BOILER STEAM TO PROCESS

A boiler is a device for heating

water and generating steam
above atmospheric pressure.
The boiler consists of a
compartment where the fuel is
burned and a compartment
where water can be evaporated
into steam. The hot water or
steam is used to transfer heat to
a process Figure: Schematic overview of a boiler room
Boiler Mountings and Accessories
Fitting and devices which are necessary for the safety and control are knows as boiler mountings
Fitting or devices which are provided to increase the efficiency of the boiler and help in the smooth
working of the plant are knows as boiler accessories
Safety valve: It is used to relieve pressure and prevent possible explosion of a boiler.
Water level indicators: They show the operator the level of fluid in the boiler, also known as a sight
glass, water gauge or water column is provided.
Bottom blow down valves: They provide a means for removing solid particulates that condense and
lay on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom
of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates
out.
Continuous lowdown valve: This allows a small quantity of water to escape continuously. Its
purpose is to prevent the water in the boiler becoming saturated with dissolved salts. Saturation
would lead to foaming and cause water droplets to be carried over with the steam - a condition known
as priming.
Hand holes: They are steel plates installed in openings in "header" to allow for inspections &
installation of tubes and inspection of internal surfaces.
Steam drums internals, A series of screen, scrubber & cans (cyclone separators).
Low- water cutoff: It is a mechanical means (usually a float switch) that is used to turn off the
burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain
point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastrophic failure.
Surface blowdown line: It provides a means for removing foam or other lightweight non-
condensable substances that tend to float on top of the water inside the boiler.
Circulating pump: It is designed to circulate water back to the boiler after it has expelled some of its
heat.
Feed water check valve or clack valve: A no return stop valve in the feed water line. This may be
fitted to the side of the boiler, just below the water level, or to the top of the boiler. A top-mounted
check valve is called a top feed and is intended to reduce the nuisance of lime scale. It does not
prevent lime scale formation but causes the lime scale to be precipitated in a powdery form which is
easily washed out of the boiler.
De super heater tubes or bundles: A series of tubes or bundle of tubes, in the water drum but
sometime in the steam drum that De-superheated steam. This is for equipment that doesn't need dry
steam.
Chemical injection line: A connection to add chemicals for controlling feed water pH.
Steam accessories
Main steam stop valve:
Steam traps
Main steam stop/Check valve: It is used on multiple boiler installations.
Combustion accessories
Fuel oil system:
Gas system:
Coal system:
Other essential items
Pressure gauges :
Feed pumps:
Fusible plug :
Inspectors test pressure gauge attachment:
Name plate:
Registration plate
WATER TUBE BOILER
A water-tube boiler is a type of boiler
in which water circulates in tubes heated
externally by the fire. Water-tube boilers
are used for high-pressure boilers. Fuel
is burned inside the furnace, creating hot
gas which heats up water in the steam
generating tubes. In smaller boilers,
additional generating tubes are separate
in the furnace, while larger utility
boilers rely on the water-filled tubes that
make up the walls of the furnace to
generate steam. Interior to the boiler
tubes is the fluid medium to be heated or
physically altered; thus the name "water
tube"
WORKING
Many water-tube boilers operate on the principle of natural
water circulation (also known as 'thermo-siphoning'). This
is a subject that is worth covering before looking at the
different types of water-tube boilers that are available.
Figure A helps to explain this principle:
Fig. A Natural water circulation in a water-tube boiler
Cooler feed water is introduced into the team drum
behind a baffle where, because the density of the
cold water is greater, it descends in the 'down comer'
towards the lower or 'mud' drum, displacing the
warmer water up into the front tubes.
Continued heating creates steam bubbles in the front
tubes, which are naturally separated from the hot water in the steam drum, and are taken off.
However, when the pressure in the water-tube boiler is increased, the difference between the densities of the
water and saturated steam falls, consequently less circulation occurs. To keep the same level of steam output
at higher design pressures, the distance between the lower drum and the steam drum must be increased, or
some means of forced circulation must be introduced.
Water-tube boiler sections
The energy from the heat source may be extracted as either radiant or convection and conduction.
The furnace or radiant section
This is an open area
accommodating the flame(s) from
the burner(s). If the flames were
allowed to come into contact with
the boiler tubes, serious erosion and
finally tube failure would occur.
The walls of the furnace section are
lined with finned tubes called
membrane panels, which are
designed to absorb the radiant heat
from the flame. Fig B Heat transfer in the furnace or radiant section
Convection section
This part is designed to absorb the heat from the hot gases by conduction and convection. Large boilers may
have several tube banks (also called pendants) in series, in order to gain maximum energy from the hot gases.

Fig. C Heat transfer in the convection section

TYPES OF WATER TUBE BOILER

Water-tube boilers are usually classified according to certain characteristics, see Table E.
Reservoir drum position For Example, Longitudinal or cross drum
Water circulation For Example, natural or forced
Number of drums For Example, two, three
Capacity For Example, 25 500kg/h, 7kg/s, 55 000 lb/h

Table E Water-tube boiler classifications

Longitudinal drum boiler
The longitudinal drum boiler was the original type of
water-tube boiler that operated on the thermo-siphon
principle (see Figure 1).
Cooler feed water is fed into a drum, which is placed
longitudinally above the heat source. The cooler water
falls down a rear circulation header into several inclined
heated tubes. As the water temperature increases as it
passes up through the inclined tubes, it boils and its
density decreases, therefore circulating hot water and
steam up the inclined tubes into the front circulation
header which feeds back to the drum. In the drum, the
steam bubbles separate from the water and the steam can
be taken off. Typical capacities for longitudinal drum
boilers range from 2 250 kg/h to 36 000 kg/h.
Cross drum boiler
The cross drum boiler is a variant of the longitudinal
drum boiler in that the drum is placed cross ways to the
heat source as shown in Figure2. The cross drum
operates on the same principle as the longitudinal drum
except that it achieves a more uniform temperature
across the drum. However it does risk damage due to
faulty circulation at high steam loads; if the upper tubes
become dry, they can overheat and eventually fail. The
cross drum boiler also has the added advantage of being
able to serve a larger number of inclined tubes due to its
cross ways position. Typical capacities for a cross drum
boiler range from 700 kg / h to 240 000 kg/h. Fig.2 Cross drum boiler
D-type boiler
This is the most common type of small-medium sized
boilers, similar to the one shown in the schematic
diagram. It is used in both stationary and marine
applications. It consists of a large steam drum vertically
connected to a smaller water drum (a.k.a. mud drum)
via multiple steam-generating tubes. These are
surrounded by walls made up of larger water filled
tubes, which make up the furnace.
Babcock & Wilcox boiler
This has a single drum, with feed water drawn from the
bottom of the drum into a header that supplies inclined
water-tubes. The water tubes supply steam back into the
top of the drum. Furnaces are located below the tubes
and drum. This type of boiler was used by the Royal
Navy's Leander class frigates. The Y160 variant used
on the Batch 3 Leanders (e.g. HMS Jupiter) also
incorporated steam atomisation equipment on the fuel supply so that the diesel fuel entering the boilers via
the three main burners was atomised into a fine spray for better flame efficiency. The superheat temperature
of the Y160 was controlled manually by the Boiler Room Petty Officer of the Watch between 7500F and
8500F and the steam supplied to the main turbines was at a pressure of 550 psi.
Advantages of Stirling construction, which eliminates fire brick entirely, are:
100% water cooled
100% gas tight
Full protection from dew point corrosion
Ease of maintenance
Maximum safety
Maximum structure rigidity
Minimum weight
Fast erection
A, D and O type Water tube Boiler
Water tube package boilers are subdivided into three classes
based on the geometry of the tubes. The .A. design has two
small lower drums and a larger upper drum for steam-water
separation. In the .D. design, which is the most common, the
unit has two drums and a large-volume combustion chamber.
The orientation of the tubes in a .D. boiler creates either a
left- or right-handed configuration. It is used in both
stationary and marine applications. It consists of a large
steam drum vertically connected to a smaller water drum (a.k.a. mud drum)
via multiple steam-generating tubes. These are surrounded by walls made up
of larger water filled tubes, which make up the furnace. For the .O. design,
the boiler tube configuration exposes the least amount of tube surface to
radiant heat. Rental units are often .O. boilers because their symmetry is a
benefit in transportation. FiguresA*-C*show tube configurations for each of
these water tube package boiler designs.
USES OF WATER TUBE BOILERS
Water-tube boilers are used in power station applications that require:
A high steam output (up to 500 kg/s).
High pressure steam (up to 160 bar).
Superheated steam (up to 550C).
Again, their ability to work at higher pressures has led to marine boilers being almost entirely water-
tube. This change began around 1900, and traced the adoption of turbines for propulsion rather than
reciprocating (i.e. piston) engines - although water-tube boilers were also used with reciprocating
engines.
Advantages of water-tube boilers
They have small water content, and therefore respond rapidly to load change and heat input.
The small diameter tubes and steam drum mean that much higher steam pressures can be tolerated,
and up to 160 bar may be used in power stations.
The design may include many burners in any of the walls, giving horizontal, or vertical firing options,
and the facility of control of temperature in various parts of the boiler. This is particularly important
 if the boiler has an integral super heater, and the temperature of the superheated steam needs to be
controlled.
Disadvantages of water-tube boilers
They are not as simple to make in the packaged form as shell boilers, which mean that more work is
required on site.
The option of multiple burners may give flexibility, but the 30 or more burners used in power stations
means that complex control systems are necessary
Lower tolerance for water quality and needs water treatment plant
FIRE TUBE
A fire-tube boiler is a type of boiler in which hot gases from a fire
pass through one or more tubes running through a sealed container
of water. The heat energy from the gases passes through the sides
of the tubes by thermal conduction, heating the water and
ultimately creating steam. A fire tube boiler can be either horizontal
or vertical. A fire-tube boiler is sometimes called a "smoke-tube
boiler" or "shell boiler" or sometimes just "fire pipe".
Vertical Fire tube boiler
Vertical Fire tube boiler (VFT) is internally fired fire-tube boiler, it
consists of vertical cylindrical shell, containing a cylinder firebox
and a number of small fire tubes.
The vertical boiler is a simple type which consists of a firebox at the
bottom and a copper barrel with a smoke tube. It typically is used to
drive stationary engines and boats. Firing is accomplished by
alcohol or solid fuel pellets. Before selecting a vertical fire-tube
boiler, you must know how much overhead space is in the building
where it will be used. Since this boiler sits in an upright position, a
room with a high ceiling is necessary for its installation.
Locomotive boiler
A locomotive boiler has
three main components: a
double-walled firebox; a
horizontal, cylindrical
"boiler barrel" containing
a large number of small
fluetubes; and a smoke
box with chimney, for the
exhaust gases. The boiler
barrel contains larger
flue-tubes to carry the
super heater elements,
where present. Forced
draught is provided in the
locomotive boiler by injecting Schematic diagram of a "locomotive" type fire-tube boiler
exhausted steam back into the exhaust via a blast pipe in the smoke box In the locomotive-type boiler, fuel is
burnt in a firebox to produce hot combustion gases. The firebox is surrounded by a cooling jacket of water
connected to the long, cylindrical boiler shell. The hot gases are directed along a series of fire tubes, or flues,
that penetrate the boiler and heat the water thereby generating saturated ("wet") steam. The steam rises to the
highest point of the boiler, the steam dome, where it is collected. The dome is the site of the regulator that
controls the exit of steam from the boiler.

Advantages of fire tube boilers

Advantages of this design are not often realized intuitively when compared to the water tube. The volume of
fluid in the unit is roughly half the physical volume of the entire boiler, thus control of water levels and
chemistry is much more forgiving and in need of less precision than for the typical water tube. Another
advantage is found in the ease with which a unit is manufactured and serviced. The heat transfer tubes of the
water tube boiler vary across the circumferential span of the drums: there may be eleven or more differing
tube types to assemble in a single water tube unit. In the fire tube, each heat transfer tube is identical to its
counterparts. Manufacture and service are readily accomplished with a stock of a single tube type. It is this
simplicity which contributes to the profound difference in life-cycle and capital cost of the fire tube
Disadvantages of the fire tube
Disadvantages of the fire tube are attributable only to ability. The limits of design pressure, supplemental
firing rates, and capacity are realized more acutely with the fire tube than with the water tube. Design
pressures above 400 psig begin to move fire tubes outside the realm of cost-effective design as do capacities
above 80,000 lb/hr and supplemental firing temperatures above 1,600 deg F. While there are project-specific
particulars which may or may not affect the overall project feasibility of the fire tube design, the water tube
often becomes the more reasonable option as operating parameters become more adverse.
The basic difference between fire tube boilers and water tube boiler are given below
Water Tube Boilers Fire Tube Boilers
Suitable for high steam pressure (above 500 psig) Ideal for low pressure steam. As seen above in the
and temperature(to 1000 F) and large capacities table, the tube thickness increases significantly at high
exceeding millions of lb/h of steam. pressures if the pressure is applied externally. The
Extended surfaces can be used in waste heat pressure can be nearly twice in water tube designs for
applications to make the boilers compact if the the same tube thickness.
gas stream is clean.
Various types of fuels can be fired with ease Suitable for high pressures as gas is contained inside
including solid, liquid and gaseous. The water tubes. Hence you see more of them in hydrogen,
cooled membrane wall construction makes an ammonia plants, where the gas pressures can be in the
excellent furnace. range of 500 to 3000 psig.
If the gas stream is dirty(as in MSW When a large duty has to be transferred at a low log-
applications)provision can be made for cleaning mean-temperature-difference as in gas turbine exhaust
using soot blowers or rapping mechanisms. HRSG applications, surface area required gets
Wide spaced tubes can be used at the gas inlet to enormous and very long tubes are required, adding to
minimize bridging of slag deposits and tube the gas pressure drop. The shell diameter becomes
spacing can be decreased as the gas is cooled. huge; hence unsuitable except in very small gas turbine
This flexibility does not exist in fire tube designs.
Horses generating low pressure saturated steam.
Super heaters if used can be located at the Economizer and superheated can be added but the
optimum gas temperature region shielded by any location for superheated is either at the gas inlet or exit,
number of screen tubes. In fire tube boiler the making it difficult to come up with a good design if
choice is at the gas inlet or exit. corrosive conditions are present.
Due to low water volume, the startup time is If slagging is a concern, then fire tube designs are
lesser and response to load changes is faster generally not suitable as the tube inlet can be plastered
compared to fire tube boilers. with slag. The gas inlet temperature has to be reduced
If the gas pressure is high, say above 5 psig, the through flue gas recirculation or the gas can be cooled
shell/casing design gets complicated and in an external water cooled furnace, making it a
expensive though it can be done. difficult design.
Due to higher heat transfer coefficients surface Cleaning the tubes is easier if there is no slagging. In
area required is lesser and hence gas pressure the case of water tube, the deposits can be formed on
drop is also lower. both the tubes and the casing, while in the fire tube it is
only inside the tubes.
For multiple pressure designs as in gas turbine A separate steam drum with internals is required if
exhaust applications, water tube is the only good steam purity(0.05 to 1 ppm) has to be achieved
choice.

Fire tube Vs water tube Boiler

Fire tubes boilers has a large volume of water, therefore more flexible and can meet the sudden
demand of steam without much drop of pressure.
Fire tubes boiler is rigid and of simple mechanical construction, so greater reliability and low in first
cost.
Fire tube boilers can be made in smallest sizes therefore simple to fabricate and transport, occupies
less floor space but more height.
Due to mostly externally fired water tubes boiler so furnace can be altered considerably to meet the
fuel requirements.
Water tubes boilers are more readily accessible for cleaning,inspection and repairs, compared to the
fire tube boilers.
Modern trend is in the favors of water tube boiler due to continuous increase in capacities and steam
pressures.
FUELS
Many different solid, liquid, and gaseous fuels are fired in boilers. Sometimes, combinations of fuels are
used to reduce emissions or improve boiler performance. Fuels commonly fired in boilers include fossil,
biomass, and RDFs as well as other types of fuels and fuel combinations. Coal, petroleum-based oils, and
natural gas are fossil fuels commonly fired in ICI boilers. However, other forms of solid, liquid, or gaseous
fuel derived from these fossil fuels are sometimes included in this category. One of these fuels, which is
referred to as tire derived fuel (TDF), consists of shredded vehicle tires. Another boiler fuel is referred to as
biomass. Biomass is renewable organic matter. Examples of biomass include fast-growing trees and plants,
wood and wood waste, agricultural crops and residue, aquatic plants and algae, animal wastes, and organic
municipal and industrial wastes. RDF is a potentially valuable energy source. It consists of MSW that has
been processed using size reduction and material recovery techniques to eliminate materials such as
aluminum, steel, glass, plastic, and rock.
Common types of fuels fired in boilers are listed in Table A*.
Fuel Description
By- Any liquid or gaseous substance produced at chemical manufacturing plants or
product/waste petroleum refineries (except natural gas, distillate oil, or residual oil) and combusted
in a steam generating unit for heat recovery or for disposal.
Biomass Organic matter that is used as fuel is called biomass; biomass is a nonfossil fuel that
includes materials such as wood, biogases, nut hulls, rice hulls, corncobs, coffee
grounds, and tobacco stems
Coal Coal is a brown-to-black combustible sedimentary rocklike material composed
principally of consolidated and chemically altered plant material that grew in
prehistoric forests; it includes all solid fuel classified as anthracite, bituminous, sub
bituminous, or lignite coal, coal refuse, or petroleum coke.
Coal refuse Waste products of coal mining, physical coal cleaning, and coal preparation
operations containing coal, matrix material, clay, and other organic and inorganic
materials Distillate oil Fuel oils that contain 0.05 wt % nitrogen or less and comply
with the specifications for fuel oil
Municipal type Refuse, more than 50% of which is waste containing a mixture of paper, wood, yard
Solid waste waste, food wastes, plastics, leather, rubber, and other noncombustible materials
and RDF such as metal, glass, and rock, which are usually removed prior to combustion
Natural gas A naturally occurring mixture of hydrocarbon gases found in geologic formations,
beneath the earths surface, of which the principal constituent is methane, or LP gas
Oil Crude oil or petroleum or a liquid fuel derived from crude oil or petroleum,
including distillate and residual oil.
Propane Propane is a heavy gaseous fossil fuel processed from crude petroleum and natural
gas. Residual oil Crude oil and fuel oil Nos. 1 and 2 that have nitrogen content
greater than 0.05 wt %, and all fuel oil
Solvent- Any solid, liquid, or gaseous fuel derived from solid fuel for the purpose of creating
derived fuel useful heat and includes, but is not limited to, solvent-refined coal, liquefied coal,
and gasified coal.
Very low Oil that contains no more than 0.5 wt % sulfur and that, when combusted without
sulfur oil SO2 emission control, has a SO2 emissions rate equal to or less than 215 ng/J (0.5
lb/MBtu) heat output.
Wood Wood, wood residue, bark, or any derivative fuel or residue thereof, in any form,
including, but not limited to, sawdust, sander dust, wood chips, scraps, slabs,
millings, shavings, and processed pellets made from wood or other forest products.
Wood residue Bark, sawdust, slabs, chips, shavings, mill trim, and other wood products derived
from wood processing and forest management operations.
Nuclear energy Nuclear fission is also used as a heat source for generating steam.