1 Introduction

This course is intended for officers and key ratings that have not
previously served on board liquefied gas tankers as part of the regular
complement. It covers mandatory minimum training requirements
prescribed by Regulation V/1, paragraph 1.2 of the International
Convention on Standards of Training, Certification and Watchkeeping
for Seafarers, STCW-95 and it includes basic safety and pollution-
prevention precautions and procedures, layouts of different types of
liquefied gas tankers, types of cargo, their hazards and their handling
equipment, general operational sequence and liquefied gas tanker
terminology.

1.1 The course

The background for and the purpose of the course as being:
- The STCW-95 Convention contains mandatory minimum requirements
for training and qualification of masters, officers and ratings of
liquefied gas tankers.
- This training is divided into two parts:
· Level 1: liquefied gas tanker familiarization – a basic safety-training
course for officers and ratings on board.· Level 2: advanced training in
liquefied gas tanker operations for masters, officers and others who
are to have immediate responsibilities for cargo handling and cargo
equipment.
- This course covers the requirements for level 1 training required by
Regulation V/1, paragraph 1.2 of the International Convention on
Standards of Training, Certification and Watchkeeping for Seafarers,
STCW-95
1.2 Development of Liquefied gas Shipping
Learning Objectives
Lists important stages in the transport of liquefied gas by ships, such
as:
· gas shipping began in the late 1920s
· the earliest ships were designed to carry liquefied gas in pressure
vessels at ambient temperature
· the first cargoes on the market were butane and propane
· development of refrigeration techniques and meta s suitable for low
temperature made it possible to carry liquefied gas at temperatures
lower than ambient
· defines terminology and explains abbreviations commonly used
aboard gas tankers and on gas terminals

In the late 1920th transportation of liquefied gases in bulk started. In

the very beginning it was transportation of propane and butane in fully
pressurised tanks. Around 1959, semi-pressurized ships entered the
market and liquefied gas was now transported under lower pressure,
which was made possible by lowering the temperature. By 1963, fully
refrigerated ships for LPG, LNG and certain chemical gases wore in
service, carrying cargo at atmospheric pressure.
Liquefied gas is divided into different groups based on boiling point,
chemical bindings, toxicity and flammability. The different groups of
gases have led to different types of gas carriers and cargo containment
system for gas carriers.
The sea transport of liquefied gases in bulk is internationally regulated
- with regard to safety through standards established by the
International Maritime Organization (IMO) and these standards are set
out in the IMO's Gas Carrier Codes, which cover design, construction
and other safety measures for ships carrying liquefied gases in bulk.

1.3 Terminology
BOILING: This is the action, which takes place when a liquid changes its
state from a liquid into a gas or vapour. The heat required to bring this
change of state about is called Latent Heat.

BOILING TEMPERATURE: This is the temperature at which a liquid boils.

As the boiling temperature rises with an increase in pressure (see
saturated vapour pressure), the boiling temperatures are usually given
for atmospheric pressure. At this pressure, water boils at + 100oC.
butane at - ½oC., ammonia at -33oC. and propane at -43oC.

CONDENSATION: This is evaporation in reverse. If a vapour becomes

supersaturated, condensation takes place and heat is surrendered. For
example, in a seawater-cooled condenser, a compressor has raised the
pressure of the vapour to such an extent that at seawater
temperature, it is supersaturated. Condensation takes place, and the
latent heat released heats up the water passing through the condenser
tubes; the heated seawater passing overboard into the sea, to be
replaced continuously by fresh cool water. The resulting condensate
will be somewhat warmer than the seawater coolant.

EVAPORATION: This is the process of converting a liquid into a vapour,

and it requires latent heat to do this. If a liquid (say liquid propane) in a
closed container at 10oC. Has a saturated vapour pressure of 5
atmospheres, and the vapour in the space above the liquid is allowed
to escape, the pressure in the container will fall. As soon as this
happens, the vapour in the space above the liquid will be
undersaturated and evaporation will take place (or the liquid boil).
Heat will be used up in the boiling process and the temperature of the
liquid will fall. The "boil off" will largely replace the vapour which has
been allowed to escape until such time as the pressure in the container
corresponds to the saturated vapour pressure of the liquid at the new
lower temperature. Continuous withdrawal of vapour means
continuous evaporation, which in turn means continuous loss of heat
(cooling).

FILLING OF CARGO TANKS: The correct maximum volume of liquid to

load in a cargo tank is such a quantity that after allowance for the
product to warm up and expand to a temperature the saturated vapour
pressure of which would lift the safety valves, 2 per cent. of the space
would remain. A tank so filled is described as Full. A tank filled above
this level is described as Overfull. A tank completely filled with liquid is
described as one hundred per cent.

FLASHOVER: Firefighting on board ships differs from firefighting ashore

in that allowance has to be made for the fact that the metal with which
a ship is constructed, conducts heat to a far greater extent than
normal shore building materials. The result is that a fire on board ship
tends to spread horizontally as well as vertically.
If the temperature of combustible material in a compartment adjacent
to one where a fierce fire is burning, is raised to above its ignition
temperature (q.v.), that material will ignite spontaneously (auto
ignition), so spreading the fire from one compartment into another,
through a bulkhead, without a spark or flame being directly involved.
Such a means of a fire spreading is termed a flash-over.

GAS/VAPOUR: Gas is a substance which has the property of indefinite

expansion. In the context of this book, it is above its critical
temperature and cannot be condensed into a liquid. If the temperature
of a gas is reduced to below its critical temperature, it then becomes a
vapour, and can be condensed into a liquid. Gases are frequently
referred to as incondensibles.

Flammable or Explosive Mixture: Petroleum as a liquid does not burn.

At ordinary temperatures, it gives off vapour, which when mixed within
certain proportions with air, will burn. The lowest proportion of
petroleum vapour in air mixture, which will burn, is termed lower
explosive limit (L.E.L.) and the strongest mixture that will burn is
termed upper explosive limit (U.E.L.). The flammable mixtures between
the lower and upper explosive limits are called the explosive range. A
mixture of vapour in air weaker than the L.E.L. is described as too lean
or over-lean whilst a mixture of vapour in air stronger than the U.E.L. is
described as too rich or over-rich. Mixtures outside the explosive range
will not burn, the words explosive and flammable within this context
being virtually synonymous.

Flash Point: This is the lowest temperature at which a flammable

mixture of air and vapour will burn when exposed to a naked flame.

Ignition Temperature: This is the temperature at which a flammable

mixture of vapour and air will ignite spontaneously (without being
exposed to a naked flame). The operation of a diesel engine depends
upon this effect.

GAS LAWS

Avogadro's Hypothesis: Equal volumes of different gases at the same

pressure and temperature contain the same number of molecules.

Boyle's Law: The volume of a given mass of gas varies inversely with
the pressure provided that the temperature remains constant:

P=

Charles's Law: The volume of a given mass of gas varies directly with
the absolute temperature provided the pressure remains constant:

Volume = or density =

Clerk Maxwell's Kinetic Theory: A gas may be imagined as a vast

number of molecules moving in all directions at irregular velocities,
colliding with one another and with the walls of the containing vessel.
The path of a molecule is zigzag in three dimensions and the mean
free path is defined as the average length between collisions, the
denser the gas, the shorter will be the mean free path.

On the assumption that the molecules are microscopic spheres, it can

be shown that the pressure and absolute temperature of a gas are
proportional to the mean kinetic energy of translation of the molecules
bombarding the walls of the vessel containing the gas. Thus, at the
same temperature the average kinetic energy of translation of the
molecules of any gas are the same whatever its mass-a "large"
molecule having low velocity and a "light" molecule having high
velocity.

This theory correlates Avogadro's Hypothesis, Boyle's Law, Charles's

Law and Gay Lussac's Law.

Dalton's Law of Partial Pressures: The pressure of a mixture of gases is

the sum of the pressures each would exert if it alone were to occupy
the containing vessel.

Gay Lussac's Law: The density of a gas at standard pressure and

temperature is proportional to its molecular weight. This is a corollary
of Avogadro's Hypothesis.

Joule's Law: When a perfect gas expands without doing external work
and without taking in or giving out heat and therefore without
changing its stock of internal energy, its temperature does not change.

HEAT
Latent Heat: This is the heat used up in changing the state of a
substance without changing its temperature. In the case of changing
the state of a substance from a solid into a liquid (melting), it is called
the latent heat of fusion, and in the case of heat changing the state of
a liquid into a gas or vapour (boiling), it is called the latent heat of
vaporisation. It takes 80 calories to change 1 gramme of ice into water
and about 539 calories to change 1 gramme of water into steam at
standard atmospheric pressure. The value of latent heat of
vaporisation varies with temperature and pressure (see critical
temperature).

Sensible Heat: This is the heat used in raising the temperature of a

substance without changing its state. 1 calorie is used to raise the
temperature of 1 gramme of water 1oC.

HEEL: This is the small quantity of liquid remaining after discharge

which it is impossible to pump out, but which is used to assist in
keeping the cargo tank cold during the ballast (unloaded) passage, and
is usually carried over to the next loading. When it is know that the
vessel will be changing grades or gas freeing, every effort should be
made to reduce this heel to the absolute minimum.

LIQUID CARRY OVER: This occurs when vapour moves swiftly over the
surface of a liquid and droplets of liquid become entrained with the
vapour and are carried over with it.
It is the entrained droplets of lubricating oil that are recovered in the
lubricating oil separator trap of the compressor, and entrained liquid
droplets which cause wet suction on a compressor.

MOLE: This is the quantity of gas the weight of which is equal to its
molecular weight in pounds or grammes. Thus a mole of hydrogen
would be 2, a mole of oxygen 32 etc. This is fairly closely related to
Avogadro's Hypothesis, a mole having the same volume for all
products at the same pressure and temperature.
PRESSURE
Absolute Pressure: This is the pressure above a vacuum. Thus a
pressure of 7 p.s.i. absolute, is really a suction pressure of 7.7 p.s.i. at
atmospheric pressure (atmospheric pressure equals 14.7 p.s.i.).

Gauge Pressure: This is the pressure above one atmosphere and is the
usual method of measuring pressures and vacuums. Absolute pressure
is therefore equal to gauge pressure plus one atmosphere.

Atmospheric Pressure: This is the pressure exerted at sea level. This

pressure varies from place to place and from time to time. The
standard atmospheric pressure is 1012.5 millibars, corresponding to
29.90 inches or 760 millimetres of mercury.

SPAN GAS: This is a laboratory-measured mixture of gases used for the

purpose of calibrating gas detectors. In gas tankers, the mixture is
usually 30 per cent. L.E.L. of the product mixed with pure nitrogen.

STRATIFICATION: This is the layering effect of two gases or vapours

with dissimilar densities, the lighter vapour floating above the heavier.

TEMPERATURE
Absolute Temperature: As a result of studying Charles's Law, it seemed
that the volume of a gas would reduce to nothing at about -273oC. (or
absolute zero). (Physicists have never been able to reach this
temperature.) It therefore follows that absolute temperature equals
temperature + 273oC.

Adiabatic Changes in Temperature: When a gas (or vapour) is

compressed, its temperature rises. When it expands, its temperature
falls. This is the adiabatic process and compression ignition (diesel)
engines rely upon this property for their operation.

Critical Temperature: This is the temperature above which it is not

possible to liquefy a gas. Saturated vapour pressure rises with an
increase in temperature. At the same time, the density of a liquid falls
with an increase in its temperature. Therefore, there must come a time
when so many atmospheres of pressure are required to liquefy the
vapour that the density of the compressed vapour and the liquid are
the same. When this state is achieved, there is virtually no difference
between the liquid and vapour phases and they freely change into
each other. The value of latent heat is reduced to zero and with any
increase in temperature, no amount of increasing the pressure will
bring about liquefaction, and the vapour is then described as a gas.
Associated with the critical temperature is the critical pressure.
VAPORISATION: This is the action of converting a liquid into a vapour.

Batch Vaporisation: This is the method of evaporation whereby vapour

is withdrawn from the top of a tank, causing the liquid in the tank to
boil, with a consequent drop in temperature. With a mixture of
products such as butane and propane, the more volatile element tends
to evaporate first, so that the proportions comprising the mixture will
change and after a time one is left with almost pure butane. This
process of altering a mixture in a tank due to the volatile constituent
evaporating first is called "weathering". However, batch vaporisation is
the simplest method and because, in L.P.G. tankers, the vapour which
has been withdrawn is condensed into a liquid and returned to the
tank, there is no tendency to alter the constituents of the mixture, so
this is used as a method of refrigeration.

Flash Vaporisation: This is the method whereby liquid is withdrawn

from the bottom of the tank and evaporated in a vaporising unit. In this
method, the constituents of a mixture remain fairly constant, as does
the temperature of the product in the tank.

VAPOUR: This is the term used for a "gas" below its critical
temperature and therefore capable of being liquefied.

Saturated Vapour Pressure (S.V.P.) All liquids tend to evaporate under

normal conditions, but if kept in a closed container, evaporation will
only take place until the atmosphere in the container becomes
saturated. In the case of water, the following experiment can be
carried out. Into the top of a barometer some water is introduced. Due
to the evaporation of the water that has been introduced, the level of
the mercury will fall. If sufficient water is introduced, the level will
virtually stop falling because the space above the mercury will be
saturated with water vapour, and a little water will show on top of the
mercury. The fall in the mercury level converted into pressure would
indicate the absolute S.V.P. at that temperature. By rising the
temperature, more water will evaporate and the level of the mercury
fall further. The new level, converted into pressure, will indicate the
new S.V.P. at the new temperature. At 100oC, the level of the
barometer will register zero. The absolute vapour pressure of water at
100oC. is therefore one atmosphere (1.0125 bar). It therefore follows
that under atmospheric conditions, a liquid will, apart from minor
evaporation, keep its state until with the addition of heat, and its
absolute S.V.P. reaches one atmosphere. From then on, all the extra
heat will be used to assist evaporation and the temperature will not
rise. In other words, the liquid boils. If the boiling action takes place in
a closed container, e.g., a boiler, as the temperature rises, so the
pressure increases. That is, the boiling temperature of the water rises
as the pressure increases. The pressure in the boiler is an indication of
the water temperature and vice versa.
If a thermometer and pressure gauge were fitted to a container
holding, say, propane, the temperature and pressure would be directly
related to each other, the pressure rising as the temperature rose and
vice versa.
A sudden release of pressure would result in continuous evaporation,
this using up latent heat so cooling the liquid until the temperature of
the liquid reached that appropriate to the S.V.P. of the product at the
new pressure. This means that if warm propane escaped onto the
deck, it would immediately evaporate and refrigerate itself down to
approximately –43oC.
Supersaturated Vapour: If the vapour pressure in a container is rapidly
increased, condensation will take place, but until the process of
condensation has been completed, the vapour will be supersaturated.

Undersaturated Vapour: This is super-saturation in reverse.

Superheated Vapour: In the absence of liquid to continue the

evaporating process and so keep the vapour saturated, the vapour
temperature can be raised to well above the temperature
corresponding to that at which the vapour would be saturated at the
pressure concerned. Any superheated vapour would have no tendency
to condense. This property is used particularly with steam. The
saturated steam coming from the boilers is heated further in the
superheater to prevent condensation taking place in the engine.

VAPOUR RETURN LINE: This is a balancing pipeline between the ship

when loading (or discharging) and the shore tank, so that the vapour
trapped in the space above the incoming liquid, and therefore being
compressed, is returned to the shore tank from which the product is
being discharged.

WET SUCTION: This occurs when liquid droplets are carried over into
the compressor suction, and get sucked into the compressor. It can
only take place if the vapour at the compressor suction is at or near
saturation.
On the compression stroke, the adiabatic increase in temperature is
used up evaporating the liquid droplets which have been sucked into
the cylinder, resulting in a dramatic drop in the discharge temperature.
The temperature of the cylinder head falls and in extreme cases can
become covered with ice.
Wet suction frequently causes damage to the compressor suction and
discharge valves, and in extreme cases, where too much unevaporated
liquid collects in the cylinder, can cause the cylinder head to be
shattered.

ZERO GAS: This is pure nitrogen used to calibrate the zero reading of
gas detectors.