Articles from this category about shipping natural gas 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.
Short description about articles category
The background for and the purpose of the articles 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:
1 Level 1: liquefied gas tanker familiarization – a basic safety-training course for officers and ratings on boards.
2 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 articles 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.
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 pressurized 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.
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 +100 °C, butane at -1/2 °C, ammonia at -33 °C, and propane at -43 °C.
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 10 °C. 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 % 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 100 %.
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:
Charles’s Law: The volume of a given mass of gas varies directly with the absolute temperature provided the pressure remains constant:
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;
- 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 1 °C.
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 %. 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 -273 °C. (or absolute zero). (Physicists have never been able to reach this temperature.) It therefore follows that absolute temperature equals temperature + 273 °C.
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 100 °C, the level of the barometer will register zero. The absolute vapour pressure of water at 100 °C 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 -43 °C.
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.