possible flame impingement, pinholes or cracks or
whatever can go wrong inside the heater. Instrument monitoring shows in detail the performance of
tubes. Each pass or coil is provided with flow recorder controller, pressure gauges, I/L and O/L
temperature gauges and inside the heater special Skin Temperature gauges to monitor the conditions
of tubes.

Q. 105 WHAT IS THE SKIN POINT TEMPERATURE AND ITS IMPORTANCE IN THE CONDUCTION OF A
HEATER ?
Skin Point is the temperature of the external surface of tubes measured at different point of their length
in the radiant zone which is normally the hottest part of a heater and also where the bulk of the heat
produced is transferred to the charge. Flue Gas leaving the radiant zone has a temperature of about
900 deg C by design whilst Flue Gas leaving the convection zone has a temperature of about 530 deg
C by design and the Flue Gas leaving air preheater has a temperature of about 250 deg C by design.
Tubes installed are 6” diameter and 7 mm thickness and the temperature of the charge ranges
between 250 deg C at the I/L of the convection zone upto 370 deg C at the O/L of the radiant zone.
In the convection zone tubes are subjected to heat at temperature between 900 deg C and 530 deg
C while in the radiant zone they are subjected to temperature at 900 deg C and above. It is evident
that tubes in the radiant zone are subjected to higher temperatures and that skin temperature is very
important to monitor their efficiency. By design radiant tube S.P.T. has been fixed at 550 deg C while
for a convection tubes, it has been fixed at 400 deg C.

Q. 106 WHAT HAPPENS IF SKIN POINT TEMPERATURE GOES UP ?

Thermal stress at which tubes are subjected specially in the radiant zone is quite consistent. In fact the
drop of temperature between S.P. and the charge is 150 deg C for convection tubes and 180 deg C
for radiant tubes at design conditions. This drop of temperature occurs within the 7 mm thickness of the
tubes. Tubes are continuously cooled by the passing charge. However the internal wall of the tubes
must have a temperature not exceeding 370 deg C. If it is exceeded, charge thermal cracking with
deposits of coke on the internal walls will be the result. As coke thermal conductivity is quite lower than
that to the tubes, heat transfer is reduced thus affecting the skin point temperature which will increase.
As the condition progresses tube plugging worsens with possible rupture of the tubes affected.

Q. 107 HOW CAN SKIN POINT TEMPERATURE GO UP ?

There are basically three ways that affect S.P.T. One is a reduction of the charge flow rate not coupled
with a correspondent reduction of the heat produced. The second is the opposite, an increase of the
heat produced without a correspondent increment of the charge flow rate. The third, but probably
the most important, is the flame impingement.

Q. 108 WHAT IS FLAME IMPINGEMENT ?

Fuel oil burners should produce a flame with a shape similar to the one produced by a candle when
not affected by any turbolence. It should be bright, not smoky and its height not more than the two /
thirds of the height of the firebox. It should be, however, equidistant from the tubes. If for any reason
the flame becomes abnormal, erratic flashing or bad shaped flame pattern will occur. Spots of tubes
are licked by this erratic flame will become hotter. Coke deposits will be the result with the
consequences already mentioned before. Fuel Gas burners produce instead a flame that is wider and
taller if compared with an equivalent fuel oil flame, so the upper part of the firebox is subjected to
higher temperatures. Since its flame is less visible than the F.O. flame, it is more difficult to detect
possible flame impingement.

Q. 109 WHAT IS A BURNER ?

A Burner is a device that mixes the fuel and air to aid complete combustion and provide a uniform
and stable flame pattern. There are two types of burners: (a) Fuel Gas Burrners and (b) Fuel Oil Burners

Q. 110 HOW IS FUEL AND AIR MIXING ACHIEVED IN A BURNER ?

In the Fuel Oil Burner the mixing of oil and air is achieved with the help of atomizing steam. A Fuel Oil
burner is constituted of two concentric pipes which end in a mixing chamber of burner tip. The burner
tip has some properly oriented holes or nozzles from where the fuel / steam mixture is spread out. The
oil is passed into the inner pipe while the steam is passed into the space between the two pipes. In the
mixing chamber, steam violently mixes and disperses the oil into very fine particles which are then
spread out of the nozzles. These fine particles mix intimately with the air coming from an opening
underneath the burner in order to aid complete combustion. The orientation of the nozzles is such that
the fuel spread out of them forms a uniform and stable flame pattern after ignition. To achieve good
dispersion in the mixing chamber it is necessary that steam in introduced very dry at a pressure of 1.5
to 2.5 kg/cm2 higher than that of the fuel oil. In the Fuel Gas Burner the gas obviously does not need
to be dispersed or atomized so there is not steam being used for this purpose. The Fuel Gas Burner is a
toric pipe containing a number of holes evenly distributed along the top surface through which gas is
flashed out. The air coming from underneath the burner mixes with it to aid complete combustion.
Q. 111 WHAT IS PRIMARY AIR AND SECONDARY AIR ?
The openings underneath the burners that allow the air to reach both Fuel Oil and Fuel Gas are
adjustable and are called air registers. The air passing through the FO / air register is the Primary Air
while the air passing through the the FG / air register is called Secondary air.

Q. 112 WHAT IS DRAFT OR DROUGHT IN A HEATER ?

Draft is a current of air induced by either natural or artificial means. In a heater the combustion of fuels
produces a very hot Flue Gas which flows to the atmosphere through the stack. This ascension of flue
gas creates a vacuum in the combustion chamber or firebox which attracts or sucks combustion air
through the air registers. This is called natural draft system. But if the flue gas for reason of heater design
cannot flow to the atmosphere through the stack naturally a forced draft system is used to maintain
an appropriate current of air to aid complete combustion of the fuel.

Q. 113 HOW IS DRAFT BEING CONTROLLED IN A HEATER ?

Draft in a heater must be controlled because it determines the combustion efficiency and in general
the heater efficiency (see question No. 90). Draft is controlled by adjusting either air registers and / or
dampers. Monitoring instruments such as Draft Gauges (DG), Oxygen analysers and heater
temperature indicators are used to control draft.

Q. 114 HOW IS FORCED DRAFT IN A HEATER ACHIEVED ?

First it is necessary to make some distinctions as there are three types of forced draft.
1. Forced draft which is achieved by means of special blowers that force air through registers into the
firebox. The amount of air is controlled by adjusting special louvers located at the suction air duct.
2. Induced draft which is achieved by means of special fans that suck flue gas from the heater thus
creating some vacuum in the firebox which in turn induce atmospheric air to pass through the air
registers. The flue gas is controlled by adjusting special louvers located at the suction flue gas duct
and which, obviously, replace the damper.
3. Balanced draft : This system is the most commonly used in refinery for heaters of consistent capacity
as flue gas heat recovery is involved (Ljungstrom) for greater heater efficiency. This system is
combination of both forced and induced drafts.

Q. 115 WHAT ARE DRAFT GAUGES ?

Draft gauges are special manometers that measure either positive or negative pressure inside the
various parts of heaters. They read pressures ranging from – 50 to + 50 mm of water column (-0.005 to
+0.005 kg/cm2)

Q. 116 WHAT IS HEAT RECOVERY SYSTEM ?

Flue gas that goes out of the heater through the stack contains very valuable heat which if not utilized
represents a lot of money wasted in fuel terms. The only way to recover this heat is to transmit it to
incoming combustion air. However it is not possible to recover all the heat from the flue gas because
of the dew point which is in this case source of acid formation and consequent equipment corrosion.

Q. 117 WHAT IS DEW POINT AND ITS RELATION WITH ACID FORMATION IN A HEATER ?
Dew point is the temperature at which air becomes saturated and produces dew, usually at night,
onto cool surfaces. Dew are water droplets condensed from the air. Flue gas originating from the
combustion process contains among other gases steam and sulphur trioxide. If the temperature of the
flue gas is allowed to cool to reach the dew point, the consequent condensation of steam will form
together with the sulphur trioxide, sulphuric acid which is highly corrosive specially in diluted form.
Therefore the temperature of the flue gas in the stack should not be less than 200 deg C as this favours
its dispersion in the atmosphere at a sufficient altitude and avoid sulphuric acid formation.

Q. 118 HOW IS HEAT RECOVERY SYSTEM ACHIEVED IN A FURNACE ?

There are mainly two types of air preheaters or heat recovery system used in furnaces and boilers. They
are called Ljungstorm air preheater and Green air preheater. The Ljungstorm type consists of a “cake”
like housing mounted on a rotating shaft. This “cake” is radially partitioned into boxes filled with
corrugated steel sheets so as to increase the heat transmitting surface. The “cake” rotates very slowly
and during its passage through the flue gas duct absorbs the heat from the outgoing flue gas and
releases it to be incoming carburant air during its passage through the air duct. The two ducts are very
close to one another and the rotating “cake” is boxed up with them so as to form a compact element.
The green air preheater consists of some finned casings placed in the flue gas duct. The flue gas passing
through them gives off its heat to the walls and fins. This heat is then absorbed by the externally passing
air. In operation it works like an air cooler but of course with a different design.

Q. 119 WHAT ARE THE PROBLEMS AFFECTING BALANCED DRAFT SYSTEMS AND AIR PREHEATERS?
Such equipment is very costly and therefore it is designed for heaters of a certain capacity. It improves
the efficiency of a heater a great deal thus saving money by saving fuel. In the past such heaters were
designed to operate at full capacity even in the event of bypassing such equipment. Nowadays to
reduce construction and operation costs such heaters cannot operate without the use of air
preheaters unless reducing their capacity substantially. Considering these factors it is imperative that
this equipment runs continuously and smoothly. All rotating parts need to be properly maintained
through checking. All lubricating points (bearings gear boxes etc) need to be inspected often to
ensure good lubrication. Any stoppage of any piece of this equipment can detemine the possible shut
down of the heater and then of the unit.

Q. 120 WHAT IS EXCESS AIR ?

Theory shows how much air is required to burn units of fuel but this is not enough to ensure a complete
combustion of fuel for the following reasons:

a) Inability to reduce fuel particles dimensions to molecular size (atomizing steam plays a great role
but it is not enough)
b) Inability to maintain a constant flow and characteristics of charge, fuel and carburant air.
c) Inability to control the atmospheric weather. Each of the above reasons, alone, is sufficient to make
perfect combustion impossible. Therefore taking into account all the above factors, a certain
amount of air, normally 15 to 20% volume above the theoretical amount required, is injected to
overcome them. In other words this air which is called excess air must be injected to maintain
control of the heat generated and transmitted to the charge.

Q. 121 WHAT HAPPENS WHEN EXCESS AIR IS VARIED OUT OF ITS NORMAL RANGE ?
Excess air absorbs heat generated by the fuel and which is meant to be transmitted to the charge.
Temperature of the charge drops and to maintain it additional fuel is required. If conduction and
performance of the heater is considered normal when excess air is within 15 to 20% volume greater
excess air involves higher fuel oil consumption with consequent lower heater performance. Lower
excess air favours incomplete combustion, lower heat generated and higher fuel consumption, thus
favouring smoky flames and black smoke out of the stack with consequent pollution of the surrounding
atmosphere. So higher excess air is preferred.

Q. 122 WHAT ARE THE STEPS TO BE TAKEN TO PREPARE A FURNACE FOR LIGHTING ?
To prepare a furnace for lighting the following steps are to be taken.
1. Clear the area around it for any debris.
2. Inspect the fire box for any possible unwanted material.
3. Seal the furnace allowing the air to go through the air registers only.
4. Check the emergency steam lines and make sure that steam is available for prompt use.
5. Check the atomizing steam lines and crack open the valves at all burners so as to allow some steam
to enter the fire box. This operation is necessary in order to check possible steam leaks and to purge
burner guns.
6. Commission the air preheater system and adjust the louvers in order to have the required draft. This
operation is necessary to ensure a constant draft and an adequate purging of the furnace. When
the furnace is not equipped with air preheater system adjust damper and air registers for the
required draft and use steam for purging.
7. Commission the F O lines making sure that all valves at the burners are closed. The circulation of FO
is necessary in order to have it ready for lighting at the right temperature. Lower temperature of FO
implies higher viscosity and therefore more difficulty to burn it efficiently.
8. Commission the FG lines feeding the pilots and the FG burners. All the valves at burners must be
closed. The checking of the FG system must start from the battery limit.
9. Check that all the instruments connected to the heater are in good working condition.
10. Ensure that the charge is being passed and well distributed through the passes.
At this point the furnace is ready for lighting.

Q. 123 HOW IS EXCESS AIR IN A FURNACE MEASURED ?

There are two main instruments to measure excess air : They are the Orsat analyzer and the Oxygen
analyzer. The Orsat analyzer is not used any more for its poor accuracy and because it requires some
time to give result. It has been replaced by electronic equipment which is not only more accurate but
can also be used continuously. The excess oxygen measured by these instrument is in volume so to
know roughly the excess air of a heater it is sufficient to multiply the oxygen percentage points by five.
For example if the oxygen analyzer reads 3.5% the excess air will be 3.5 x 5 + 16.6% by volume.

Q. 124 WHAT IS A SOOT BLOWER ?

A soot blower is a piece of equipment installed in heaters and blowers whose function is to remove
soot from tubes.

Q. 125 WHAT IS SOOT ?

Soot is a carbon deposit originated from the combustion of fuel oil and which settles on tubes in
heaters, mainly on those areas not directly exposed to the burner flames, like the convection zone.

Q. 126 WHY IS IT NECESSARY TO REMOVE SOOT FROM TUBES ?

The carbon deposit or soot settling on tubes creates a sort of insulation, thus reducing the effects of
heat transfer through the tubes to the charge.

Q. 127 HOW IS IT POSSIBLE TO DETERMINE WHEN SOOT IS TO BE REMOVED ?

Due to the soot, efficiency of heat transfer is reduced, therefore more fuel is required to maintain the
temperature of the charge and as consequence the temperature of the convection zone increases.
Monitoring the increment of this temperature is possible to determine the frequency of soot blowing.
However once a day soot blowing is a normal practice.

Q. 128 WHAT IS THE MEDIUM USED FOR SOOT BLOWING ?

Medium pressure steam (15 to 18 kg/cm2) is generally used to blow off the soot from the tubes.

Q. 129 HOW ARE SOOT BLOWERS CLASSIFIED ?

Soot blowers are classified as follows:
a) Stationary rotary type which is mainly used in boilers.
b) Long retractable type which is used in the superheater section of boilers and in the convection
section of heaters.

Q. 130 WHAT IS STATIONARY ROTARY TYPE SOOT BLOWER ?

This soot blower is composed of a tube or lance that passes through the walls of a boiler and reaches
across the tubes. Along its length the lance has a number of holes through which steam is blown onto
tubes and soot is removed. An electric motor is provided to rotate the lance so as to reach as much
tubes area as possible. At the end of soot blowing cooling air is injected into the lance to prevent
overheating.

Q. 131 WHAT IS LONG RETRACTABLE SOOT BLOWER ?

This type of soot blower differs from the stationary type as the lance, after performing soot blowing, is
retracted out completely (just the tip remains inside). This is because the heat of the flue gas in this area
is the hottest. The other feature of this soot blower is that it does not have holes along the length of the
lance but just at the tip. The electric motor connected to it has the double function of advancing the
retracting the lance as well as rotating it.

Q. 132 HOW IS THE SOOT BLOWING SYSTEM BEING OPERATED ?

The system is fully automatic and can be operated from the control room. However operators must be
familiar with the system so as to operate it manually in the event of instrument failure.

Q. 133 WHAT IS MEANT BY HEATER PROTECTION ?

Heat is necessary to perform all the various refinery processes. Heat is produced and transferred to
charge in heaters. Heaters must be adequately protected against any possible dangerous
occurrence. Heaters are made out of materials that have been designed to support a calculated
stress. When the designed limits are not respected a dangerous occurrence is likely to occur. There are
three factors that determine a possible dangerous occurrence. They are:

a) Equipment failure under normal conditions

b) Equipment failure under abnormal conditions
c) Human error

With the evolution of the modern technology it has been possible to protect the heaters more
efficiently as far as the cases a) and b) are concerned. Against human error there is no protection, but
continuous education and full consciousness of what is being done in respect of safety.

Q. 134 WHAT IS MEANT BY EQUIPMENT FAILURE UNDER NORMAL CONDITION ?

Equipment in a refinery is composed of many pieces each one of them designed and constructed to
perform a certain and specific job. For example a centrifugal pump is equipped with a strainer that
removes solid particles from the stream to avoid erosion of the pump impeller and to avoid plugging
 of the items downstream the pump. When the strainer is full the pump itself starts cavitating and
constant flow of liquid is not ensured anymore. This situation becomes a hazard when this flow of liquid
constitutes the charge of a heater. In short a piece of equipment has failed to perform under normal
conditions.

Q. 135 WHAT IS MEANT BY EQUIPMENT FAILURE UNDER ABNORMAL CONDITIONS ?

Equipment efficiency declines with the time. In fact overhauling is foreseen after a given running time
to restore the initial efficiency. In fact throughput is linked among other factors by the mechanical state
of equipment. Equipment is normally divided into three classes:

a) Primary equipment or those items which play a vital role in refinery operations.
b) Secondary equipment or those items which play an important role in refinery operation.
c) Normal equipment or those items which play a marginal role in refinery operations. For example an
heater feeding pump is considered a piece of primary equipment, while a reflux pump pertains to
be second group. Now abnormal conditions can be considered the following:
i. Operating a unit above equipment limits.
ii. Failure of items (with spares available, pertaining to the primary and secondary equipment)
Any further failure is the Equipment failure (Shut down) under abnormal conditions.

Q. 136 WHAT IS A PREFLASH?

Preflash is a small column which is used to separate light ends from crude oil. By light ends are intended
those hydrocarbons boiling up to 80 to 90 deg C.

Q. 137 WHERE IS PREFLASH NORMALLY INSTALLED ?

to operate a preflash, a temperature between 120 to 180 deg C is needed depending on the design
conditions. It is normally installed after the desalting process.

Q. 138 WHAT IS THE PURPOSE OF PREFLASHING CRUDE OIL ?

There are two reasons to preflash crude oil
a) Light ends removed in the preflash column and which are conveyed to the main fractionator
reduce the thermal load of critical exchangers and of the heater.
b) Light ends removed in the preflash and not conveyed to the main fractionator but to the stabilizer
section reduce the operating pressure of the main fractionator.

Q. 139 WHAT IS TEMPERED WATER SYSTEM ?

Tempered water system is a close circuit in which water maintained at a controlled temperature of
about 60 deg C is being circulated through exchangers. It is a cooling system used for a particular
purpose.

Q. 140 WHAT IS THE PURPOSE OF TEMPERED WATER SYTEM?

In the refinery there are three cooling systems: 1) Cooling water system 2) Air coolers & 3) Tempered
water system. Cooling water system and air coolers are used for the same purpose and for the same
products in well distributed combination. Tempered water system is used to cool, but avoiding
excessive cooling, heavy oils coming from the main fractionator and from the vacuum column. These
heavy oils contain paraffines and waxes which solidify at ambient temperature. If the cooling is
excessive they tend to stick flowing and reducing heat exchange efficiency.

Q. 141 WHAT IS A VACUUM DISTILLATION ?

Separation of crude oil into fractions is achieved in the atmospheric fractionator by using the fractional
condensation technique. The limit of this technique is that hydrocarbons are subjected to cracking
when heated above their mechanical resistance. Light hydrocarbons have higher resistance to
cracking then they heavy ones at high temperature. To achieve separation of heavy fractions the only
way possible is to artificially reduce their boiling points so that high temperatures are avoided. The
technique consists in creating vacuum conditions by means of special ejectors. Therefore vacuum
distillation unit is a tower where vacuum is being created and fractioning of heavy oils achieved
avoiding unwanted cracking.

Q. 142 WHAT IS CRACKING ?

Cracking is the thermal decomposition of a complex compound such as petroleum achieved at high
temperature and with or without the help of catalyst. It alters the structure of hydrocarbons in a way
that if saturation of cracked molecules is not possible the result is formation of coke with consequent
plugging of equipment.

Q. 143 AT WHICH TEMPERATURE DOES CRACKING OCCUR ?

The temperature / pressure equilibrium is the main factor which determines the cracking phenomena.
Transfer temperatures of 350 to 360 deg C in atmospheric distillation units involve some cracking, which
is negligible. Increasing these temperatures, cracking increases considerably and is not acceptable.
Transfer temperatures around 400 deg C in vacuum distillation units involve some cracking but this is
tolerable.

Q. 144 WHAT IS THE PURPOSE OF INJECTING STEAM IN VACUUM UNIT ?

The cracking phenomena is enhanced by the length of time at which hydrocarbons are kept at high
temperature, so the shorter this time the better to reduce unwanted cracking. To reduce this time it is
necessary to increase the velocity of the hydrocarbons flow by means of stem injection. Steam is
normally injected in the hottest parts of equipment such as the last coils of the heater or in the transfer
line. Steam is also injected at the bottom of the vacuum column as stripping steam and in the stripping
section if such equipment is provided.
Q. 145 WHAT IS THE PURPOSE OF VACUUM DISTILLATION UNIT?
The most widely used applications of vacuum units is to produce the following:
a) Bitumens
b) Feedstock for cracking plants
c) Special fractions for the production of lubricants.

Q. 146 WHAT IS THE FEED GENERALLY USED FOR VACUUM UNITS?

The feedstock for vacuum units is generally residue from atmospheric distillation units, which contains
mixtures of all kind of complex hydrocarbons from paraffines to aromatics with Boiling Points ranging
from 350 deg C to above 600 deg C. Certain crude oils are so heavy that themselves constitute the
feed to vacuum units. In this case the light fractions recovered from the top are conveyed to other
units for further fractioning.

Q. 147 WHAT IS THE BASIC PRINCIPLE OF THE VCUUM COLUMN ?

The basic principle of the vacuum column is similar to that of atmospheric distillation column, but with
the following differences:

a) The number of trays is quite limited.

b) The gas oil draw off trays are crossed by riser pipes so as to allow rising vapours not to touch the
liquid.
c) Demister pads are installed in order to stop very heavy fractions from rising and which constitute a
source of carbon residue and metals which is not desirable in the cracking unit feedstock.

Like in the atmospheric column refluxes are used with the same purpose, thus making heat balance
by subtracting it.

Q. 148 HOW IS VACUUM ACHIEVED IN VACUUM COLUMN ?

The vacuum is obtained by extracting and condensing the hydrocarbon vapours and the steam with
the help of steam ejectors. The overhead product is conveyed into a condenser or water cooler.
Liquids (hydrocarbons and water) obtained from all condensable products fall into a far below placed
separator. The condensation itself causes volume shrinking, thus creating partial vacuum. To achieve
maximum vacuum the incondensable gases tapped from the condenser are sucked by the ejectors.
Ejectors are in stages and to achieve maximum efficiency in vacuum pulling from two to three stages
are required.

Q. 149 WHAT ARE THE FUNCTIONS OF THE VACUUM OVERHEAD SEPARATOR ?

The separator receives the liquid hydrocarbons and water from the condenser where they are
separated by difference of gravity and transferred to slop (hydrocarbons) and to sour water stripper
(water). But the separator has not only this function it also ensures a proper seal to avoid vacuum
breaking in the vacuum tower. The separator is placed at least 13 metres below the condenser. The
outlet of the condenser is connected with a pipe that dips into the liquid in the separator. When the
ejectors are operated this pipe contains a column of liquid which is in equilibrium with the vacuum
pulled and ensures the sealing of the vacuum system. Remember that a 10 metres column of water
corresponds to one atmosphere or 1 kg / cm2 pressure.
Q. 150 WHAT IS VACUUM GAS OIL ?
Vacuum Gas oil is a distillate with a boiling range between 370 deg C and 550 deg C which is extracted
from both VDI and VD2and is suitable as feedstock for FCC units. In fact its characteristics corresponds
to the specifications demanded for FCC feedstock and which are mainly based on the Conradson
Carbon Residue value (not more than 0.25% wt) and metals value (at the lowest possible).

Q.151 Vacuum residue is what remains after the extraction of the heavy distillates. Is a very
heavy fraction with the following characteristics :

a) High specific Gravity (around 1 or 10.5 API).

b) Boiling range from 550 C upwards.
c) Sulphur content of about 0.6% wt.
d) Conradson Carbon Residue of about 15% wt.
e) Very high viscosity (1,100 cst at 98.8 C).
f) Metals content of about 70 ppm.

Vacuum residue is suitable for bitumen purpose as it only needs some oxidation through an Asphalt
Blowing Unit, otherwise it is blended with lighter products in order to reduce its viscosity and then used
as Fuel to be fired in furnaces, boilers etc.

Q.152 WHAT IS CONRADSON CARBON RESIDUE ?

The weight percent of carbonaccous residue remaining after a standard laboratory destructive
distillation (burning) or the sample constitutes the Conradson Carbon Residue. This value is taken as the
non catalytic coke forming tendency of products such as Topping and vacuum Heavy Gasoils. A value
of about 0.25 or lower usually indicates a product that has a low coke forming tendency.

Q. 153 WHAT IS VISCOSITY ?

The measure of the ability of a fluid to flow is called viscosity and it is expressed in seconds. To give an
example at a temperature of 37.8 deg C (100 F) a given volume of vacuum residue employs more
than half million seconds to flow through a calibrated orifice compared to one second employed by
a same amount of water through the same orifice. Increasing the temperature of the vacuum residue
up to 100 deg C (230 F) its flowing time is reduced to about 1,000 seconds.

Q. 154 WHAT IS THE IMPORTANCE OF VISCOSITY IN REFINERY OPERATIONS ?

As viscosity reduces with temperature increase, it is necessary to know the viscosity of i.e. a fuel oil, to
determine the best operating temperature at which it should be maintained in order to achieve the
best flowing and combustion conditions. However heavy oils such as vacuum residue need some
blending in order to reduce their excessively high viscosity so as to avoid too high operating
temperature. In lubricants, viscosity determines instead their suitability for any particular application.

Q. 155 WHAT IS SOUR WATER ?

Sour water is water that contains impurities such as hydrocarbons, ammonia, hydrogen sulfide, phenols,
cyanides etc. which give it a characteristics tangy smell. Twelve units in the Refinery discharge sour
water which is conveyed to S.W.S. for a preliminary treatment.
 Q.156 WHAT IS AMMONIA ?
Ammonia or NH3 is a colourless alkaline gas with characteristic pungent odour and suffocating effects.
It causes lacrimation and irritates the skin. Is highly soluble in water, at 0 one litre of water absorbs 1.150
litres of NH3, the maximum concentration in water is however 35%. Ammonia boils at –33.4 deg C and
freezes at –77.7 deg C. Its density is 0.59 (air =1.0). Copper and copper / alloys are greatly attack by
NH3.

Q.157 WHAT IS HYDROGEN SULFIDE ?

H2S or hydrogen sulfide is the only hydrogenated compound of the sulphur, is colourless, has the odour
of rotten eggs and, when inhaled, is very poisonous. It is fairly soluble in a feebly acidic solution. It boils
at –60.3 deg C and freezes at –85.6 deg C. Its density is 1.1857 (air =1.0). Concentrations of few ppm of
hydrogen sulfide in atmosphere (up to about 10 ppm) are sensed, higher concentrations. However
inhibits the sense of odour making this gas extremely dangerous. Crude oil contains little amount of H2S
as this gas is mainly obtained from the processes that eliminate sulphur through hydrogenation.

Q.158 WHAT IS PHENOL ?

Phenol also called “Carbolic Acid” is a caustic, poison, white crystalline compound derived from
benzene and used in various resins, plastics, disinfectants and pharmaceutical compounds having at
least one hydroxyl group attached directly to the benzene ring. It melts at 42 deg C, boils at 182 deg
C and has a density of 1.006 (water =1.0). It is soluble in alchoal, very little in water and has feebly
acidic property. In Refinery phenols are originated from FCC process.

Q.159 WHAT IS CYANIDE ?

Cyanide is any of various salts or esters of hydrogen cyanide containing a CN group; especially, the
extremely poisonus compounds potassium cyanide and sodium cyanide. Cyanides are found in small
quantities in the sour water from FCC process.

Q.160 WHAT IS THE PURPOSE OF SOUR WATER STRIPPER UNIT ?

The purpose of this unit is to remove hydrocarbons, ammonia and hydrogen sulfide from the sour water
discharges by various units of the Refinery, by stripping them off with the reboiler type sour water
stripper. The treated water can be reused in the process plant (desalting process) or sent for further
treatment before being disposed of (waste water treatment).

Q.161 WHAT ARE THE PROCESS PRINCIPLES OF S.W.S. ?

H2S and NH3 are present in sour water in the form of dissolved ammonia hydrosulphide (NH 4 SH). In this
aqueous solution this salt, with the help of heat, reforms H 2S and NH3 which are then stripped off in the
vapour phase. Temperature and low pressure favour the release of these gases. On the other hand
low temperature and high pressure favour the formation of NH4SH with possible plugging of equipment.

Hydrocarbons are removed in two steps :

i. The waters from desalting process contain some entrained oil and sludge which are mostly removed
in the desalter slop oil separator.
ii. The residual oil entrained from the slop oil separator as well as the other traceable oil contained in
the other sour water streams are stripped off in the sour water stripper together with NH3 and H2S.

An oil skimming system and an oil separator are provided in the S.W.S. in order to remove hydrocarbons
which cannot be stripped off in the stripper so as to reduce to the minimum the possibility of foaming
 trouble caused by these oils. The oil skimming operation is performed occasionally depending upon
the accumulation of oil in the strippers and in the feeding drum.

Q. 162 WHAT IS ACID GAS ?

The acid gas from the S.W.S. is mostly composed of water, hydrogen sulphide and ammonia. It is called
acid gas because the acidic action of the hydrogen sulphide is predominant over the alkaline action
of the ammonia.

Q. 163 WHAT ARE THE MAJOR OPERATING CONDITIONS OF S.W.S. ?

The most important operating conditions are the pressure of the acid gas K.O. drum and the
temperature of the vapours going into it. Other operating conditions are the stripping temperature and
the stripping steam flow rate.

Q. 164 WHAT ARE THE EFFECTS OF PRESSURE IN S.W.S ?

Low pressure favours the removal of acid gas from the sour water so the operating pressure is kept at
the minimum required to send the acid gas to CDU 1 and VDU 1 heaters. A value of 0.7 kg/cm 2
measured in the acid gas K.O. drum is the minimum best operating condition.

Q. 165 WHAT ARE THE EFFECTS OF TEMPERATURE IN S.W.S. ?

Stripping of the acid gas from the sour water is better achieved at relatively high temperature.
However, since it affects the operating pressure is a variable not independent. The acid gas
temperature in the KO drum should be maintained at 85 deg C for the following reasons :

i.Lower temperature may lead to annonium hydrosulphide formation with consequent plugging or
faouling.
ii.Higher temperature increases the entrained steam flow rate which results in an increase of the cooling
water heat duty.

Q.166 WHAT ARE THE EFFECT OF THE STRIPPING STEAM RATE ON S.W.S. ?
Stripping steam is injected in the reboiler at a rate of 15 + 20% of the feed. In general increasing steam
flow rate will increase the removal of acid gas from the sour water, however higher rates do not
substantially contribute to the effective removal of acid gas.

Q. 167 WHAT IS THE SPENT CAUSTIC SODA TREATING UNIT ?

Is a unit where spent caustic soda, discharged from the FCC Gasolline Merox Unit, is being neutralized
by Sulfuric acid (H2SO4).

Q. 168 WHAT IS SPENT CAUSTIC SODA ?

Spent Caustic Soda is derived from Caustic Soda or Sodium hydroxide (NaOH) which is a white,
translucent, sealed and strongly alkaline compound very soluble in water with release of heat and
forming a strongly alkaline solution. Caustic solution is utilized in FCC Gasoline Merox Unit for the
removal of Sulphur compounds, contained in FCC Gasoline, by reacting with them and forming the
corresponding salts. After the reaction the original strength of the caustic solution is reduced
considerably. In other words it becomes Spent Caustic Soda. However its pH is still quite high as it ranges
between 9-12.

Q. 169 WHY IS IT NECESSARY TO NEUTRALISE SPENT CAUSTIC SODA ?

All the sour water streams, after being processed in the S.W.S., are discharges and conveyed to the
Waste Water Treatment Section for further treatment before disposal. It is a necessary step so as to
avoid contamination of the natural environment of the Refinery. This treatment involves a biological
process in which special bacteria are used to degrade the contaminants of the water. These bacteria
are very sensitive to high pH, therefore the feeding water has a pH which should not exceed 9. As spent
caustic soda has a pH well above this value, a neutralization treatment is necessary in order to reduce
its pH below the 9 value and consequently to avoid undesirable influence on the biological treatment.

Q. 170 HOW IS NEUTRALISATION OF SPENT CAUSTIC SODA ACHIEVED ?

Sulphuric acid (H2SO4) is injected to reduce the pH of spent soda. A pH meter is installed in the unit to
indicate the value of the neutralization effect of the sulphuric acid. This operation is carried out in
batches of 10 m3 each and is not frequent as the amount of spent caustic produced by FCC Merox
Gasoline is 2 tons a day by design. In other units the neutralization is achieved by allowing some flue
gas extracted, by means of ejectors, from the Topping Heaters Stack. To mix with spent caustic soda.
In this case Flue Gas contains Sulphur dioxide (SO 2) and Sulphur trioxyde (SO3) which in presence of
water forms sulphuric acid.

Q.171 WHAT IS THE PURPOSE OF THE FLARE SYSTEM ?

The Flare system has been designed and constructed to dispose of the released gases from the various
process units. To avoid contamination of the atmosphere, these gases are burnt off at the tips of the
flare.

Q. 172 WHAT ARE THE WORKING OF THE FLARE SYSTEM ?

The working conditions of the flare system are based on 100% throughout in all of the Refinery.

They are as follows :

- Surplus
- When SRU is down 3.4 t/h
- Cooling water failure about 200 t/h
- Total power failure about 580 t/h

Q.173 WHAT IS MEANT BY SURPLUS FUEL IN THE FLARE SYSTEM ?

Pressure control in all Refinery processes is a primary condition for the stability of the processes
themselves and, as consequence, under normal operating conditions, some excess gas is discharged,
through the pressure control system into the flare system. This gas has been calculated to be about 2.3
tons per hour or about 0.4% of the total through out.

Q. 174 WHAT IS THE IMPACT OF THE ACID GAS ON THE FLARE SYSTEM WHEN SRU IS DOWN ?
Acid gas is produced by FCC unit and GI unit. It contains mainly hydrogen sulphide (H2S) and is sent to
SRU for sulphur recovery. But when this unit is down the acid gas is disposed of at the flare system. In
therms of flow its impact is minimal, just about 1.1 tons/h, but in therms of corrosion would be quite
substantial. Flare headers up to the Flare Seal Drums are separated for acid gas and sweet gas. The
acid gas line is steam traced to prevent severe corrosion due to wet concentrated H 2S gas.

Q. 175 WHAT ARE THE EFFECTS OF COOLING WATER FAILURE IN THE FLARE SYSTEM ?
The function of the cooling water system is to remove all the heat produced or developed in the various
processes so that the products can be safely handled and stores. A failure of the cooling water system
means that this heat is not removed and the consequence is a tremendous pressure build up in all
processes and an enormous quantity of hydrocarbons is released to the flare system. This quantity has
been calculated to be about 200 tons per hour or over one –third of the refinery design capacity.
Q. 176 WHAT ARE THE SEAL DRUMS IN THE FLARE SYSTEM ?
Before being released to the stack, the gas, from the knockout drums, is passed through a water barrier.
This water barrier is created in two Seal Drums. Because of this barrier, air of atmosphere is prevented
from entering the system through the stacks, thus making flashback impossible. The water barrier is 150
mm in one drum and 250 mm in the second drum. Under normal conditions released gas will pass
through the lower water barrier drum only. Under abnormal conditions (high gas discharge) the gas
will be able to pass through the higher water barrier drum also.

Q. 177 WHAT ARE THE FLARE STACKS IN THE FLARE SYSTEM ?

Flare stacks are two elevated ducts or risers. One stack is 28” size and normally functions under normal
operating conditions, while the second stack is 54” size and functions during emergencies. The gas
released from the Seal Drums is discharges and burnt at the tip of the flares. An anti flash back device
is installed near the flare tip. In order to maintain the flame continuously on, as the released gas from
the seal drums is often pulsating, some gas pilots are also installed at the flare tip. A special device
through which low pressure steam is injected permits to maintain a smokeless flame.

Q. 178 WHAT ARE THE FLARE HEADERS IN THE FLARE SYSTEM ?

In the Refinery there are two types of gas which can be flared, thus sweet gas and acid gas. Sweet
gas is gathered into two main headers, thus one 32” from Lubes Section and one 40” from Fuels
Section. This solution makes the two sections independent from each other. Acid gas is produced in
the Fuel Section and is gathered into a 16” header separated from the others in order to reduce at the
minimum corrosion problems.

Q. 179 WHAT ARE THE SLOW DOWN KNOCKOUT DRUMS IN THE FLARE SYSTEM ?
The gases released from the various processes entrain liquids and / or condensable vapours. The three
flare headers are connected to three special drums which have the function of separating (knocking-
out) the entrained liquids from the gas. These liquids are recovered by means of reciprocating pumps
thus saving some hydrocarbons from being wasted by flaring.

Q. 180 WHAT ARE THE EFFECTS OF TOTAL POWER FAILURE IN THE FLARE SYSTEM ?
This can be considered the worse operating conditions for the flare system. In case of total power
failure it is possible that for a while the whole design capacity of the Refinery is discharged to the
atmosphere through the flare system. In short, about ten thousands kg of hydrocarbons are flared every
minute. However, considering statistics, this possibility is very remote.

Q. 181 WHAT IS THE FLARE SYSTEM OUTLINE ?

Obviously the flare system outline reflects the Refinery design and as there are two main independent
sections it consists of

1. Three Flare Headers.

2. Three Blow Down Knockout Drums.
3. Two Flare Seal Drums.
4. Two Flare Stacks.
5. Auxiliary Service and Special Equipment.

Q. 182 WHAT IS THE “STERILISED AREA” IN THE FLARE SYSTEM ?

Flare stacks are placed in a safe position which is far from the refinery process units. The areas
surrounding them with an average radios of 110 m with respect to the center of the emergency flare
is called Sterile Area. The flare stacks are elevated 65 m above ground level and this has been designed
to maintain the radiant heat developed by the flaming gas under the worse conditions within
acceptable limits at the boundary of the Sterile Area.

Q. 183 WHAT IS AUXILIARY SERVICE AND SPECIAL EQUIPMENT IN THE FLARE SYSTEM ?
Auxiliary Service consists of :

a. Water supply for the water barrier in the seal drums.

b. Steam supply for smokeless flame purpose and for purging.
c. Fuel gas supply for gas pilots.
d. Nitrogen supply for purging the flare system when commissioning.
Special Equipment consists of an ignitor which is used to light the gas pilots at the tip of the flares.

Q. 184 WHAT IS HEAT EXCHANGER ?

Heat Exchanger is a piece of equipment used for transferring between two fluids having different
temperature, by bringing them into indirect contact using tubular walls.

Q. 185 WHAT IS THE PURPOSE OF USING HEAT EXCHANGERS ?

Head is used in all processes of a Refinery. At the completion of these processes the various products
obtained from them need to be cooled to a convenient safe temperature for handling purposes. Most
of the heat contained in these products is recovered by transmitting it to the fresh charge in the heat
exchangers. The effect in this case is that heat is not wasted and therefore a considerable amount of
fuel is saved. In addition the size of heaters is greatly reduced and money is saved from capital
investments. However, capital investment paid for the construction of heat exchangers is rapidly repaid
by the fuel saving which is in the range of 60 +70%.

Q. 186 WHAT IS HEAT EXCHAGER SURFACE AREA ?

The surface area of the tabular walls through which heat is exchanged between two fluids is called
Heat Exchanger Surface Area. Within limits determined by the nature and characteristics of the fluids
used to exchange heat, the surface area is designed to be the highest possible.

Q. 187 HOW ARE HEAT EXCHAGERS CLASSIFIED ?

There are different types of heat exchangers and depending on their construction design, can be
classified as follows :

1. Shell-and Tube.
2. Double pipe.
3. Submerged pipe coil.
4. Air cooler.
5. Others.

All the above mentioned types, depending on their use, can be names as follows :
For heating = a) Evaporator b) Preheater c) Heater
d) Superheater e) Reboiler.
For cooling= a) Condenser b) cooler C) Chiller.

Q. 188 WHAT IS SHELL AND TUBE HEATER EXCHAGER ?

The shell and tube heat exchanger is one of the most common types of heat exchangers used in
refinery. It consists of a bundle of small diameter tubes into which the fluid to be heated is passed and
a shell to contain the fluid to be cooled which passes externally the tubes bundle. Basically there are
three types of shell and tube heat exchangers, thus :

1. Fixed-tube-plate.
2. Floating Head.
3. U-tube.

Q. 189 WHAT IS FIXED-TUBE-PLATE HEAT EXCHAGER ?

Fixed-tube-plate heat exchanger consists of a bundle of tubes arranged in a cylindrical shell. The ends
of the tubes are sealed into tube place or headers whose section correspond to the internal section
of the shell and which prevents the fluid from leaking. Covers are provided at the ends of the shell. This
type of heat exchanger does not allow for metal heat expansion and therefore it is mainly used where
low differential temperatures are involved such as water coolers, small condensers, lube oil coolers etc.

Q. 190 WHAT IS FLOATING HEAD HEAT EXCHAGER ?

This type of heat exchanger differs from the fixed-tube plate heat exchanger as its tubes bundle is fixed
into a header on one end only, leaving the other end to float or expand according to temperature
changes, inside of the shell. Also in this type of exchanger covers are provided at both ends of the
shell. Among the Shell and Tube heat exchangers it is the most widely used in Refinery operations.

Q. 191 WHAT IS U-TUBE HEAT EXCHANGER ?

This type of exchanger like the floating head type allows for heat expansion. Its main feature is that the
tubes bundle is folded like a U shape so that its ends are sealed into a unique tubes plate which is
provided with a special baffle that separates the fluid entering the exchanger from the out-going fluid.
One cover only is necessary for the shell. This type of exchanger is mainly used as reboiler, heater or
superheater for steam generation.

Q. 192 WHAT IS A DOUBLE PIPE HEAT EXCHANGER ?

Double pipe Heat Exchanger consists of two concentrical tubes in which the hot fluid flowing through
the inner tube releases heat to the cold fluid flowing in between the two tubes. This type of heat
exchanger has comperatively the smallest surface area and therefore has little application in refinery.
It is mainly used in Chemical Laboratories or for particular reasons.

Q. 193 WHAT IS A SUBMERGED PIPE COIL HEAT EXCHAGER ?

This heat exchanger consists of a coil designed in a serpentine form and placed at the bottom of tanks
which contain particular or heavy and very viscous liquids that need to be maintained at a sufficiently
high temperature to allow regular flowing. The heating fluid passed through the coil is either steam or
Hot Oil (heavy gasoil). This type of heating coils is also widely used in vessels where the liquid collecting
at the bottom must be vaporized like i.e. a Gas KO drum.

Q. 194 WHAT IS AN AIR COOLER ?

The air cooler is an exchanger where forced air constitutes the cooling medium which passes externally
through a bank of finned tubes in is being cooled. Air coolers are widely used in refinery
operations to cool products whose heat content cannot be economically recovered with heat
exchangers, but must be removed for handling reasons. Air coolers are normally employed where
water availability is a problem as a cooling medium.

Q. 195 WHAT IS A TRIM COOLER ?

Trim cooler is a fixed-tube-plate exchanger where water is used as temperature stabilizer medium. Trim
coolers are normally placed downstream air coolers so that the product passed through them reaches
a stable temperature. The reason is that air coolers are affected by weather conditions changes, while
trim coolers are not.

Q. 196 WHAT IS A CONDENSER ?

Condenser like trim cooler is a fixed-tube-plate exchanger. Its function is to condense condensable
vapours by removing their latent heat. This type of exchanger is mainly used for top columns vapours
which contain light hydrocarbons and in steam turbines for exhaust steam condensing.

Q. 197 WHAT IS LATENT HEAT ?

Latent heat is the heat required to vaporize a boiling liquid. During vaporization the temperature of the
boiling liquid is constant and since it is not possible to measure it with a thermometer it is called latent
heat.

Q. 198 WHAT IS SENSIBLE HEAT ?

Sensible heat is the heat required to bring the temperature of a liquid up to its boiling point. This heat is
called sensible because it is possible to “sense” or measure with a thermometer.

Q. 199 WHAT IS MEANT BY PASSAGES IN A HEAT EXCHANGER ?

Passages are the U turns the heat receiving fluid is obliged to make after entering an exchanger and
before leaving it. Bundles of exchangers can be split into two or more groups or sub bundles and the
heat receiving fluid is obliged to pass in each one of them. These groups are called passes or passages.
The number of passages in a heat exchanger is designed to have the maximum efficiency for heat
exchange, taking also into account the drop of pressure caused by them.

Q. 200 WHY IS IN A HEAT EXCHANGER, THE HEAT RECEIVING FLUID NORMALLY PASSED THROUGH
THE BUNDLE ?
Charges are normally passed through the tube bundle and the reason is linked with the “Dirty Factor”
of the heat exchanging fluids. Charges are normally dirtier than the heat releasing fluids and therefore
they are “forced” to pass through the tubes bundle. In this way the impurities, sediments or dirt have
no time to settle and plug the tubes and the dead end areas are minimized. In the shell instead the
possibility of dirt settling is much higher. In a very few exchangers of the refinery the charge is passed
through the shell because the heat releasing fluid is dirtier than the charge itself.
Q. 201 WHAT IS CO-CURRENT FLOW IN A HEAT EXCHANGER ?
Flow is said to be co-current in a heat exchanger when both fluids enter it at the same end, thereafter
flowing parallel to each other. The differential temperature between the hot and the cold fluids is, in
this type of exchanger, at the maximum at the inlet and at the minimum at the outlet and the result is
that the maximum temperature attained by the cold fluid is always lower than the exit temperature
of the hot fluid.

Q. 202 WHAT IS COUNTER-CURRENT FLOW HEAT EXCHANGER ?

Flow is said to be counter-current in a heat exchanger when fluids are arranged to enter it at opposite
ends, thereafter crossing each other. The differential temperature between the hot and the cold fluids
is, in this type of exchanger, almost constant and the result is that the maximum temperature attained
by the cold fluid may be higher than the exit temperature of the hot fluid. Obviously counter-current
flow heat exchangers are much the more efficient if compared to co-current flow heat exchangers.

Q. 203 HOW IS IT POSSIBLE TO DETERMINE AN INTERNAL LEAKAGE IN A HEAT EXCHANGER ?

When the heat exchanger is internally leaking it means that some of the fluid flowing at higher pressure
is passing through a leak and mixes with the fluid flowing at lower pressure. The amount of fluid passing
through this leak depends on the size of the leak itself and on the pressure differential between the two
fluids. The effect may be undesirable contamination of the fluid at lower pressure and to determine its
extent a sample of the contaminated fluid should be taken for checking. If a visual checking results
negative the sample should be taken to the Chemical Laboratory for analysis, the nature of which
depends on the nature and characteristics of the sample. If the contamination is so heavy than can
adversely affect the process operation the heat exchanger should be isolated and in case this is not
possible and process shut down will be the consequence. In some cases it is possible to minimize the
leakage by reducing at the minimum the pressure differential between the two fluids so that
contamination is reduced to acceptable limits. It is obvious that as far as safety is concerned the above
operation can be performed only if the safety aspect of it is fully compiled with.

Q. 204 WHAT IS PUMP ?

Conveying fluids is greatly simplified by their ability to change their shape, which enables them to be
pumped. However a pump is a device which exploiting this ability adds energy to the fluid so as to
make its transfer possible. Probably there are more devices like pumps to transfer fluids than to do any
other task. The reason is because the factors that imply the use of a pump are so numerous as to make
this true.

Q. 205 HOW ARE PUMPS CLASSIFIED ?

Pumps are classified according to their duty. A few consideration can give the idea of what type of
pump is necessary to use, thus the nature of the liquid to be pumped (viscosity, temperature,
corrosiveness etc. ) and the pumping duty (flow rate, delivery head etc.). To add energy to a fluid in a
pump there are basically two ways, therefore pumps are classified as follows :
A – Persuasive (centrifugal pumps)
B - Compelling ( positive displacement pumps).
Q. 206 WHAT ARE PERSUASIVE OR CENTRIFUGAL PUMPS ?
Centrifugal pumps are devices where a rotating impeller, in a stationery casing, creates a low pressure
zone in the center, called eye, where liquid is drawn in. The Kinetic energy imparted by the impeller is
converted to pressure energy in the casing and the liquid is induced or persuaded to leave. If the liquid
is prevented from leaving the casing by closure of the discharge valve no more low pressure in the eye
of the impeller is created and then no further liquid is drawn in, but the pump will not be damaged.
That is why centrifugal pumps are essentially persuasive rather than compelling in their action.

Q. 207 WHAT ARE COMPELLING PUMPS ?

Compelling pumps are basically reciprocating pumps and rotary pumps. These are positive-acting,
i.e., they are compelling rather than persuading in their action. Fluid once drawn in must be provided
with an outlet, or high discharge pressure will occur, sufficient to burst the pump or the discharge leave.
Pumps of this type are all fitted with relief valves.

Q. 208 WHAT IS THE IMPELLER IN A CENTRIFUGAL PUMP ?

Impeller is the part of the centrifugal pump that actually rotates. It is composed of a series of common
shaped vanes which depart from a center called suction eye. When the liquid enters the impeller
through the eye the vanes cause it to rotate and transmit to it the centrifugal force. The impeller is
mounted on a shaft which transmits the motion received from a driver. The impeller is said to be closed
or open depending on whether the vanes have disc plated or shrounds on both sides or not.

Q. 209 WHAT IS THE CASING IN A CENTRIFUGAL PUMP ?

Casing in a centrifugal pump is the outer cover of the impeller. The centrifugal force imparted by the
impeller to the liquid is contained and accumulated by this cover in form of pressure. The casing is
provided with an opening connected with piping through which are liquid with the accumulated
energy is passed.

Q.210 WHAT IS CENTRIFUGAL FORCE AND HOW IS IT TRANSFORMED INTO PRESSURE ENERGY IN A
CENTRIFUGAL PUMP ?
Centrifugal force is the force which acts on a body that moves circularly and tends to move it away
from its center. In a centrifugal pump the impeller is the body that moves in a circular path and its
rotation is the force, called centrifugal force, which tends to move it away from its center. Any liquid
reaching, therefore, the center or eye of the impeller is by this centrifugal force moved away from it.
The liquid has acquired what is called velocity or kinetic energy. By reason of this velocity the liquid
tends to move away in all directions. It is prevented from doing so by the casing which conveyes the
liquid and move it in an established direction called discharge nozzle. As the liquid is obliged by the
casing to follow a fixed direction it builds up some pressure which enables it to move away.

Q. 211 HOW ARE CENTRIFUGAL PUMPS CLASSIFIED ?

In general centrifugal pumps can handle large volume, are compact, fairly inexpensive, can handle
some suspended solids and to a certain extent can handle hot liquids. They can be installed horizontally
or vertically following the position of the shaft to which the impeller is mounted and fixed. But, whatever
is the case, centrifugal pumps are classified by their delivery head as follows :
A – Single – stage for low delivery pressure.
B – Multi – stage for medium and high delivery pressure.

Q. 212 WHAT IS SINGLE – STAGE CENTRIFUGAL PUMP ?

When a centrifugal pump has one impeller only is said to be single stage. These types of pumps are
widely used for small to large capacity rates being the only limitation the discharge head. The added
energy to the liquid measured in form of pressure is generally about 10 kg / cm2.

Q. 213 WHAT IS A MULTI – STAGE CENTRIFUGAL PUMP ?

If the discharge nozzle of a single –stage centrifugal pump is connected to the suction of a second
pump with the same characteristics, then we have obtained a multi –stage pump. However in a multi
– stage pump the impellers are fitted on one shaft and the casing covering them is of a special design
so as to achieve the same result of a series of single – stage pumps. This type of pumps are generally
used for low to medium capacity rates but with a discharge head which can be very high. It is related
to the number of impellers mounted on the shaft.

Q. 214 WHAT IS THE SUCTION PRESSURE IN A CENTRIFUGAL PUMP ?

It has been said that when an impeller rotates, a low pressure zone is created at the center or eye, so
that liquid is drawn in to be then displaced. This low pressure is actually the pressure created by the
rotating impeller and is lower than the pressure exerted on the eye when the impeller is stationery.
Therefore suction pressure is the pressure exerted on the eye of a stationery impeller. It can be
measured in kg /cm2 or expressed in height of column of water measured in metres. The formula is as
follows :
Suction pressure = kg / cm2 ; kg / cm2 X 10 = metres of column of water.

Metres of column of water = metresof coumn of the liquid

Liquid density at pump temperature to be pumped.

Q. 215 WHEN IS THE SUCTION PRESSURE WORKING AGAINST A CENTRIFUGAL PUMP ?

Suction pressure or suction head can be positive, equivalent or negative if referred to atmospheric
pressure. When suction head is negative the rotating impeller creates a vacuum which is higher than
the negative head in order to lift the liquid. It is said, in this case, that the suction pressure works against
the pump, thus the pump lifts the liquid. An example of this type of pumps is the one that transfers
hydrocarbons accumulating in the close drain system.

Q. 216 WHAT IS THE MAXIMUM NEGATIVE HEAD POSSIBLE IN A PUMP ?

The atmosphere surrounding our earth and in which we breath exerts a pressure equivalent to a column
of water measuring 10.33 m which is called Bar (barometric) and which is about the equivalent of 1
kg/ cm2. In short one atmosphere is equal to one Bar and equal to about 1 kg/ cm2. It is cleat that
without the atmosphere our earth would join the conditions of the outer space, thus would be joining
absolute vacuum conditions. As there is nothing lower than absolute vacuum, the maximum vacuum
possible is that corresponding to 10.33m of column of water. In theory if there is a pump to lift water
situated 10.33m below its suction level, that would be the maximum negative head possible for the
pump to lift it. And if the liquid is different from water then its density is to be taken into account to find
the equivalent negative head expressed in metres. In the case of Mercury (Hg) whose density is 13.59
(or 13.59 times heavier than water) the maximum negative head possible is
Negative head + 10.33m = 0.76m = 76 cm.
13.59

But practically it is not possible to reach this limit for two reasons, thus the impeller cannot induce
perfect vacuum and second, there must be a differential pressure between the negative head and
the vapour pressure of the liquid. In short the negative head value must be higher than the vapour
pressure of the liquid to avoid vaporization and hence cavitation. Temperature is another factor that
 determines the lifting capacity of a pump. Under normal conditions the maximum operating lifting
head is about 50% of the maximum theoretical one.

Q. 217 HOW DO TEMPERATURE AND PRESSURE AFFECT THE SUCTION HEAD ?

The main physical property of liquids is their boiling point which is the temperature at which they turn
to vapour with respect to pressure. If a liquid is brought to its boiling point either by reducing pressure
or increasing temperature the ability of the pump to transfer it is annulled because of the formation of
vapours. Therefore whether the suction head of a pump is positive (above atmospheric pressure) or
negative (below atmospheric pressure) it is essential that its value be well above the vapour pressure
of the liquid at the pumping temperature. In other words the following considerations must be kept in
mind while operating a pump :

i. At constant suction pressure an increase of liquid temperature determines an increase of its vapour
pressure, thus favouring vaporization and then cavitation. A reduction of the temperature favours
the performance of the pump ; however the viscosity of the liquid must be put into consideration.
ii. At constant temperature an increase of suction pressure improves the performance of the pump
while a reduction will deteriorate it.

The difference between the suction pressure and the vapour pressure of the liquid to be pumped is
call Net Positive Section Head (NPSH).

Q. 218 WHAT IS CAVITATION AND ITS EFFECTS IN A CENTRIFUGAL PUMP ?

When the suction pressure in a pump lowers to equalize the vapour pressure of the liquid, formation of
vapour bubbles occurs. This happens exactly in the eye of the impeller where the pressure is at its lowest
value. The kinetic energy imparted by the impeller vanes to the bubbles obliges them towards the
outlet of the pump, where the pressure is at its highest value. The impact against the liquid in the higher
pressure area makes the bubbles to collapse (this phenomenon is called implosion) with great force.
As the bubbles continue to form and collapse the efficiency of the pump is annulled. This process is
called cavitation. The effect of the bubbles collapsing is a sort of very noisy kick against mainly the
vanes of the impeller with consequent removal of metallic particles or, in short, pitting. In addition
cavitation, apart from the noise generates vibration which increases the stress on the bearings with
possible damage.

Q. 219 WHAT ARE THE POSSIBLE CAUSES IN A PUMP WHICH DELIVERS LESS LIQUID THAN EXPECTED
The following are some of the possible causes :

1. Air enters the pump during operation.

2. Operating speed too low.
3. Wrong direction of rotation.
4. Faulty measuring instruments.
5. Available NPSH too low.
6. Viscosity of the liquid too high.
7. Partial clogging of the impeller and casing.
8. Partial clogging of suction line, strainer etc.
9. In a system with more than one pump, the operation of one pump may affect the other
pumps operations.

Q. 220 WHAT ARE THE POSSIBLE CAUSES IN A PUMP WHICH DOES NOT DEVELOP ENOUGH PRESSURE
?
The causes are the same as mentioned in question 207.

Q. 221 WHAT ARE THE POSSIBLE CAUSES IN A PUMP WHICH CONSUMES TOO MUCH POWER ?
The following are some of the possible causes :
1. Speed too high.
2. Pumped liquid with higher specific gravity and or viscosity.
3. Misalignment of mechanicals parts.
4. Damaged bearings.
5. Either lack or excessive lubrication of bearings.
6. Bent shaft.
7. Wrong direction of rotation.
8. Faulty instruments.

Q. 222 WHAT ARE THE POSSIBLE CAUSES IN A PUMP WHICH VIBRATES ?

The following are some of the possible causes :

1. Misalignment of mechanical parts.

2. Rotating parts rubbing against stationery parts.
3. Worn out, damaged or improper lubrication of bearings.
4. Wrong direction of rotation.
5. Cavitation.
6. Available NPSH too low.
7. Piping, strainer partially clogged.
8. Air enters pump during operation.
9. Critical speed.
10. Rotating elements not balanced.
11. Bent shaft.
12. Pump operating at very low flow rate.
13. Various types of resonance.
14. Loose bolts.

Q. 223 WHAT ARE THE POSSIBLE CAUSE IN A PUMP WHICH IS NOISY ?

The possible causes are the same as mentioned in question 212.

Q. 224 WHAT ARE THE POSSIBLE CAUSES IN A PUMP WHICH OVERHEATS ?

The following are some of the possible causes :
1. Pump allowed to run dry.
2. Vapour pockets inside pump.
3. Pump operates near shutdown.
4. Internal rubbing.
5. Worn out bearings.
6. Poor lubrication.

Q. 225 WHAT ARE THE POSSIBLE CAUSES IN A PUMP WHERE BEARINGS OVERHEAT ?
The following are some of the possible causes :
1. Damaged impeller.
2. Rotary elements not balanced.
3. Excessive radial or axial loads.
4. Bent shaft.
 5. Misalignment.
6. Pump operates for a prolonged time at a very low flow rate.
7. Improper installation of bearings.
8. Excessive grease in bearings.
9. Excessive suction pressure.
10. Inadequate coling of bearings and or lubricant.

Q. 226 WHAT ARE THE POSSIBLE CAUSES OF FLOW RATE FLUCTUATION IN A PUMP ?
The following are some of the possible causes :
i. Fluctuation of liquid level in suction vessel.
ii. Fluctuation of pressure in suction vessel.
iii. Fluctuation of flow controlling instruments.
iv. Fluctuation of conditions in the down stream receiving equipment.

Q. 227 WHAT IS THE SEAL SYSTEM IN A CENTRIFUGAL PUMP ?

The seal system in a centrifugal pump is a way to prevent liquid from leaking cut of the casing between
the casing and the shaft. There are two types of seals, thus packing which are flexible materials that fit
tightly against the shaft inside a cylindrical housing mounted on the casing and mechanical seal which
is basically composed of two washers, one fixed on the shaft and therefore rotating and the other fixed
in the cylindrical housing mounted on the casing and therefore stationery, which by means of springs
are pushed one against the other so that liquid cannot escape from the casing. However there is a
small amount of liquid let out to lubricate and cool the two washers. Mechanical seals, though more
expensive, replace packing in almost all modern pumps as being more effective.

Q. 228 WHAT IS COOLING SYSTEM IN A CENTRIFUGAL PUMP ?

Cooling in a centrifugal pump is necessary to prevent over heating of parts where friction is involved,
thus bearings, packing mechanical seal and which can be affected by hot pumped liquid. Cooling is
normally achieved by water circulation through jackets surrounding these parts. However with the
modern technology is possible to eliminate cooling in pumps handling liquids up to about 300 C. In
pumps handling very hot liquids cooling of seals is achieved by steam injection as water circulation
would give unwanted thermal shock.

Q. 229 HOW ARE COMPELLING PUMPS CLASSIFIED ?

Compelling positive displacement pumps are designed for small capacity rates but where high
pressure may be required therefore their classification depends on their design rather
than their pumping duty.
They are classified as follows :

i. Reciprocating pumps.
ii. Rotary pumps.

Q. 230 WHAT IS A RECIPROCATING PUMP ?

A reciprocating pump is so called because of the to –and-fro motion of the working parts, a piston in
a cylinder. There are two main types of such pump differing in the method of providing a seal between
the piston and the cylinder, thus :
1. Piston pump. 2. Plunger pump.

Q. 231 WHAT IS A PISTON PUMP ?

Piston pump is a reciprocating pump where the seal between piston and the cylinder is by rings
embedded in the walls of the piston like i.e. the piston of a petrol engine.

Q. 232 WHAT IS A PLUNGER PUMP ?

Plunger pump is a reciprocating pump where the seal between piston or plunger and the cylinder is
by a stationery packing in the cylinder wall past which the moving element, the plunger, slides. An
application of this type of seal is the common hydraulic jack.

Q. 233 WHAT IS THE MAIN CHARACTERISTIC OF RECIPROCATING PUMPS ?

Reciprocating pumps are characterized by a pulsating discharge ; the liquid leaves in squirts and not
as continuous stream. This type of pump cannot handle liquids containing abrasive solids in suspension
owing to its construction.

Q. 234 WHAT IS A ROTARY PUMP ?

Rotary pump is a positive displacement pump where the major feature is rotary elements which ensure
a continuous discharge. This type of pump is tending to displace reciprocating pumps, being cheaper
and more compact. All the rotary pumps utilize the principle of the rotary piston by which even heavy
and viscous liquids are caught between a rotor and a stator and are carried out round to the
discharge.
1. Gear pump 2. Sliding-vane pump 3. Screw pump.

Q. 235 WHAT IS A GEAR PUMP ?

Gear pump is a rotary pump where the rotating elements are two gears meshed or engages each
other. The space between the gear teeth and the casing walls is occupied by the liquid which is in this
way forced out continuously. This type of pump is widely used for lubricating purposes where lubricating
oil must be forces to ensure lubrication.

Q. 236 WHAT IS A SLIDING-VANE PUMP ?

Sliding –vane pump is a rotary pump where the rotating element is a small eccentrically rotating drum
inside a cylindrical casing. The drum has some radial slots where vanes fitted into them slide towards
the walls of the casing for the centrifugal force imparted to them by the rotation of the drum or / and
special springs accommodated at the bottom of the slots . This ensures a proper contact and seal
between the vanes and the casing walls. Due to the eccentrical position of the drum with respect to
the axis of the cylinder the volume of spaces between two consecutive vanes changes from a
minimum to a maximum and from a maximum to a minimum during a whole revolution. This type of
pump is used for gases. An example of it is the air compressor which sends air to control the opening /
closing of the slide valves in the FCC unit.
Q. 237 WHAT IS A SCREW PUMP ?
Screw pump is a rotary pump where the rotating elements are two threaded cylinders, engaged each
other and made to turn by special gear, inside of a special casing. Like in the gear pump the liquid is
forced out through the threaded space lift between the scres and the casing. High pumping duty is
possible with this type of pump. An example of its application is on board of crude oil tankers.

Q. 238 WHAT IS A VALVE ?

A valve is any of the various devices that regulate the flow of fluids or loose materials through structures
such as piping, ducts, canals etc. Valves normally consist of a holed body, with attachments for piping
connection, inside which a mobile element, externally operated, is accommodated and which is used
to close, open or control the passage of fluid through the body.

Q. 239 WHAT ARE THE CRITERIA USED IN SELECTING VALVES ?

The physical characteristics of fluids (whether liquid, gaseous, corrosive, erosive etc.) and the
conditions at which they are transferred are the main factors that determine the design and the
material constituting the valve. The service required by the valve (whether isolating or controlling the
fluid) is another important factor that determines the design and operability of the mobile element.
Another important factor is the pumping duty (flow rate) which determines the size of the passage
through the body of the valve. As the applications are many, many are the types of valves designed
and employed as they have to correspond to precise and specific requirements. The following are
generally the guidelines used to select valves :
1. Types 2. Materials. 3. Class or nominal pressure.
4. Normal size 5. Attachment 6. Others.

Q. 240 WHAT IS MEANT BY TYPES OF VALVES ?

This classification permits to distinguish the various valves used according to the nature of fluid (whether
liquid or gas) and the type of service required (whether regulating or isolating).
1. Gate valves.
2. Globe valves or Disc valves or noedle valves.
3. Plug valves or Cock valves or Ball valves.
4. Check valves or Non return valves.
5. Safety valves or Relief valves.
6. Pheumatic valves.
7. Others.

Q. 241 WHAT IS A GATE VALVE ?

This type of valve is probably the most common valve used in refinery operations. It sizes from 3/8” up
to over 60”. For its particular design, however very simple, thus a lens which slides between two guides
inside the body and which, near the closing point, as it causes turbulent flow with erosion effects, is
used for isolating purposes only. In fact during operations, this valve is kept either fully close or fully
open.
Q. 242 WHAT IS A CLOSE VALVES OR A DISC VALVE ?
This type of valve is generally constituted of a spherical body inside which an externally actuated disc,
stops or regulates the flow of fluids. The disc obliges the fluid to flow in a circular path, thus reducing its
turbulence and hence erosion effects, during passage through the valve. For this reason disc valves
are generally used to control flow of fluids. As an example, all the lines which bypass pneumatic control
valves are fitted with this type of valve.

Q. 243 WHAT IS A PLUG VALVE OR COCK VALVE OR BALL VALVE ?

Valves are normally actuated by rotation of wheel connected to the mobile element through the
steam. The rotation of the wheel for many turns causes the opening or closing of the passage. In thee
case of plug valves there is no need to use a wheel since the opening or closing of the passage is
achieved in a quarter of a turn only. A simple movable handle is therefore used. The mobile element
is either a tapered cock or plug or a spherical ball which fits in the body of the valve. The name of the
valve is after the shape of the mobile element, though the operating system is the same. The mobile
element has a hole which corresponds to the size of the piping (in the ball valves it is exactly the same
) and this reduces flow friction losses at the minimum. A special grease injection system has the function
to reduce friction between the mobile element and the body, to properly seal the valve against
leakages, to provide a “jacking” action, and to protect the seating surfaces from corrosion and
erosion. Foe their characteristics these valve are highly suitable for handling gases and where high
pressures are involved. These valves are not suitable for regulating purposes.

Q. 244 WHAT IS A CHECK VALVE ?

The importance of check valves is mainly seen in pump operations. A check valve is a particular valve
where the mobile element is actuated by the movement of the fluid passing through it. The mobile
element is generally a disc hinged upon inside the body. The passage of fluid causes its opening in one
direction and its closure at any backflow. There are many types of check valves and they are named
after the shape of the mobile element (ball, piston, disc etc.).

Q. 245 WHAT IS A SAFETY VALVE ?

Refinery equipment is designed to perform a certain duty under certain particular conditions of
pressure, temperature, chemical and mechanical wear. Materials constituting equipment are selected
accordingly. Mechanical constrains include pressure limitations and therefore special devices such as
relief valves and safety valves are provided to prevent equipment from being over pressurized. IN
designing equipment two are the pressures considered, thus the operating and the design pressures.
Safety valves are always related to the design pressure because it refers to the maximum resistance
(to pressure) tolerated by equipment. The mobile element is generally a disc with a spring or weight
applied to it so that the orifice in the body is properly sealed. The spring or weight is calculated so that
the disc lifts when the designed pressure is reached and the fluid is discharges either into atmosphere
like, i.e. steam, or into a close system such as Blow Down. The importance of safety valves is such that
any modification required to change their pressure lifting set must be authorized by the highest
technical authority of the refinery.

Q. 246 WHAT IS A PNEUMATIC VALVE ?

Pneumatic valves or Automatic Process Control Valves are valves, generally of the plug type, that are
actuated by a pneumatic signal. The Pneumatic signal is dry compressed air, called Instrument Air,
which acts on a large surface area diaphragm connected to the mobile element through a steam.
The surface area of the diaphragm is calculated to give the necessary thrust to move the plug using
low pressure air. As an example a variation of one PSI (one pound per square inch corresponds to a
pressure of 70 gr / cm 2) of the instrument air on a 30 cm diameter diaphragm produces a thrust
equivalent to about 50 kg. Since variations are up to twelve PSIs the total thrust possible is about 600
kg. Pneumatic valves control the flow rates of fluids and doing so they, directly or indirectly, control the
desired parameter. FRC (Flow Recorder Controller) is an example of direct control. TRC (Temperature
Recorder Controller) is an example of indirect control because temperature is indirectly controlled by
controlling the flow rate of the heating / cooling liquid.

Q. 247 ARE THERE ANY OTHER VALVES TO BE MENTIONED ?

As a matter of fact other types of valves are too numerous to mention, Just it is necessary to bear in
mind that any device that is designed to control or to stop or to let whatsoever flow of whatsoever
fluid, material etc. can be called valve.

Q. 248 WHAT ARE THE MATERIALS USED IN VALVE CONSTRUCTION ?

The materials used in valves construction are conform to the duty the valves are designed for. Valves
which are installed in a process cannot be replaced unless by using spares with the same
characteristics.
Materials used are generally the following :
1. Cast – Iron.
2. Carbon steel.
3. Low-alloy steel.
4. Stainless steel.
5. Copper and its alloys (bronze, brass etc.)
6. Special Metals and alloys.
7. Non metallic materials (PVC, PTFE, etc.)

Q. 249 WHAT IS CAST – IRON ?

Cast – Iron is an alloy of Iron (about 85 + 90%) Carbon (2.2 + 4.8%), Silicon ( 5%) Manganese ( 3.5% )
Phosphorus ( 2.5% ) Sulphur (0.1% ) and traces of Nichel, Copper, Chromium, Molybdenum etc.
However this composition is what is obtained from the blast-furnaces which is a process used to treat
iron ore . Varying its composition it is possible to vary its characteristics . Its melting point ranges from
1150 c and 1300 c. Cast Irons have generally a crushing strength of over 77 kg per square millimeter
and are normally used to resist compressive loads, e.g. for engine mountings, supports for many kinds
of machinery and valves of considerable dimensions. Its limiting factor is the operating temperature
which should never be above 150 c as it causes brittleness. Tendency is to replace cast-iron with forged
steel.

Q. 250. WHAT IS CARBON STEEL ?

Reducing the carbon contents in cast iron to 0.1 + 1.5% and the other elements to desired ranges the
result is Carbon steel or commonly steel. Small variations in its composition determine important
variations in its characteristics. Carbon steel is the material used the most all over the world for large-
scale engineering.

Q. 251 WHAT IS LOW ALLOY STEEL ?

Low alloy steels are a special steels containing carbon in the range of 0.2 + 0.6% and Chromium,
Molitoenum and Nichel in various proportions. It is possible to obtain steels with high resistance to heat,
corrosion etc. Equipment of the refinery where high temperature is involved is made from low alloy
steel.
Q. 252 WHAT IS STAINLESS STEEL ?
Stainless steel is any of various steels alloyed with sufficient chromium to resist corrosion, oxidation or
rusting associated with exposure of ordinary steel to water and moist air. Nichel is also added to
produce the standard “18-10” stainless steel (Chromium 18%, Nichel 8 +10%) which is used for aircraft
parts, blading of steam turbines, etc. Stainless Steel is also used as the material for equipment where
resistance to naphthenic acids and temperature is require (CDU-1 VDU-1).

Q. 253 WHAT IS COPPER AND ITS ALLOYS ?

Copper is a ductile, malleable, reddish-brown metallic element that is an excellent conductor of heat
and electricity and is widely used for electrical wiring, water piping, and corrosion resistance parts
either pure or in alloys such as brass, bronze, monel etc. Brass is an alloy of copper and zinc and is
largely used for ornaments, objects etc. Bronze is an alloy of copper and tin and is largely used for
constructing valves, of small dimensions (up to 4”). Monel is an alloy of copper and nichel and in
refinery is largely used for lining the interiors of columns, vessels, etc. which are subjected to acid attack.
Copper and its alloys are resistant at low pH. While they are practically dissolved in alkaline ambient.

Q. 254 WHAT IS INTENDED BY SPECIAL METALS AND THEIR ALLOYS IN VALVES CONSTRUCTION ?
Metals such as Cobalt, Molybdenum, Titanium etc. and their alloys are employed where very high
resistance to erosion, corrosion as well as stability to high temperatures are required. These materials
are very expensive and therefore their use is limited to very special cases. Satellite, for example, is an
alloy of cobalt and molybdenum and is employed in the fabrication of the slide valves and plug valves
of the FCC Unit.

Q. 255 WHAT ARE THE NON METALLIC MATERIALS USED IN VALVES CONSTRUCTION ?
It refers to those materials (mainly plastics) which are used in particular cases but where high pressure
is not involved. They are many but the two main ones are :

1. Polyvinyl chloride or PVC which is a plastic used to make valves and piping resistant to acioity or
alkalinity. Its limiting factor is the temperature which cannot exceed 80c.
2. Polytetrafluorethylene or PTFE or Teflon which is a plastic material having exceptional properties
such as high resistance to organic solvents, boiling acids and is stable at temperatures up to 250 c

Q. 256 WHAT IS INTENDED BY CLASS OR NOMINAL PRESSURE OFA VALVE ?

Externally on the body of a valve there is a number which can be 150 or 300 or 600 or 900 or 1500 or
3000. This number represents the Nominal Pressure expressed in PSI (Pound Square Inch equivalent to
70 gr / cm 2) at which the valve has been designed. For instance 300 is equivalent to a pressure of
21kg / cm2 and a valve bearing this number can be used in processes with pressures up to 21 kg / cm2.
However this is applicable to valves from 1 ½” above.