013 --- the physics of liquefied gases

7/31/2019 013 --- The Physics of Liquefied Gases

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A VIDEOTEL PRODUCTION

The Physics ofLiquefied Gases

VIDEOTEL PRODUCTIONS

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The Physics of

Liquefied GasesA VIDEOTEL PRODUCTION

The Producers would like to thank the following for their help with this programme:

Bibby-Harrison Management Services Ltd

BP Shipping Ltd

BV United Gas Carriers

Calor Gas Ltd

Chevron Shipping

Dorchester Maritime Ltd

The Institute of Marine Engineers

ITS Testing Services Ltd

SIGTTO

CONSULTANT: CHRIS CLUCAS

PRODUCER: ROBIN JACKSON

DIRECTOR: GEORGE BEKES

PRINCIPAL PRINT AUTHOR: ROBIN JACKSON

Warning:Any unauthorised copying, hiring, lending, exhibition diffusion, sale, public performance

or other exploitation of this video is strictly prohibited and may result in prosecution.

COPYRIGHT Videotel 1999

This video is intended to reflect the best available techniques and practices at the time ofproduction, it is intended purely as comment. No responsibility is accepted by Videotel,

or by any firm, corporation or organisation who or which has been in any way

concerned, with the production or authorised translation, supply or sale of this video foraccuracy of any information given hereon or for any omission herefrom.


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CONTENTS Page

Introduction 1

Module 1: The Properties of Gases 1

Module 2: The Gas Laws 3

Module 3: Sensible and Latent Heat 6

Module 4: Enthalpy Pressure Relationships The Mollier Chart 7

Module 5: The Laws of Thermodynamics 8

Module 6: Cargo Systems 8

Module 7: Two Stage Compression 10

Module 8: Refrigerated Systems 12

Module 9: Summary 16

Videotel Productions The Physics of Liquefied Gases


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THE PHYSICS OF LIQUEFIED GASES

Introduction

In recent years there has been a rapid growth in the shipment of cargoes of liquefied gas. Both LNG (liquefiednatural gas) and LPG (liquefied petroleum gas) are now shipped between continents in considerable

quantities. Ammonia and other chemical feedstocks are also carried in liquefied form. The ships that carrythese cargoes are obviously very different from the vessels that carry containers and dry cargoes. It is

essential that those who sail on them have a clear understanding of what is involved if they are to carry thesecargoes safely and efficiently and without affecting their specification.

Training in basic physics as it relates to the safe carriage of liquefied gases in bulk is mandatory under therequirements of the IMO International Convention on Standards of Training, Certification and Watchkeeping

for Seafarers 1995 (STCW95). This training is compulsory for Masters, Chief Engineers, Chief Officers, SecondEngineers and any other Officer responsible for handling of bulk liquefied gas cargoes.

This training package complies with the STCW95 requirements and looks at the basic physics involved incarrying the main liquefied gas cargoes as this can help us understand how to ensure that we avoid anyproblems. The package can be used for group training or individual self-study.

The programme is presented in a lecture format and is broken up into distinct modules. It is suggested

that you refer to the relevant section of this booklet as you finish viewing each module. It is not advisableto view the whole programme in a single session.

1. The Properties of Gases

A gas is a collection of molecules flying around in space. If we were to add up the volume of each individualmolecule it would be quite insufficient to fill the space which contains them. As the physicist Oliver Lodgesummed it up: A gas is something in chaotic movement which is bounded by the surfaces of the container

around it. In fact the very word gas comes from the Greek for chaos.

Within a container, we can think of gas molecules or atoms as balls, bouncing off each other and off thesurface of the container. Their speed of movement depends on the energy level or temperature of the gas.

As we cool the gas down, the speed decreases and eventually the molecules will stick together so that thegas becomes a liquid. This liquid is bounded by the lower surfaces of its container but the upper surface

reflects gravitational attraction.

Cooled still further the molecules will stick together even more closely and form a solid. This hascompletely self-contained boundaries and is self-defining. So by condensing a volume of gas and turning it

into a liquid, we can reduce its volume many hundreds of times. This is the basis of transporting gases inliquid form. For example, propane gas occupies approximately 250 times the volume of propane liquid at

15C .

Much work was done on the subject of gases at the beginning of the 19th Century. The Irish scientist,

Robert Boyle, for example, measured the volume of a gas while he varied the pressure on it, keeping thetemperature constant. The French physicist, Jacques Charles, kept the pressure constant and then measured

the volume while varying the temperature. He found that as the temperature rose, the volume increased ina straight line relationship. Although scientists then could not produce extremely cold temperatures, the

graph showed that, in theory, there would be a very low temperature when the volume of gas was zero.


1


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C is the point where the extension of the line AB meets the temperature scale in degrees Celsius.

This point is the start of the absolute temperature scale.

Repeating these experiments with a variety of gases showed that whatever the gas, the temperature at

which the volume became zero was the same. This temperature was defined as the absolute zero oftemperature. The absolute scale was named in honour of the physicist Lord Kelvin and this temperaturescale is given the origin point of zero degrees Kelvin. The Kelvin scale has the same interval degrees as the

Celsius scale but the zero point is 273 degrees below zero Celsius. It is not entirely clear what happens toatoms at the absolute zero of temperature. It can only be approached, never actually reached.

x

x

x

C

-273.15C -100C 0C 100C Temperature

0K 173.15K 273.15K 373.15K

Volume

A

B


2

Illustration of Absolute Temperature


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2. The Gas Laws

The work of the early scientists on gases resulted in a number of Gas Laws which defined the relationships

between different properties of the gas.

Boyles Law, for example, states that Pressure x Volume is constant if the gas is held at constanttemperature. P x V = a constant

P

ressure,

P

Volume, V

P x V = constant


3

Illustration of Boyles Law

Volume, V

PxV = constant

P

ressure,

P


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Charles Law states that Volume divided by Temperature is constant if the pressure is constant.

V/T = a constant

Volume,

V

Temperature, T

V

T= constant


4

Illustration of Charles Law

Temperature, T

VT

= constant

Volume,

V


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The Pressure Law, sometimes referred to as the Constant Volume Law, states that Pressure divided by

Temperature is constant if the volume is kept constant. P/T = a constant

Another scientist who contributed a great deal to our understanding of this area was the Italian, Amedo

Avogadro. In the early 19th century he proposed that the volume of a gas could be related to its mass. Atzero degrees Celsius and one atmosphere pressure he found that for different gases, a volume of 22.4 litres

always held a quantity of gas equivalent to its molecular weight in grams. Avegadro defined these conditionsas normal temperature and pressure or NTP. So 16 grams of methane or 44 grams of propane occupied

the same volume at NTP.

Furthermore, Avogadro proposed that this volume contained the same number of atoms or molecules for

any gas - a number he calculated as 6.023 x 1023. He defined this quantity as a mole, (from the Latin for alarge number of molecules), and defined the volume occupied by a mole of a gas at NTP as the molar

volume. Note that Avogadros hypothesis is independent of mass units - so it is possible to define amolecular number and volume for kilograms, pounds and so on. However, the gram-mole is the most

commonly used, so it is the unit assumed if no other is stated.

By putting these various Gas Laws and Avogadros hypothesis together, it is possible to derive a GeneralGas Equation that relates the pressure, volume and mass of a gas to its temperature.

P x V = n R x T

n is the number of moles of the gas and R is called the Universal Gas Constant.

Pressure,

P

P

T= constant


5

Illustration of Pressure Law

Temperature, T

PT

= constantPressure,

P


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The other law which describes the behaviour of gases is known as Daltons Law. Dalton found that if he

took a mixture of gases which did not react with each other chemically, then the pressure they exerted in aclosed container was independent of the mixture - that is each gas behaved as if the other components of

the mixture were not present. This can be simply illustrated for a mixture of hydrocarbon gases as:

P = p1 + p2 + p3 + p4 + ....

where P is the total pressure of the mixture, p1 is the partial pressure of the methane present, p2 is the

partial pressure of the ethane present, etc.

Finally, we need to be clear that in calculations involving the gas laws, we use absolute units ofpressure, whereas in general industrial practice we measure pressures relative to atmospheric pressure. So

gauge pressure shows pressure above atmospheric pressure and the absolute pressures used in gas lawcalculations are 1 atmosphere higher than gauge pressures.

3. Sensible and Latent Heat

The carriage of liquefied cargoes also requires us to understand what happens if and when these liquidschange phase - that is cool down and become solids or heat up and become gases. Lets consider the example

of water.

Ice, the solid form of water exists from a temperature of absolute zero -273C. If we start to add heat we

can use a thermometer to measure the rise in temperature. This heat can be measured or sensed so isdefined as sensible heat. But at 0C, although the rate of heating remains constant, the temperature has

stopped rising. This is because the energy being put in to the ice is being used to break up the bonds betweenthe ice molecules and form the more loosely bonded liquid we call water. Because this heat input cannot be

sensed, it is called the latent heat and because the ice is melting it is called the latent heat of fusion.

As the water is heated, the movement of the water molecules steadily increases. The temperature rises andso we have more sensible heat. At 100C the molecules start to fly off into the space above the liquid surface

and the water boils. For the molecules to move into the gas phase, the interaction between the liquidmolecules has to be overcome and this requires more latent heat energy - the latent heat of vaporisation.

Once all the water has been converted into steam then this itself can be heated further. It becomes a

superheated gas. When the gas is in equilibrium with the liquid we call the gas saturated.

When we are handling gases on board ship, however, we need to consider not just the temperature but

also the volume and pressure. These define the total heat content of the gas or its enthalpy.

ICE

WATER

STEAM

Sensible heat Latent heat offusion (355 kj/kg)

Sensible heat(420 kj/kg)

Latent heat ofvaporisation(2260 kj/kg)

Superheating

TEMPERATUREC100

0

HEAT


6

Behaviour of Water When Heated


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4. Enthalpy Pressure Relationships - The Mollier Chart

In our example of water we know that the transition temperatures - the freezing and boiling points - are 0C

and 100C respectively at atmospheric pressure. But these temperatures can be altered by changes in

pressure. For instance, on board ship we often reduce the pressure and then boil sea-water to distil it andobtain fresh water at jacket water temperatures. Conversely in steam boilers or autoclaves, the pressure isincreased and the boiling point is raised. Furthermore, the latent heat of vaporisation varies according to

whether the boiling is being undertaken at reduced or increased pressure.

When the relationships of enthalpy against pressure are plotted on a chart the resulting diagram is known

as a Mollier Chart. This is an essential aid to understanding what may happen when we liquefy or condenseour cargo gases. A simplified version of a Mollier Chart is shown below.

The diagram plots the enthalpy along the x-axis and the absolute pressure on the y-axis. From the bottomleft a curved line runs up towards the right, levels off and then turns down again to the x-axis. The left hand

portion of the line represents the saturated liquid line and the right hand portion the saturated vapour line.

In the region between and below these curves there is a mixture of liquid and vapour. The transition point,at the top, is called the Critical Point.

The horizontal distance between the curves denotes the latent heat at a particular pressure and this

decreases as the pressure increases until, at the Critical Point, it becomes zero.

The other lines shown within the envelope run through points with the same dryness fraction. Thefigures show the fraction of gas in the mixture - i.e. 0.4 means that 40% of the mixture is liquid and 60% gas.

To the left of the saturated liquid line there will be sub-cooled liquid and to the right of the saturated

vapour line will be super-heated vapour.

Critical point100C90C50C0C 50

40

30

20

10

6

5

4

Pressure(barAbsolute)

Satu

rated

vap

ourli

ne

Latent heat

0.2

0.4

0.6

0.8

Saturate

dliquidlin

e Dryne

ss(x

)

Constanttemperature

SUB-COOLEDLIQUID

LIQUID/VAPOURMIXTURES

SUPERHEATEDVAPOUR


7

Enthalpy: Simple Mollier Chart


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The dashed lines are lines of constant temperature and by looking at these it is clear that at certain

temperatures it is not possible to liquefy a gas just by increasing the pressure. The gas must first be cooledto the critical temperature or below. For some of the cargoes commonly carried, such as ethylene and

ethane, this critical temperature is in the ambient range.

5. The Laws of Thermodynamics

In using the cargo systems on board we also need to understand some of the laws of thermodynamics. There

are four of these laws although only three concern us here.

The Zero(th) Law introduces the concept of thermal equilibrium between bodies. It states that:-

If a body A is in thermal equilibrium with body B, and body B is also in thermal equilibrium with bodyC, then body A is also in thermal equilibrium with body C.

The First Law introduces the concept that heat and work are equivalent. It states that:-The heat lost from a source is equal to the total heat gained and work done on the bodies that receive that

heat.

The Second Law introduces the concept of directional heat flow. It states that:-Heat always flows from a hot body to a cooler one.

6. Cargo Systems

With an understanding of the Gas Laws and the Laws of Thermodynamics we can follow the operation of a

simple cargo system on board a liquefied gas ship. A simplified cycle is shown below.

Cargovapour

Liquid separator

Compressor

Expansion

valve

Sea water

cooling

1

5

4

2 3

cargo tank

condenser

collecting vessel


8

Single Stage Direct Compression Cycle


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The cargo tank contains cold liquid. Although it is insulated, some heat will still come through from

outside. This is a consequence of the Second Law of Thermodynamics. This heat will overcome the latentheat of vaporisation and so some of the cargo will boil off. The vapour formed will raise the pressure in the

tank.

The vapour is drawn off and passed to a compressor. The work done by the compressor raises thetemperature of the vapour according to the First Law of Thermodynamics. This temperature rise is necessary

so that it becomes hotter than the cooling medium that is subsequently used to cool it again.

The hot gas or vapour now passes to a condenser where it is cooled by sea water. The removal of heat

means that the molecules slow down and the vapour condenses to form a warm liquid. This liquid is alsounder pressure.

The liquid is then passed through an expansion valve that reduces the pressure. This results in a partial

evaporation which causes the temperature of the bulk liquid to fall back down to tank temperature. So coldcondensate is returned to the tank. (These valves are sometimes referred to as Joule-Thompson valves after

the physicists who discovered the cooling-expansion phenomenon.)

It is important to note that the Joule-Thompson expansion results in cooling because the small amount ofgas generated absorbs its latent heat of vaporisation from some of the liquid. The real cooling effect of this

cycle is the removal of sensible heat and the latent heat of vaporisation from the full flow of gas as it iscondensed and cooled.

In summary, we can see that the heat flowing into the tank has been returned to the environment via the

sea-water, together with the additional work done by the compressor. This can also be followed on theMollier Chart.

Point 1 represents the vapour in the dome of the tank. As it passes to the compressor it picks up some heat

(Point 2). The compressor raises the temperature and pressure of the gas to Point 3. It is then cooled and

condensed and becomes a saturated liquid (Point 4) in the condenser. The expansion valve then produces

cold liquid with a proportion of vapour and this is returned to the tank (Point 5).

This is the same cycle as that of a domestic refrigerator but on the ship the working fluid is the cargo boil-

off. If the cargo were totally pure it would be easy to use a Mollier Chart but in reality, few cargoes arechemically pure and in cases such as LPG, the cargo is usually a mixture of several hydrocarbons. This can

give rise to problems due to the preferential evaporation of the compound with the lower boiling point.

12

34

5

Pressure(absolute)

Enthalpy


9

Mollier Chart: Single Stage Direct Compression Cycle

Pressure(absolute)

Enthalpy


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For example, take a cargo of liquefied propane that contains 4% ethane. When the cargo vaporises,

however, the gas will contain 22% ethane. Variations like this can affect the mass flow through thecompressor. This in turn affects the cooling performance of the system. Ethane also raises the condensing

pressure. So increasing proportions of ethane will effectively reduce the plants cooling capacity. Knowing

the composition of your cargoes will help you understand how plant performance can vary.

7. Two Stage Compression

The single stage system we have just described has certain limitations. For example, if the cargo tank

pressure is low and the sea-water temperature is high, the compressor has to do much more work to makethe cycle operate - in other words it will need a high compression ratio. But as the Compression Ratio in a

typical compressor is increased, the valve performance becomes less efficient.

1.0

0.9

0.8

0.7

0.5

0.6

0.41 2 3 4 5 6 7 8 9 10 11 12

Effect of Compression Ratio on Valve Efficiency

Compression Ratio

Efficiency

Gas

Liquid

0 4 22 50100

-40

-50

-60

-70

-80

-90

-40

-50

-60

-70

-80

-90

TemperatureC


10

Effect of Compression Ratio on Valve Efficiency

MOLE % ETANEPressure = 1.1 bar (A); Mole % propane = 100 - Mole % ethane

Propane - Ethane Equilibrium Diagram


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To overcome this problem, it is normal to split the compression into two stages.

The principle is the same as before with cargo vapour from the tank passing to the first and second stage

compressors. It may be necessary to cool the suction vapour entering the second stage compressor by passingit through an intercooler fed by cold liquid on its way back to the cargo tank. However, care must be taken

not to cool this vapour too much or otherwise there could be liquid entering the second stage compressor.

The Mollier Chart shows the process with the numbers on the Chart corresponding to those on the diagram

of the plant.

5

3

21

4

67

8

Pressure(absolute)

Enthalpy

Schematic mollier chartTwo stage direct compression cycle

5

6

7

43

2

8

1Sea water

cooling

Condenser

1st stagecompressor

2nd stagecompressor

Intercooler

Expansionvalve

Collectingvessel

Liquidreturn

Cargo vapour

Liquid separator

Two stage direct compression cycle

Cargotank


11

Two Stage Direct Compression Cycle

Schematic Mollier Chart:

Two Stage Direct Compression Cycle

Pressure(absolute)

Enthalpy


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8. Refrigerated Systems

In many ships carrying refrigerated cargoes, the cargo cooling is actually performed by an intermediate

refrigerant such as R22. This enables us to reduce the pressure gradient across the cargo vapour compressor.

The refrigerant circulates in a separate, closed cycle in which it is condensed by sea water. This system isknown as the Cascade Cycle.

The Mollier Charts can again be related to the plant, with one chart representing the cycle for the cargo,the other the cycle for the refrigerant.

5

3

21

4

6

9

10

7

8

Pressure(absolute)

Enthalpy

Pressure(absolute)

Enthalpy

Schematic mollier chartCascade cycle

Cargo R22

Sea water

coolingR22 condenser

2

5

4

10

9

8 7

3 6

R22 LiquidseparatorCargo

condenser

Cargo vapourcompressor

Cargo vapour

Cargo liquidseparator

Cargotank R22 Collecting

vessel

R22Compressor

Liquid

1

CargoTank


12

Cascade Cycle

Mollier Chart:

Cascade Cycle

Pressure(absolute)

Pressure(absolute)

Enthalpy Enthalpy


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This system involves more machinery and is more complex but it is needed for cargoes such as ethylene

or ethane being shipped through areas where the temperature of the sea-water is higher than the CriticalTemperature of the cargo. The relationship between pressure and temperature of the cargo is particularly

relevant in this context.

The graph below shows the relationship between absolute pressure and boiling point for various liquefiedgases.

For butane, for example, the boiling point at atmospheric pressure is approximately 0Celsius. When thetemperature rises to about 45 Celsius, the pressure is increased to approximately 4 bar absolute (or 3 bar

gauge). For propane the atmospheric pressure boiling point is approximately -42Celsius and at about0Celsius the pressure rises to about 4.5 bar absolute (or 3.5 bar gauge).

The temperature at which a liquid gas turns into vapour is often called the saturation temperature to

differentiate it from the term boiling point - which is most often used to specify vaporisation at

atmospheric pressure.

For world-wide trade at sea, the maximum temperature for sea is assumed to be 32Celsius, and for theambient air, 45Celsius. So it can be seen from the chart, that to keep polypropylene cargoes liquid at those

temperatures, the pressure in the tank needs to be about 19 bar absolute, (or 18 bar gauge).

The graph also shows that cargoes such as methane, ethylene and ethane will always be above theircritical temperatures in high ambient conditions so they must be cooled below the critical temperature in

order to liquefy them. Pressure alone will not achieve this liquefaction. So these cargoes are alwaystransported in cryogenic conditions at atmospheric pressure, about -104Celsius for ethylene, and

-160 Celsius for methane.

CRITICAL TEMPERATURE/CRITICAL PRESSURELINE

PROPAN

E

BUTA

NE

BUTYLEN

E

A

MM

ONIA

VINYL

CHLO

RIDE

METHA

NE

PROPY

LENE

ETHAN

E

ETHYLE

NE

70

60

50

40

30

20

1098

7

6

5

4

3

2

1

-160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)

SATUR

ATEDVAPOURPRESSURE(Kg/cm2

absolute)-LOGSCALE


13

Pressure vs Temperature - Various Liquefied Gases


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As we mentioned earlier in the context of LPG mixtures, the fact that the cargoes are seldom pure cancause complications due to differential evaporation. Its quite common, for example, to leave a small

quantity of cargo or heel in a tank at the end of a discharge. This may be used to keep the tank cool or, in

the case of LNG ships, used for fuel during the ballast voyage. If this heel is allowed to evaporate usingnatural evaporation, the lighter components will be drawn off first and, as time goes on, the heel will containmore of the heavier components. This process is known as flash vaporisation or weathering.

To prevent this happening it is necessary to pump some of the liquid into a heat exchanger and evaporateit all in one go. This is known as batch vaporisation.

The heat that penetrates the insulation and enters the cargo tank can also produce another physical

phenomenon known as Stratification.

Convection

current

Heat entersthrough insulation

Heat transferBOIL OFF Hot layer

Boil off removed

STRATIFICATION

WEATHERING

OR FLASK VAPOURIZATION

Heat enters

through insulation

Lighter components

Spray liquidvaporizes -gives offlightercomponents

Spray liquidrundown(heaviercomponents)

Liquid Heel boils off(lighter components first)


14

Weathering

or Flask Vaporization

Stratification


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The heat causes the liquid near the tank surface to heat up very slightly - perhaps by no more than 1 or

1.5Celsius. But this warming reduces the density of the liquid so that it rises to the top of the liquid in thetank and forms a warm layer at the liquid surface. This promotes evaporation of liquid from the surface. The

bulk of the liquid in the tank is not affected.

In certain cases, temperature differentials can cause more severe effects although the instance quotedhere happened in a shore tank and is unlikely to occur on board ship. The tank was initially filled with a light

LNG mixture. Subsequently a heavier LNG from a different source was also added. The heavier layer lay atthe bottom and the two fractions remained very separate.

As the boil off was drawn away from the upper layer, this became richer in heavier components and itsdensity increased. At the same time, the lower layer gradually warmed up and its density decreased. A point

was reached when the layer on top was heavier than the layer below and the layers spontaneously rolledover. This caused a massive increase in the rate of evaporation and lifted the tank roof. As the roof had been

designed to relieve under excess pressure, no mechanical damage was caused, but a potentially hazardoussituation had occurred.

On board ship, procedures may be specified for circulation of the cargo using the pumps to ensure that if

cargoes, which are nominally the same but have come from different sources, are loaded in the same tank,then this unstable roll-over situation does not occur.

LIGHTERLIQUIDLAYER

HEAVIERLIQUIDLAYER

WEATHERING:

DENSITY INCREASES

THERMAL EXPANSION

DENSITY DECREASES

BOILING OFF

HEAT

SUDDEN ROLL OVER, CAUSING

MIXING & MASSIVE VAPORIZATION

HEAT

HEAT


15

Roll Over


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9. Summary

The safe carriage of liquefied gas cargoes on your ship requires careful observance of all the appropriate

regulations and safety requirements defined in the IGC Code, Marpol Annex II, SOLAS Chapter II, the ICS

Liquefied Gas Tanker Safety Guide, and elsewhere. The Cargo Data Sheets also carry much vital information.In this booklet and the accompanying programme we have described some of the basic physics that shouldhelp you to understand the physical behaviour of the cargoes and cargo systems and help you to avoid any

mishaps. The key aspects to remember are:

The various Gas Laws and the derivation of the General Gas Equation.

The concept of Sensible and Latent Heat and the transitions between the phases of matter.

The meaning of Enthalpy and the use of the Mollier Chart. This makes it clear that a gas cannot condenseuntil it is below its critical temperature.

The Laws of Thermodynamics and their application in cargo systems.

The various systems used to achieve compression and liquefaction. The possible consequences of carrying gas mixtures with components that evaporate at different rates.

For further information you may also wish to refer to the following training packages available from Videotel:

The Chemistry of Liquefied Gases

An Introduction to Liquefied Gas Carriers


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013 --- the physics of liquefied gases

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