Preparation and Execution Cargo Operations LNG and LPG

Cargo operations involving LNG and LPG require careful planning and execution to ensure safety and efficiency. These operations typically include loading, transportation, and unloading of liquefied natural gas (LNG) and liquefied petroleum gas (LPG) from tankers or storage facilities.

Specialized equipment and procedures are used to handle the unique properties of these liquid gases, such as cryogenic temperatures and high pressures. Strict safety protocols and regulations are followed to minimize the risk of accidents and protect personnel, the environment, and the cargo.

General Cargo Handling Operations

Tank inspection

Before any cargo operations are carried out it is essential that cargo tanks are thoroughly inspected for cleanliness; that all loose objects are removed; and that all fittings are properly secured. In addition, any free water must be removed. Once this inspection has been completed, the cargo tank should be securely closed and air drying operations may start.

Drying

Drying the cargo handling system in any refrigerated ship is a necessary precursor to loading. This means that water vapour and free water must all be removed from the system. If this is not done, the residual moisture can cause problems with icing and hydrate formation within the cargo system. (The reasons are clear when it is appreciated that the water vapour contained in a cargo tank atmosphere, on an average-size carrier, may be almost half a tonne; of course, this depends on the ambient conditions).

Whatever method is adopted for drying, care must be taken to achieve the correct Dew Point – Definition and Pronunciationdew point temperature. Malfunction of valves and pumps due to ice or hydrate formation can often result from an inadequately dried system. While the addition of antifreeze may be possible to allow freezing point depression at deep-well pump suctions, such a procedure must not substitute for thorough drying. (Antifreeze is only used on cargoes down to -48 °C; propanol is used as a deicer down to -108 °C but below this temperature, for cargoes such as LNG, no deicer is effective). Tank atmosphere drying can be accomplished in several ways. These are described below.

Drying using inert gas from the shore

Drying may be carried out as part of the inerting procedure when taking inert gas from the shore, this is now commonly done. This method has the advantage of providing the dual functions of lowering the moisture content in tank atmospheres to the required dew point and, at the same time, lowering the oxygen content. A disadvantage of this and the following method is that more inert gas is used than if it is simply a question of reducing the oxygen content to a particular value.

Drying using inert gas from ship’s plant

Drying can also be accomplished at the same time as the inerting operation when using the ship’s inert gas generator but satisfactory water vapour removal is de-pendent on the specification of the inert gas system. Here, the generator must be of suitable capacity and the inert gas of suitable quality – but the necessary speci-fications are not always a design feature of this equipment. The ship’s inert gas generator is sometimes provided with both a refrigerated dryer and an adsorption drier which, taken together, can reduce dew points at atmospheric pressure to -45 °C or below.

On board air-drying systems

An alternative to drying with inert gas is by means of an air-drier fitted on board. The principle of operation is shown in. In this method, air is drawn from the cargo tank by a compressor or provided by the on board inert gas blower (without combustion) and passed through a refrigerated drier. The drier is normally cooled by R22 refrigerant. Here the air is cooled and the water vapour is condensed out and drained off. The air leaving the drier is, therefore, saturated at a lower dew point. Further reduction of the dew point can be achieved by a silica gel after-drier fitted downstream. Thereafter, the air may be warmed back to ambient conditions by means of an air heater and returned to the A Study on Support Arrangement of a Cargo Tank for Tank Type A LPG Shipscargo tank. This process is continued for all ship tanks (and pipelines) until the dew point of the in-tank atmosphere is appropriate to carriage conditions.

Inerting – before loading

Inerting cargo tanks, cargo machinery and pipelines is undertaken primarily to ensure a non-flammable condition during subsequent gassing-up with cargo. For this purpose, oxygen concentration must be reduced from 21 per cent to a maximum of five per cent by volume although lower values are often preferred.

However, another reason for inerting is that for some of the more reactive chemical gases, such as vinyl chloride or butadiene, levels as low as 0,1 per cent may be required to avoid a chemical reaction between oxygen and the incoming vapour. Such low oxygen levels can usually only be achieved by nitrogen inerting; provided from the shore.

There are two procedures which can be used for inerting cargo tanks: displacement or dilution. These procedures are discussed below.

Inerting by displacement

Inerting by displacement relies on stratification of the cargo tank atmosphere based on the difference in vapour densities between the gas entering the tank and the gas already in the tank. The heavier gas is introduced beneath the lighter gas at a low velocity to minimize turbulence. If good stratification can be achieved, with little mixing at the interface, then just one tank volume of the incoming inert gas is sufficient to change the atmosphere. In practice mixing occurs and it is necessary to use more than one tank-volume of inert gas. This amount may vary by up to four times the tank volume, depending on the relative densities of the gases together with tank and pipeline configurations. There is little density difference between air and inert gas; inert gas from a combustion generator is slightly heavier than air while nitrogen is slightly lighter. These small density differences make inerting by displacement difficult to achieve and usually the process becomes partly by displacement and partly by dilution (discussed below). Combustion-generated inert gas is usually introduced through the liquid loading line with the effluent being exhausted through the vapour line into the vent header.

Inerting by displacement is an economical procedure as it uses the least amount of inert gas and takes the shortest time. However, it is only practical when mixing with the initial tank vapour can be limited. If the tank shape and the position pipe-entries are suitable for the displacement method, then results will be improved by inerting more than one tank at a time. This should be done with the tanks aligned in parallel. The sharing of the inert gas generator output between tanks reduces gas inlet speeds, so limiting vapour mixing at the interface. At the same time the total inert gas flow increases due to the lower overall flow resistance. Tanks being inerted in this way should be monitored to ensure equal sharing of the inert gas flow.

Inerting by dilution

When inerting a tank by the dilution method, the incoming inert gas mixes, through turbulence, with the gas already in the tank. The dilution method can be carried out in several different ways and these are described below:

Dilution by repeated pressurisation

In the case of Type “C” tanks inerting by dilution can be achieved through a process of repeated pressurisation. In this case inert gas is pressurised into the tank using a cargo compressor. This is followed by release of the compressed gases to atmos-phere. Each repetition brings the tank nearer and nearer to the oxygen concentration of the inert gas. Thus, for example, to bring the tank contents to a level of five per cent oxygen within a reasonable number of repetitions, inert gas quality of better than five per cent oxygen is required.

It has been found that quicker results will be achieved by more numerous repetitions, each at low pressurisation, than by fewer repetitions at higher pressurisation.

Dilution by repeated vacuum

Type “C” tanks are usually capable of operating under considerable vacuum and, depending on tank design, vacuum-breaking valves are set to permit vacuums in the range from 30 per cent up to 70 per cent. Inerting by successive dilutions may be carried out by repeatedly drawing a vacuum on the tank. This is achieved by using the cargo compressor and then breaking the vacuum with inert gas. If, for instance, a 50 per cent vacuum can be drawn, then, on each vacuum cycle, half the oxygen content of the tank is removed. Of course, some of the withdrawn oxygen will be replaced by the oxygen content of the inert gas.

Of all the dilution processes, this method can be the most economical as only the minimum quantity of inert gas is used to achieve the desired inerting level. The overall time taken, however, may be longer than with the pressurisation method because of reduced compressor capacity when working on vacuum and a slow rate of vacuum-breaking due to limited output of the inert gas generator.

Continuous dilution

Inerting by dilution can be carried out as a continuous process. Indeed, this is the only diluting process available for Type “A” tanks which have very small over-pressure or vacuum capabilities. For a true dilution process (as opposed to one aiming at displacement) it is relatively unimportant where the inert gas inlet or the tank efflux are located, provided that good mixing is achieved. Accordingly, it is usually found satisfactory to introduce the inert gas at high speed through the vapour connections and to discharge the gas mixture via the bottom loading lines.

When using the continuous dilution method on ships with Type “C” tanks, increased inert gas flow (and thereby better mixing and reduced overall time) may be achieved by maintaining the tank under vacuum. This is accomplished by drawing the vented gas through the cargo vapour compressor. Under these circumstances care should be taken to ensure good quality inert gas under the increased flow conditions.

Where a number of tanks are to be inerted, it may be possible to achieve a reduction in the total volume of inert gas used, and the overall time taken, by inerting tanks one after the other in series. This procedure also inerts pipelines and equipment at the same time. (On some ships, cargo and vapour pipeline arrangements may prevent more than two tanks being linked in series.) The extra flow resistance of a series arrangement will decrease the inert gas flow rate below that achievable when inerting tanks singly.

As can be seen from the foregoing discussion the optimum arrangement for inerting by dilution will differ from ship to ship and may be a matter of experience.

Inert gas-general considerations

It can be seen from the preceding paragraphs that inert gas can be used in different ways to achieve inerted cargo tanks. No one method can be identified as the best since the choice will vary with ship design and gas density differences. Generally, each individual ship should establish its favoured procedure from experience. As already indicated, the displacement method of inerting is the best but its efficiency depends upon good stratification between the inert gas and the air or vapours to be expelled. Unless the inert gas entry arrangements and the gas density differences are appropriate to stratification, it may be better to opt for a dilution method. This requires fast and turbulent entry of the inert gas upon which the efficiency of dilution depends.

Whichever method is used, it is important to monitor the oxygen concentration in each tank from time to time from suitable locations using the vapour sampling connections provided. In this way the progress of inerting can be assessed and, eventually, assurance can be given that the whole cargo system is adequately inerted.

While the above discussion on inerting has centered on using an inert gas generator, the same principles apply to the use of nitrogen. The use of nitrogen may be required when preparing tanks for the carriage of chemical gases such as vinyl chloride, ethylene or butadiene. Because of the high cost of nitrogen, the chosen inerting method should be consistent with minimum nitrogen consumption.

Inerting prior to loading ammonia

Modern practice demands that ships’ tanks be inerted with nitrogen prior to loading ammonia. This is so, even although ammonia vapour is not readily ignited.

Inert gas from a combustion-type generator must never be used when preparing tanks for ammonia. This is because ammonia reacts with the carbon dioxide in inert gas to produce carbamates. Accordingly, it is necessary for nitrogen to be taken from the shore as shipboard nitrogen generators are of small capacity.

The need for inerting a ship’s tanks prior to loading ammonia is further underscored by a particular hazard associated with spray loading. Liquid ammonia should never be sprayed into a tank containing air as there is a risk of creating a static charge which could cause ignition. (Mixtures of ammonia in air also introduce an additional risk as they can accelerate stress corrosion).

Gassing-up

Neither nitrogen nor carbon dioxide, the main constituents of inert gas, can be condensed by a ship’s reliquefaction plant. This is because, at cargo temperatures, each is above its critical temperature and is, therefore, incondensible. Accordingly, removal of inert gas from the cargo tank is necessary. This is achieved by Gassing Up (Purging) of Cargo System on Liquefied Gas Carriersgassing-up using vapour from the cargo to be loaded and venting the incondensibles to atmosphere so that subsequently the reliquefaction plant can operate efficiently.

Similarly, on changing grade, without any interveninginerting, it may first be necessary to remove the vapour of the previous cargo with vapour of the cargo to be loaded. The basic principles discussed previously in respect of inerting methods apply equally to this type of gassing-up. However, when gassing-up there is usually a greater density difference between cargo vapours than is the case when inerting from air.

Gassing-up at sea using liquid from deck storage tanks

Gassing-up at sea is a procedure normally only available to fully refrigerated, or semi-pressurised ships. Such carriers are often equipped with deck tanks which may have a compatible cargo in storage. In this case, either vapour or liquid can be taken from the deck tanks into the cargo tanks. Liquid can be taken directly from deck storage through the tank sprays (with the exception of ammonia). This is done at a carefully controlled rate to avoid cold liquid striking warm tank surfaces. In this case vapour mixing occurs in the cargo tanks and the mixed vapours can be taken into other tanks (when purging in series) or exhausted to the vent riser.

Alternatively, liquid from the deck storage tanks can be vaporized in the cargo vaporizer and introduced gradually into the top or bottom of the cargo tank, depending on vapour density, to displace the existing inert gas or vapour to other tanks or to the vent riser.

Only when the concentration of cargo vapour in the tanks has reached approximately 90 per cent (or as specified by the compressor manufacturer) should the compressor be started and cool-down of the system begin.

Gassing-up alongside

Gassing-up operations which take place alongside are undertaken using cargo supplied from the shore. At certain terminals, facilities exist to allow the operation to be carried out alongside but these terminals are in a minority. This is because the venting of hydrocarbon vapours alongside a jetty may present a hazard and is, therefore, prohibited by most terminals and port authorities.

Thus, well before a ship arrives in port with tanks inerted, the following points must be considered by the shipmaster:

• Is venting allowed alongside? If so, what is permissible?
• Is a vapour return facility to a flare available?
• Is liquid or is vapour provided from the terminal for gassing-up?
• Will only one tank be gassed-up and cooled down initially from the shore?
• How much liquid must be taken on board to gas-up and cool-down the remaining tanks?
• Where can the full gassing-up operation be carried out?

Before commencing gassing-up operations alongside, the terminal will normally sample tank atmospheres to check that the oxygen is less than five per cent for LPG cargoes (some terminals require as low as 0,5 per cent) or the much lower concentrations required for chemical gases such as vinyl chloride.

Where no venting to atmosphere is permitted, a vapour return facility must be provided and used throughout the gassing-up operation. In this case, either the ship’s cargo compressors or a jetty vapour blower can be used to handle the efflux. Some terminals, while prohibiting the venting of cargo vapours, permit the efflux to atmosphere of inert gas. Thus, if a displacement method of gassing-up is used the need for vapour return to shore may be postponed until cargo vapours are detected at the vent riser. This point may be considerably postponed if tanks are gassed-up one after the other in series.

Where a terminal supplies a liquid for gassing-up, it should be loaded at a carefully controlled rate. It is then passed through the ship’svaporiser. Alternatively, the liquid may be allowed to vaporise in the ship’s tanks. If vapour is supplied, this can be introduced into the tank at the top or bottom depending on the vapour density.

When a ship arrives alongside with tanks containing a cargo vapour which requires to be replaced with the vapour of a different grade, then the terminal will normally provide a vapour return line. The vapours taken to the shore will be flared until the desired vapour quality is achieved in the tanks. At this point cool-down can begin.

If facilities, such as a vapour return line, are not available for the ship to gas-up alongside, it is common practice for the ship to prepare one cargo tank and to take sufficient liquid to complete the operation elsewhere. The ship then leaves the berth for a designated anchorage or proceeds to sea. The ship returns to the berth after having gassed-up and cooled-down all cargo tanks.

Cool-down

Cool-down-refrigerated ship

Cooling down is necessary to avoid excessive tank pressures (due to flash evaporation) during bulk loading. Cool-down consists of spraying cargo liquid into a tank at a slow rate. The lower the cargo carriage temperature, the more important the Cooldown of Cargo System on the Liquefied Gas Carrierscool-down procedure becomes.

Before loading a refrigerated cargo, ship’s tanks must be cooled down slowly in order to minimise thermal stresses. The rate at which a cargo tank can be cooled, without creating high thermal stress, depends on the design of the containment system and is typically 1 aoc per hour. Reference should always be made to the ship’s operating manual to determine the allowable cool-down rate. Hle normal cool-down procedure takes the following form. Cargo liquid from shore (or from deck storage) is gradually introduced into the tanks either through spray lines, if fitted for this purpose, or via the cargo loading lines. The vapours produced by rapid evaporation may be taken ashore or handled in the ship’s reliquefaction plant. Additional liquid is then introduced at a rate depending upon tank pressures and temperatures. If the vapour boil-off is being handled in the ship’s reliquefaction plant, difficulties may be experienced with incondensibles, such as nitrogen, remaining from the inert gas. A close watch should be kept on compressor discharge tempera-tures and the incondensible gases should be vented from the top of the condenser as required.

As the cargo containment system cools down, the thermal contraction of the tank and, the drop in temperature around it, together tend to cause a pressure drop in the hold and interbarrier spaces. Normally, pressure control systems supplying air or inert gas will maintain these spaces at suitable pressures but a watch should be kept on appropriate instruments as the cool-down proceeds.

Cool-down should continue until boil-off eases and liquid begins to form in the bottom of the cargo tanks. This can be seen from temperature sensors. At this stage, for fully refrigerated ammonia for example, the pool of liquid formed will be at approximately -34 °C while the top of the tank may still be at -14 °C. This gives a temperature difference of 20 °C. The actual temperature difference depends on the size of the cargo tank and the spray positions.

Difficulties that may occur during cool-down can result from inadequate gassing-up (too much inert gas remaining) or from inadequate drying. In this latter case, ice or hydrates may form and ice-up valves and pump shafts. In such cases antifreeze can be added, provided the cargo is not put off specification, or the addition will not damage the electrical insulation of a submerged cargo pump.

Throughout the cool-down deepwell pump shafts should be turned frequently by hand to prevent the pumps from freezing up.

Once the cargo tanks have been cooled down, cargo pipelines and equipment should be cooled down.

Cool-down – semi-pressurized ships

Most semi-pressurized ships have cargo tanks constructed of steels suitable for the minimum temperature of fully refrigerated cargoes. However, care must be taken to avoid subjecting the steel to lower temperatures. It is necessary to maintain a pressure within the cargo tank at least equal to the saturated vapour pressure corresponding to the minimum allowable steel temperature. This can be done by passing the liquid through the cargo vaporizer and introducing vapour into the tank with the cargo compressor. Alternatively, vapour can be provided from the shore.

Loading

As with procedures for loading preparation and loading, procedures prior to unloading should be explained in detail to the trainees. Unloading can be done in different ways, the ship type, cargoes and terminal conditions being decisive in choosing which method is used. The learning objectives indicate the level of knowledge required.

Before starting, the pumps must be moved manually to ascertain that they are not frozen. Pumps have to be started with the valve on the pressure side almost closed, to prevent overload and pressure surge and to minimize accidents in the event of faulty lining up. When the pump is running normally, the pressure valve is opened and finally the discharge valve at the manifold is opened. This has to be done with care and caution.

In some ships, when starting to discharge, the piping is arranged for recirculating, i. e. pumping back to the ship’s tank, before opening up the shore connection. This might prevent major pockets of vapour being forced through piping to shore tanks. It is advisable at first only to start one pump and to let it run smoothly before starting other pumps that will discharge to the same shore line.

When everything is functioning normally, the ampere reading is noted and the switch ammeter is set for 80 % of the ampere reading, thus securing the correct function of the automatic stop when the pump is empty or is cavitating.

When the tank is empty the pump will stop and the valves are closed. If a remainder is left in the pump-well, the pump can be forced to run for about 10 seconds by pressing the starter; this may be done several times at short intervals. The discharge valve has to be closed immediately afterwards, to prevent liquid running back to the tank and forcing the pump to run backwards and cause damage to the motor.

Remember that the bearings in the discharge pipe in the tank are lubricated by the product. Bearings therefore must never get dry when the pump is running.

Loading refrigerated ships

When liquefied gas is being loaded, it is necessary to consider the location, pressure, temperature and volume of the shore tanks as well as the terminal’s pumping procedures. Fully refrigerated ships usually load from fully refrigerated storage where tanks typically operate at a pressure of approximately 60 millibars.

This pressure will allow the cargo at the bottom of a full shore tank to sustain a temperature perhaps one degree Centrigrade warmer than its atmospheric boiling point.

When this cargo is pumped to the jetty, the pumping energy required for transfer is dissipated in the liquid as heat, to which must be added the heat flow into the liquid through the pipelines. The cargo may, therefore, arrive on the ship at an even warmer temperature. When loading without a vapour return line being used, the vapour which is displaced by the incoming liquid must be reliquefied on board. The power required for this, plus the heat flux through the insulation, may leave little capacity for cooling the cargo during loading.

Therefore, as can be seen from the foregoing paragraphs, the early stages of loading can be critical, particularly where significant distances exist between the storage tank and jetty. The ship’s tank pressures must be regularly checked and on no account should relief valves be allowed to lift.

Loading rates should be reduced, and if necessary stopped, when difficulties are experienced in maintaining acceptable tank pressures. In some ports in hot countries, where the terminal has long pipelines, this feature can be difficult to overcome. Under these circumstances cargo stoppage would allow the pipeline contents once again to rise in temperature. Accordingly, in such ports cargo flow should be maintained as long as it is safe to do so until cold product can be received on board at which time tank pressures will fall.

A rise in ship’s tank pressure in the early stages of loading can also be controlled to some extent by loading limited quantities of liquid into the cargo tank via the top sprays, if fitted. This will help to condense some of the cargo vapours.

Loading pressurized ships

Pressurized ships normally arrive at a loading terminal having cargo tanks at atmospheric pressure. Firstly, the ship requests vapours from the shore to purge any remaining nitrogen or contaminants from the tanks. This also allows the equalization of ship and shore pressures. Thereafter, the method used at the beginning of loading is to allow only very slow flow so giving time for the incoming liquid to expand safely at the first valves in the ship’s system.

In this case, as the liquid is allowed through, local flash-cooling can occur and it is important to ensure that at no time are tank or pipeline temperatures allowed to fall below design limitations.

Loading pressurized ships from refrigerated storage

The cargo tanks on fully pressurized ships are made from carbon steel which is only suitable for a minimum temperature of between 0 °C and -5 °C. Consequently, some refrigerated cargoes require considerable heating prior to loading on such ships. Given that fully pressurized ships may not have cargo heaters fitted on board, all heat input must be provided by pumping through heaters fitted on shore.

Of course, on a pressurised ship, having loaded a cargo at close to 0 °C, the cargo may warm up further during the voyage in accordance with ambient conditions. The Gas Codes only allow cargo to be loaded to such a level that the tank filling limit will never be more that 98 per cent at the highest temperature reached during the voyage. This means that, during pre-loading discussions, tank topping-off levels must be established to allow sufficient room for liquid expansion into the vapour space while on voyage.

Loading semi-pressurized ships from refrigerated storage

The cargo tanks on semi-pressurised ships are usually constructed of low temperature steels able to accommodate fully refrigerated propane at temperatures of between -40 °C and -50 °C – or even for ethylene carriers at -104 °C. Refrigerated cargoes can therefore be loaded directly to such ships without heating. In addition, these ships can usually maintain fully refrigerated temperatures on voyage and this is often done to gain more space so that a greater weight of cargo can be carried. However, when discharge to pressurised storage is planned, this is conditional on the ship having suitable equipment to warm the cargo. On semi-pressurized ships the cargo is occasionally allowed to warm up during the loaded voyage and in this case a similar procedure to that described for fully pressurized ships applies.

The loaded voyage

Cargo temperature control

For all refrigerated and semi-pressurised gas carriers it is necessary to maintain strict control of cargo temperature and pressure throughout the loaded voyage. This is achieved by reliquefying cargo boil-off and returning it to the tanks.

During these operations incondensibles must be vented as necessary to minimise compressor discharge pressures and temperatures. In LNG ships the boil-off is burned as fuel in the ship’s main boilers.

Frequently, there are occasions when it is required to reduce the temperature of an LPG cargo on voyage. This is necessary so that the ship can arrive at the discharge port with cargo temperatures below that of the shore tanks, thus minimising the amount of flash gas. Depending on the cargo and reliquefaction plant capacity it can often take several days to cool the cargo by one or two degrees centigrade, but this may be sufficient. The need for this will often depend on the contractual terms in the charter party.

In this respect poor weather conditions can sometimes present problems. Although most reliquefaction plants have a suction knock-out drum to remove liquid, there is a risk in gale conditions that entrained liquid can be carried over into the compressor.

For this reason it is preferable not to run compressors when the ship is rolling heavily, if there is risk of damage. In calm weather conditions if the condensate returns are passed through the top sprays, because of the small vapour space in the tank and poor circulation in the tank, it is possible that a cold layer can form on the liquid surface.

This enables the compressors to reduce the vapour pressure after only a few hours running, when in fact the bulk of the liquid has not been cooled at all. To achieve proper cooling of the bulk liquid the reliquefaction plant should be run on each tank separately and the condensate should be returned through a bottom connection to ensure proper circulation of the tank contents. After the cargo has been cooled, reliquefaction capacity can be reduced to a level sufficient to balance the heat flow through the tank insulation.

If the reliquefaction plant is being run on more than one tank simultaneously, it is important to ensure that the condensate returns are carefully controlled in order to avoid the overfilling of anyone tank.

Discharging

When a ship arrives at the discharge terminal, cargo tank pressures and temperatures should be in accordance with terminal requirements. This will help maximum discharge rates to be achieved. Before the discharge operation begins, the pre-operational ship/shore procedures should be carried out along similar lines to the loading operation previously outlined.

The method of discharging the ship will depend on the type of ship, cargo specification and terminal storage. Three basic methods may be used:

• Discharge by pressurising the vapour space;
• Discharge with or without booster pumps;
• Discharge via booster pump and cargo heater.

Stopping the cargo compressor should always be carried out in accordance with the manufacturer’s instructions. Generally, the first action is to stop the compressor. This is followed by closure of the suction and discharge valves. The glycol/water system is left running to provide crankcase heating or, alternatively, the lubricating oil heater should be left switched on.

Discharge by pressurising the vapour space

Discharge by pressure using either a shore vapour supply or a vaporizer and compressor on board is only possible where Type “C” tanks are fitted. It is an inefficient and slow method of discharge and is restricted to small ships of this type. Using this system, the pressure above the liquid is increased and the liquid is transferred to the terminal. An alternative method is to pressurise the cargo into a small deck tank from which it is pumped to the shore.

Discharge by pumps

Starting cargo pumps

A centrifugal pump should always be started against a closed, or partially open, valve in order to minimise the starting load. Thereafter, the discharge valve should be gradually opened until the pump load is within safe design parameters and liquid is being transferred ashore.

As the discharge proceeds, the liquid level in the cargo tanks should be monitored. Discharge and ballasting operations should be carefully controlled, bearing in mind ship stability and hull stress. Removal of liquid from the cargo tank may cause changes in interbarrier space pressures and these should be monitored throughout the discharge.

Discharging by centrifugal cargo pumps, either alone or in series with booster pumps, is the method adopted by most ships and an understanding of the centrifugal pump is essential for efficient cargo discharge.

Increasing the flow rate increases the back pressure. This varies approximately as the square of the flow rate, giving the shape of system characteristic curve as shown. The point where the two curves intersect is the flow rate and head at which the pump will operate.

The actual system characteristic applicable at any terminal should be known to shore personnel and they should have such curves available. In preparing such graphs, personnel should note, as mentioned above, that the system characteristic can vary with the size of the chosen pipeline and with variation in the pipe-lengths from the jetty when alternative shore tanks are used. If a range of pipelines and tanks are available at anyone terminal, then it may be appropriate for terminal personnel to have a number of system characteristics, already pre-calculated and available, for use during pre-transfer discussions.

In any case during the pre-transfer discussions such matters should be covered and the optimum transfer rate should be agreed.

It also may be desirable to throttle a cargo pump discharge when it is used in conjunction with a booster pump. This may be done in order to reduce the pressure in the booster module. Any additional control of flow, however, should be carried out by throttling the booster pump discharge, by opening the main pump recirculation or by a combination of the two. It should be noted that control of flow solely by throttling the main pump discharge may cause loss of booster pump suction.

As liquid is being pumped from the ship, tank pressures tend to fall. Boil-off due to heat flow through the tank insulation takes place continuously and this generates vapour within the tank. The boil-off is usually insufficient to maintain cargo tank pressures at acceptable levels but this ultimately depends upon discharge rate, cargo temperature and ambient temperature. Where vapours produced internally are insufficient to balance the liquid removal rate, it is necessary to add vapour to the tank if discharge is to continue at a constant rate. This vapour may be provided either by using the ship’s cargo vaporizer or from the terminal (via a vapour return line). When using the cargo vaporizer the liquid is normally taken from the discharge line and diverted through the vaporizer.

Discharge via booster pump and cargo heater

Where cargo is being discharged from a refrigerated ship into pressurized storage, it is necessary to warm the cargo (usually to at least a °C). This means running the cargo booster pump and cargo heater in series with the cargo pump. To operate the booster pump and heater, it is necessary to first establish sea water flow through the heater. Thereafter, the booster pump and heater may be slowly cooled down (prior to full operation) by very slow throughput of liquid from the cargo pump dis-charge. Once cooled down, the discharge valve can be opened until the desired outlet temperature is reached. It is important to ensure that the cargo pumps maintain adequate flow to the booster pump at all times.

Heating cargo during discharge always entails a risk of freezing the circulating water in the heater. In addition to checking the cargo outlet temperature and the booster pump suction during operation, attention should also be paid to the sea water inlet and outlet temperatures and pressures. The sea water outlet temperature must not be allowed to fall below the manufacturer’s recommended limit. A low temperature switch should stop cargo flow through the heater in case of low sea water discharge temperature.

As will be noted, this method of cargo heating depends on a suitable sea water temperature. In cold sea water areas the efficiency of the system can be seriously affected and slow discharge rates can result and if sea water temperatures are below SOC the risk of freezing becomes much greater. To cover such possibilities sometimes thermal oil heaters are fitted to ships.

Changing cargo (and preparation for drydock)

Of all the operations undertaken by a gas carrier, the preparation for a change of cargo is the most time consuming. If the next cargo is not compatible with the previous cargo, it is often necessary for the tanks to be gas-freed to allow a visual inspection. This is commonly the case when loading chemical gases such as vinyl chloride, ethylene or butadiene.

When a ship receives voyage orders, a careful check must be made on the compatibility of the next cargo. (It is also necessary to check compatibilities and the ship’s natural ability to segregate, if more than one cargo grade is to be carried. On such occassions special attention must be given to the ship’s reliquefaction system).

There may also be a need, when changing cargoes, to replace the lubricating oil in compressors for certain cargoes.

In order to obtain a gas-free condition, the full process is as shown below, however, depending on the grade switch, it may not be necessary to include all these steps:

1. First, make the tank liquid free.
2. Then, warm the tank with hot cargo vapours (if necessary).
3. Next, inert the tank;
4. and finally, ventilate with air.

These procedures are preliminary to tank entry for inspection or when gas freeing the ship for drydock.

Removal of remaining liquid

Depending upon cargo tank design, residual liquid can be removed by pressurisation, normal stripping or, in the case of fully refrigerated ships with Type “A” tanks, by using the puddle heating coils fitted for this purpose. (An older method of warming Type “A” tanks with hot vapours from the compressor – but without puddle heating – is now generally out of favour due to the extended time taken, although on some ships, and particularly those in LNG trades, there may be no other choice).

The first operation to be carried out is the removal of all cargo liquid remaining in the tanks or in any other part of the cargo system. Due to enhanced evaporation in a non-saturated atmosphere, residual liquid can become super-cooled to a temperature which could result in brittle fracture of the tank. Furthermore, any liquid retention will frustrate the future inerting operation.

As an aid to liquid removal many general purpose LPG ships are provided with special pressure vessels mounted on deck. These tanks can be used for the recovery of liquid and vapour from the cargo tanks. The contents of the deck tanks may also be used, at some future time, to provide vapour for gassing-up purposes when changing grades.

When all cargo tank liquid has been removed, the tanks can be inerted either with inert gas from the ship’s supply or from the shore, as required by the next cargo. Alternatively, gassing-up using vapour from the next cargo may be carried out – but this is increasingly unusual.

Liquid stripping for Type “C” tanks

For ships having Type “C” cargo tanks a cargo stripping line is often provided. By pressurising the cargo tanks on these ships, (using the cargo compressor) residual liquid can be lifted from the tank sump into the stripping line and thence to deck level. It may then be stored temporarily in a chosen cargo tank for returning to the shore. Alternatively, it may be stored in a deck-mounted pressure vessel provided for the purpose. This draining should continue until all liquid cargo is removed from the cargo tanks, as checked through the bottom sampling line. The compressor pressure necessary to remove residual liquid will depend on the specific gravity of the cargo and the depth of the tank.

Liquid freeing for other tank types

For ships with Type “A” or “B” tanks the removal of all cargo liquid residues is not possible by pressurisation. Instead, cargo liquid residues must be vaporised. This is normally achieved using puddle heating coils.

When puddle heating coils are used, the heat source in the coils is hot gas discharged from the cargo compressor. Vapour is drawn from the cargo tank atmosphere and passed through the compressor where the heat of compression causes increased vapour temperatures. By by-passing the condenser, hot vapour can be led directly to the heating coil system and heat is transferred to the liquid cargo residue. In this way remaining liquid is evaporated and an effect of the heat transfer is to turn the hot vapour in the coils into liquid which is then normally piped to a deck-mounted tank. An alternative to the use of puddle heating coils is to supply hot cargo vapours (from the compressor) directly to tank bottoms. However, as already covered earlier in this section, this results in much slower evaporation of remaining liquids than the method described above as the hot gas only flows over the surface of the liquid pool rather than causing boiling within it.

This method is used, however, on LNG carriers not fitted with puddle heating and on some smaller ships where increasing temperature on special cargo grades could be problematic.

When a ship is at sea, in order to finalise either type of operation, cargo tank vapour is normally sent to the vent riser. Alternatively it may be condensed and pumped into deck storage or overboard. If the ship is in port, as venting to atmosphere is seldom allowed, the condensate is usually pumped to the shore or put into deck storage.

When all tanks have been satisfactorily liquid-freed, pipework and other in-line equipment must be blown free from liquid and drained through the appropriate drain valves.

Warming-up

When cargo tanks have to be fully ventilated with fresh air it is often necessary, depending on tank temperatures and design considerations, to warm-up the tanks prior to inerting. This is achieved by controlled circulation of warm cargo vapours through the tanks and is done before inerting takes place.

As for the cool-down, the rate of warm-up should be carefully controlled in accordance with the shipbuilder’s guidance. Warming up is vital where cargo tanks are at very low temperatures, for example on board LNG ships.

On such ships, compressors and heaters are operated to circulate warm gas. First, this evaporates any residual liquid and, thereafter, the whole tank structure is warmed to ambient conditions.

If warming up to ambient temperature is not carried out, freezing of carbon dioxide from within the inert gas can result. (Moreover, greater volumes of inert gas will be required at low temperatures).

Inerting – after discharge

Removal of cargo vapours with inert gas is carried out to reduce gas concentrations to a level where aeration can take place without the tank atmosphere passing through the flammable envelope. The level to which the hydrocarbon vapour must be reduced varies according to the product and details of the varying flammable envelopes for each product are given.

Read also: Preparation of loading and unloading operations for LNG/LPG carriers

In general, when inerting in this way it is necessary to reduce the hydrocarbon content in the inert atmosphere to about 2 per cent before air blowing can begin. (Although this is conservative for methane, it is in accord with common practice).

Ammonia-special procedures

Certain cargoes present particular difficulties when trying to remove all traces of the product. Ammonia is one such case. When a ship is switching from ammonia to LPG, most traces of vapours must be removed from the system. Prior to loading the next cargo an allowable concentration of ammonia vapour in a tank atmosphere is usually quoted at 20 parts per million by volume. This results in a time consuming operation which is covered in more detail below.

The first operation when switching from ammonia is to remove all liquid ammonia from the system. This is important as ammonia, when evaporating to air, is particularly likely to reach super-cooled conditions. Therefore, unless all liquid is removed, dangerously low liquid temperatures can result and tank fractures could ensue. Confirmation that all liquid has been removed can be established, during warming-up, by carefully observing tank temperature read-outs.

Once cargo tank temperatures have been warmed to above the dew point of the air, the ammonia vapours are usually dispersed by blowing warm fresh air through the system. (For ammonia the inert gas plant must not be used as the ship’s inert gas plant is not suitable for this purpose due to carbamate formation). The continued use of warm dry air should avoid water vapour condensation, thus limiting the seepage of ammonia into porous tank surfaces. The ventilation of tanks and the cargo system at the highest practical temperature is advantageous as this encourages release of ammonia from rusty surfaces. (Ammonia is released ten times faster at 45 °C than at 0 °C).

Washing with fresh water to remove ammonia is sometimes carried out. This can be most effective as ammonia is highly water soluble. However, the following points should be noted:

• The benefit of water washing is limited to certain types of tank. (This technique is not always practical for large fully refrigerated ships with prismatic tanks).
• When switching from ammonia to LPG, water can hold ammonia in solution and this can be a contaminant for future cargoes. Accordingly, water washing is only recommended for cargo tanks which are completely clean, rust-free and have minimum internal structure, so allowing full and effective drainage.
• All traces of water must be removed at the end of washing to stop the formation of ice or hydrates. The high solubility of ammonia in water (300:1) can lead to dangerous vacuum conditions being created within a tank. It is, therefore, essential to ensure adequate air entry into the cargo tank during the water washing process.
• After water washing it is essential that all water residues are removed using either fixed or portable pumps. Subsequently, tanks and pipelines must be thoroughly dried before further preparations for cargo loading are made. In order to maintain maximum dryness it is important to continue ventilation of the tanks using air with a dew point lower than the tank atmosphere for the reasons discussed above.

Overfilling of Cargo Tanks

During the loading of a semi—pressurized LPG vessel of approximately 5 000 cbm, a spill occurred due to a cargo tank being over-filled under the following circumstances:

• The vessel was lying head up at a river jetty with a 3 knot ebb current, wind blowing from bow to stern.
• The loading at the time of the incident was in the charge of the Chief Officer and the rate during the previous hour had indicated that the tank would be in the fully loaded condition in some 2,5 hours time.
• This particular vessel is fitted with a gas blower which produces a flash cooling effect in the cargo tank by transferring large volumes of gas to the shore.
• It was stated by the Chief Officer that he heard a sound from the blower, and while going to investigate it, there was an eruption of liquid from the vent stack.
• The action by the Officer was to stop the blower, close the filling valve to the tank in question and advise verbally the jetty operator.
• The emergency shutdown button was not pushed because of the automatic closing of the vapour line with a corresponding surge in tank pressure.
• During this period, liquid gas erupted from the vent stack with large drops and collected on the vessel’s main deck. The fire fighting hoses, at the ready during loading or unloading operations, was used to flush the liquid gas overboard and cause more rapid evaporation.
• It was noted that although the liquid had ceased to come out of the vent stack the tank safety valve did not re-seat and a dense cloud of vapour was being exhausted.
• A full-scale emergency procedure was initiated and a seawater hose was directed on the safety valve in order to impart heat and hopefully free the seized spindle.
• In view of the density of the gas around the vessel, a decision was made by the Master to close down the auxiliary generating plant, i. e. shut down the entire ships machinery.
• he crew with the exception of senior officers, were sent ashore. After a period of between one to two hours the valves re-seated.
• The jetty supervisor, after consultation with the Harbour Authority, had closed the river to all traffic, and the adjacent traffic lanes were also closed.
• Following exhaustive checks using portable explosion meters the Auxiliary plant was restarted and the vessel completed the loading operation by using the gas blower to effect rapid reduction in tank pressures by transferring vapours to shore.

Tanker Terminology

Absolute temperature

The fundamental temperature scale with its zero at absolute zero and expressed either in kelvin or degrees Rankine. One kelvin is equal to one Celsius degree or one centigrade degree; one Rankine degree is equal to one Fahrenheit degree. To convert Celsius to Kelvin, add 273. To convert Fahrenheit to Rankine, add 460.

Absolute zero

The temperature at which the volume of a gas theoretically becomes zero and all thermal motion ceases. Generally accepted as being – 273,16 °C or – 459,69 °F.

Acute toxic effect

The effect on man of a single exposure of short duration to high concentrations of toxic compound or toxic vapour.

Administration

The government of the country in which the ship is registered.

Adiabatic

Without transfer of heat. Adiabatic expansion is volume change in a liquid or gas with no heat loss or gain involved.

Airlock

A separation area used to maintain adjacent areas at pressure differential; e. g. an electric motor room airlock on a gas carrier is used to maintain pressure segregation between a gas-dangerous zone on the open weather deck and the pressurized gas-safe motor room.

«Alcohol-type» foam

A fire-fighting foam effective against many water-soluble cargoes. It is also effective against many non-water-soluble cargoes.

Anaesthesia

A total loss of feeling and consciousness or the loss of power or feeling over a limited area of skin.

Anaesthetics

Chemicals which produce anaesthesia.

Antistatic additive

A substance added to a petroleum product to raise its electrical conductivity above 100 picosiemens/metre (pS/m) to prevent accumulation of static electricity.

Approved equipment

Equipment of a design that has been tested and approved by an appropriate authority such as a Government or classification society. The authority should have certified the equipment as safe for use in a specified hazardous atmosphere.

Aqueous

Indicating that the compound is in solution in water.

Asphyxia

The condition arising when the blood is deprived of an adequate supply of oxygen, so that loss of consciousness may follow.

Asphyxiant

A gas or vapour which, when inhaled, leads to asphyxia.

Auto ignition

The ignition of a combustible material without initiation by spark or flame, when the material has been raised to a temperature at which self¬-sustaining combustion occurs.

Auto-ignition temperature (Autogenous ignition temperature)

The lowest temperature to which a solid, liquid, or gas requires to be raised to cause self- sustained combustion without initiation by a spark, flame or other source of ignition.

Avogadro’s Law

Avogadro’s Hypothesis. Equal volumes of all gases contain equal numbers of molecules under the same conditions of temperature and pressure.

BLEVE

Boiling Liquid Expanding Vapour Explosion. Associated with the rupture under fire conditions of a pressure vessel containing liquefied gas.

Boil-off

Vapour produced above the surface of a boiling liquid.

Boiling point

The temperature at which the vapour pressure of a liquid is equal to atmospheric pressure. Boiling points, as quoted on the data sheets, are correct at a pressure of 760 mmHg, unless indicated to the contrary.

Boiling range

Some liquids which are mixtures, or which contain impurities, boil over a range of temperatures known as the boiling range. When this occurs, the range will be stated on the relevant data sheet. The lower temperature is that at which the liquid starts to boil.

Bonding

The connecting together of metal parts to ensure electrical continuity.

Booster pump

A pump used to increase the discharge pressure from another pump (e. g. a main cargo pump).

Brittle fracture

Fracture of a material caused by lack of ductility in the crystal structure resulting from low temperature.

Bulk cargo

Cargo carried in cargo tanks and not shipped in drums, containers or packages.

Canister-type breathing apparatus

A respirator consisting of mask and replaceable canister filter through which toxic air is drawn by the breathing effort of the wearer and the toxic elements are absorbed. A filter dedicated to the specific toxic contaminant gas must be used. May be referred to as «cartridge» or «filter» respirator.

Cargo area

That part of the ship which contains the cargo-containment system, cargo pump and compressor rooms, and includes the full beam deck area over the length of the ship above the cargo containment. Where fitted, cofferdams, ballast or void spaces at the after end of the aftermost hold space or the forward end of the forwardmost hold space are excluded from the cargo area.

Cargo conditioning

The maintaining of the cargo quantity without undue loss, of the cargo tank pressure within its design limits, and of the desired cargo temperature.

Cargo containment system

The arrangement for containment of cargo, including, where fitted, a primary and secondary barrier, associated insulation, interbarrier spaces and structure required for the support pf these elements.

Cargo handling

The loading, discharging and transferring of bulk liquid cargo.

Cascade reliquefaction cycle

A process whereby vapor boil-off from cargo tanks is condensed in a cargo condenser in which the coolant is an evaporating refrigerant such as Freon 22. The evaporating refrigerant is then passed through a conventional seawater-cooled condenser.

Catalyst

A substance that starts a reaction or changes its speed without being itself chemically changed. A catalyst which reduces the speed of a reaction is known as a negative catalyst.

Cathodic protection

The prevention or corrosion by eletromechanical techniques. On tankers it may be applied either externally to the hull or internally to the surfaces of tanks. At terminals, it is frequently applied to steel piles and fender panels.

Cavitation

A process occurring within the impeller of a centrifugal pump when pressure at the inlet to the impeller falls below that of the vapour pressure of the liquid being pumped. Bubbles of vapour which are formed collapse with considerable impulse force in the higher-pressure regions at the impeller. Significant damage can occur to the impeller surfaces.

CEFIC

The European Council of Chemical Industries.

Certified gas-free

Certified gas-free means that a tank, compartment or contained has been tested using an approved testing instrument and proved to be sufficiently free, at the time of the test, of toxic or explosive gases for a specified purpose, such as hot work, by an authorized person (usually a chemist from shore) and that a certificate to this effect has been issued. If an authorized person is not available, the test should be carried out by the Master or his appointed deputy and the certificate will take the form of an entry in the tanker’s logbook.

Certificate of Fitness

A certificate issued by the Administration of a country confirming that the structure, equipment, fittings, arrangements and materials used in the construction of a gas carrier are in compliance with the relevant IMO Gas Code. Such certification may be issued on behalf of the Administration by approved Classification Societies.

Chemical absorption detector

An instrument for the detection of gases or vapors working on the principle of reaction occurring between the gas being sampled and a chemical agent in the apparatus.

Chronic toxic effect

The cumulative effect on man of prolonged exposures to low concentrations or of intermittent exposures to higher concentrations of a toxic compound or toxic vapour.

Clingage

Oil remaining on the walls of a pipe or on the surfaces of tank interiors after the bulk of the oil has been removed.

Closed gauging system (closed ullaging)

A system whereby the content of a tank can be measured by means of a device which penetrates the tank, but which is part of a closed system and keeps tank contents from being released. Example are the float-type systems, electric probe, magnetic probe and protected sight glass.

Coefficient of cubical expansion

The fractional increase in volume for a 1 °C rise in temperature. The increase is 5/9 of this for a 1 °F rise.

Cofferdam

The isolating space between two adjacent steel bulkheads or decks. This space may be a voided spaced or ballast space.

Combustible-gas detector (explosive meter)

An instrument used to detect combustible hydrocarbon gases, generally using a heated filament of a special metal to oxidize the gas catalytically and measure the gas concentration as a percentage of its Lower Flammable Limit. No single instrument is suitable for all combustible vapours.

Compatibility

The ability of two or more compounds to exist in close and permanent association.

Combination carrier

A ship which is designed to carry either petroleum cargoes or dry bulk cargoes.

Combustible (also referred to as «flammable»)

Capable of being ignited of burning. For the purpose of these guidance notes, the terns «combustible» and «flammable» are synonymous.

Corrosive liquids

Liquids which corrode normal constructional materials at an excessive rate. Usually they also cause serious damage to human tissue and to the eyes.

Critical temperature

The temperature above which gas cannot be liquefied by pressure alone.

Critical pressure

The pressure of saturated vapour at the critical temperature, i. e. the pressure required to cause liquefaction at that temperature.

Cryogenics

The study of the behavior of matter at very low temperatures.

Cyanosis

A bluish discoloration of the skin, particularly about the face and extremities, which usually occurs when the blood is not properly oxygenated by the lungs, and materials itself particularly in the area of the mouth and ears.

Dalton’s Law and Partial Pressures

The pressure exerted by a mixture of gases is equal to the sum of the separate pressure which each gas would exert if it alone occupied the whole volume.

Dangerous area

An area on the tanker which, for the purpose of the installation and use of electrical equipment, is regarded as dangerous.

Dangerous cargo endorsement

Endorsement to a certificate of competency of a responsible officer for him to serve as such on a dangerous cargo carrier (i. e. oil or chemical or gas carrier).

Deepwell pump

A type of centrifugal cargo pump commonly found on gas carriers. The prime mover, usually but not always an electric motor, is flange-mounted on top of the cargo tank drives, through a long transmission shaft, the pump assembly located in the bottom of the tank. The discharge pipe surrounds the drive shaft and the bearings of the shaft are cooled and lubricated by the liquid by the liquid pumped.

Density

The mass per unit volume of a substance at specified conditions of temperature and pressure.

Dewpoint

The temperature at which the water vapour present in a gas saturates the gas and begins to condense.

Dry chemical powder

A flame-inhibiting powder used in fire-fighting.

Earthing (also referred to as «grounding»)

The electrical connection of equipment to the main body of the earth to ensure that it is at earth potential. On board ship the connection is made to the main metallic structure of the ship, which is at earth potential because of the conductivity of the sea.

Endothermic

Referring to a process which is accompanied by absorption of heat.

Entry permit

A document issued by a responsible person permitting entry to a space or compartment during a specific time interval.

Explosimeter

See «Combustible-gas indicator».

Explosion-proof (flame-proof)

Electrical equipment is defined and certified as explosion-proof (flame-proof) when it is enclosed in a case which is capable of withstanding the explosion within it of a hydrocarbon gas/air mixture or other specified flammable gas mixture. It must also prevent the ignition of such a mixture outside the case either by spark or flame from the internal explosion or as a result of the temperature rise of the case following the internal explosion. The equipment must operate at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.

Exothermic

Referring to a process which is accompanied by evolution of heat.

Explosive limit/range

See «Flammable range».

Filling density (for liquefied gases)

The «filling density» is defined as the percent ratio of the weight of the liquid gas in a tank to the weight of water the tank will hold at 15,56 °C (60 °F).

Filling ratio (for liquids)

That volume of a tank, expressed as a percentage of the total volume, which can be safely filled, having regard to the possible expansion of liquid.

Flame arrester

A permeable matrix of metal, ceramic or other heat-resisting materials which can be cool a deflagration flame and any following combustion products below the temperature required for the ignition of the unreacted flammable gas on the other side of the arrester.

Flame-proof

See «explosion-proof».

Flame screen

A portable or fitted devise incorporating one or more corrosion-resistant wire-woven fabrics of very small mesh used for preventing sparks from entering a tank or vent opening or, for a short time, preventing the passage of flame. (Not to be confused with a flame arrester, see Instructor Manual section 1.4).

Flammable (also referred to as «combustible»)

Capable of being ignited and of burning. For the purpose of these guidance notes, the terms «flammable» and «combustible» are synonymous.

Flammable range (also referred to as «explosive range»)

The ranger of hydrocarbon gas concentrations in air between the lower and upper flammable (explosives) limits. Mixtures within this range are capable of being ignited and of burning.

Flashlight (also referred to as «torch»)

A battery-operated hand lamp. An approved flashlight is one which is approved by a competent authority for use in flammable atmosphere.

Flashpoint

The lowest temperature at which a liquid gives off sufficient gas to form a flammable gas mixture near surface of the liquid. It is measured in the laboratory in standard apparatus using a prescribed procedure.

Foam (also referred to as «froth»)

An aerated solution which is used for fire prevention and fire-fighting.

Foam concentrate (also referred to as «foam compound»)

The full-strength liquid that is received from the supplier, which is diluted and processed to produce foam.

Foam solution

The mixture produced by diluting foam concentrate with water before processing to make foam.

Free fall

The unrestricted fall of liquid into a tank.

Freezing point (melting point)

The temperatures at which the liquid state of a substance is in equilibrium with the solid state, i. e. at a higher temperature the solid will melt and at a lower temperature the liquid will solidify. Freezing point and melting point may not always coincide, but they are sufficiently close to enable the difference between them to be ignored for the purpose of this Guide. (See «Supercooling»).

Froth

See «Foam».

Gas

This term is used to cover all vapour of vapour/air mixtures.

Gas absorption detector

An instrument used for the detection of gases or vapours which works on the principles of discoloring a chemical agent in the apparatus.

Gas Codes

The codes for the construction and equipment of ships carrying liquefied gases in bulk, prepared and published by the International Maritime Organization.

Gas-dangerous space or zone

A space or zone within the cargo area which is not arranged or equipped in a approved manner to ensure its atmosphere is at all time maintained in a gas-safe condition, or an enclosed space outside the cargo area through which any piping passes which may contain liquid or gaseous products unless approved arrangements are installed to prevent any escape of product vapour into the atmosphere of that space.

Gas-free

A tank, compartment or container is gas-free when sufficient fresh air has been introduced into it to lower the level of any flammable, toxic, or inert gas to that required for a specific purpose, e. g. hot work, entry, etc.

Gas-free certificate

A certificate issued by an authorized responsible person confirming that, at the time of testing a tank, compartment or container, it was gas-free for a specific purposes.

Gas-safe

A space not designated as a gas-dangerous space.

Gauze screen (sometimes called «flame screen»).

A portable or fitted device incorporating one or more corrosion-resistant wire-woven fabrics of very small mesh used for preventing sparks from entering an open deck hole, or FOR A SHORT PERIOD OF TIME preventing the passage of flame, yet permitting the passage of gas.

Grounding

See «Earthing».

Halon

A halogenated hydrocarbon previously used in fire fighting which inhibited flame propagation.

Hard arm

An articulated pipework arm used in terminals to connect shore pipework to ship manifold.

Harmful

A general descriptive term for injurious effects on health that may be caused by chemicals.

Hazardous area

A hazardous are is one in which vapour may be present continuously or intermittently in sufficient concentrations to create a flammable atmosphere or an atmosphere which is dangerous for personnel.

Hazardous zone

See «hazardous area».

Health hazard

A general descriptive term for the danger to the health of personnel presented by some chemicals.

Heat of fusion

Quantity of heat required to effect a change of state of a substance from solid to liquid without change of temperature. (Latent heat of fusion).

Heat of vaporization

Quantity of heat required to effect a change of state of a substance from liquid to vapour without change of temperature. (Latent heat of vaporization).

Hold space

The space enclosed by the ship’s structure in which a cargo containment system is situated.

Hot work

Work involving sources of ignition or temperature sufficiently high to work cause the ignition of a flammable gas mixtures. This includes any work requiring the use of welding, burning or soldering equipment, blow torches, some power-driven tools, portable electrical equipment which is not intrinsically safe or contained within an approved explosion-proof housing sand-blasting equipment, or internal-combustion engines.

Hot-work permit

A document issued by a responsible person permitting specific hot work to be done during a specific time interval in a defined area.

Hydrate

White, snow-like, crystalline substance formed at certain pressure and temperatures by hydrocarbons containing water.

Hydrate inhibitors

An additive to certain liquefied gases that is capable of depressing the temperature at which hydrates begin to form. Typical depressants are:

• methanol;
• ethanol;
• isopropyl alcohol, etc.

Hydrocarbons gas

A gas composed entirely of hydrocarbons.

Hydrolysis

The decomposition of a compound by the agency of water (H-OH) into two parts, one part them combining with hydrogen (H) from the water and the other part with the hydroxyl (OH).

Hygroscopic tendency

The tendency of substance to absorb moisture from the air.

IACS

International Association of Classification Societies.

IAPH

International Association of Ports and Harbours.

ICS

International Chamber of shipping.

IEC

International Electromechanical Commission.

IMO

International Maritime Organization, the United Nations specialized agency dealing with maritime affairs.

Incendive spark

A spark sufficient temperature and energy to ignite a flammable vapour.

Inert condition

A condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8 % or less by volume by addition of inert gas.

Inert gas

A gas or a mixture of gases, such as flue gas, containing insufficient oxygen to support the combustion of hydrocarbons.

Inert gas distribution system

All piping, valves and associated fittings to distribute inert gas from the gas plant to cargo tanks, to vent gases to atmosphere and to protect tanks against excessive pressure or vacuum.

Inert gas plant

All equipment specially fitted to:

• supply;
• cool;
• clean;
• pressurize;
• monitor;
• and control delivery of inert gas to cargo tank systems.

Inert gas system (IGS)

An inert gas plant and inert distribution system together with means for preventing back-flow of cargo gases to the machinery space, fixed and portable measuring instruments and control devices.

Inerting

The introduction of inert gas into a tank with the object of attaining the inert condition.

Ingestion

The act of introducing a substances into the body via the digestive system.

Inhibited chemical

A chemical used to which an inhibitor or additive has been added.

Inhibitor

A substance use to prevent any chemical reaction.

Insulating flange

A flanged joint incorporating an insulating gasket, sleeves and washers to prevent electrical continuity between pipelines, hose strings or loading arms.

Interbarrier space

The space between a primary and a secondary barrier of a cargo containment system, whether or not completely or partially occupied by insulation or other material.

Interface detector

An electrical instrument for detecting the boundary between oil and water.

INTERTANKO

International Association of Independent Tanker Owners.

Intrinsically safe

An electrical circuit or part of a circuit is intrinsically safe if any spark or thermal effect produced normally (i. e. breaking or closing the circuit) or accidentally (e. g. by short circuit or earth fault) is incapable, under prescribed test conditions, of igniting a prescribed gas mixture.

Irritating liquid

A liquid which, on direct contact with the eyes or skin, will cause, injury, burns or severe irritation.

Irritating vapour

A vapour which will cause irritation of the eyes, nose, throat and respiratory tract. Such vapours generally are immediately evident.

ISGOTT

International Safety Guide for Oil tankers and terminals. Published jointly by ICS, OCIMF and IAPH.

Isothermal

When a gas passes through a series of pressure and/or volume variations without change of temperature, the changes are called «isothermal».

Latent heat

The heat required to cause a change in phase of a substance from solid to liquid (latent heat of fusion) or from liquid to vapour (latent heat of vaporization). These phase changes for single-component systems occur without change of temperature at the melting point and the boiling point respectively.

Liquefied gas

A liquid which has a saturated vapour pressure exceeding 2,8 bar absolute at 37,8 °C and certain other substances specified in the IMO Codes.

LNG

Liquefied natural gas, the principal constituent of which is methane.

Loading overall

The loading of cargo or ballast «over the top» through an open-ended pipe by means of an openended hose entering a tank through a hatch or other deck opening, resulting in the free fall of liquid.

Lower flammable limit (LFL)

The concentration of a hydrocarbon gas in air below which there is insufficient hydrocarbon to support and propagate combustion. Sometimes referred to as «lower explosive limit (LEL)».

LPG

Liquefied petroleum gas. Mainly propane and butane, and can be shipped separately or as a mixture.

Main deck

The steel deck forming the uppermost continuous watertight deck.

Manifold valves

Valve in a tanker’s piping system immediately adjacent to the ship/shore connecting flanges.

MARVS

Maximum Allowable Relief Valve Setting of a cargo tank.

MmHg

The abbreviation for «millimeters of mercury» used as units of pressure.

Molar volume

he volume occupied by one molecular mass in grams (g mole) under specific conditions. For an ideal gas at standard temperature and pressure it is 0,0224 m³.

Mole

The mass that is numerically to the molecular mass. It is most frequently expressed as the gram molecular mass (g mole) but may also expressed in other mass units, i. e. kg mole. At the same pressure and temperature the volume of one mole is the same for all perfect gases. It is practical to assume that petroleum gases are «perfect» gases.

Mole fraction

The number of moles of any component in a mixture divided by the total number of moles in the mixture.

Mooring winch brake design capacity

The percentage of the breaking strength (when new) of the mooring rope, or of the wire it carries, at which the winch brake is designed to yield. May be expressed as a percentage or in tonnes.

Mooring winch design heaving capacity

The power of a mooring winch to heave in or put a load on its mooring rope or wire. Usually expressed in tonnes.

Mucous membranes

Those surfaces lined with secretion; for example, the inside of the nose, throat, windpipe, lungs and eyes.

Naked lights

Open flames or fires, lighted cigarettes, cigars, pipes or similar smoking materials, any other unconfined sources of ignition, electrical and other equipment liable to cause sparking while in use, and unprotected light bulbs.

Narcosis

A condition of profound insensibility, resembling sleep, in which the unconscious person can only be roused with great difficulty but is not entirely indifferent to sensory stimuli.

Narcotics

Substances which produce narcosis.

NGL

Natural Gas Liquids. Liquids fractions found in association with natural gas. Ethane, propane, butane, pentane and pentanes plus are typical NGLs.

Non-volatile petroleum

Petroleum having a flashpoint of 60 °C (140 °F) or above as determined by the closed-cup method of test.

OBO, OIL/ORE

See «Combination carrier».

OCIMF

Oil Companies International Marine Forum.

Odoriser

Stenching compound added to liquefied petroleum gas to provide a distinctive smell. Ethyl mercaptan is commonly used for this purpose.

Odour threshold

The smallest concentration of gas or vapour, expressed in parts per million (ppm) by volume in air, that most people can detect by smell.

Open gauging

A system which does noting to minimize or prevent the escape of vapour from tanks when the contents are being measured.

Oral administration

The introduction of a substance into the body via the mouth.

Oxidizing agent

An element or compound that is capable of adding oxygen or removing hydrogen; or one that is capable of removing one or more electrons from an atom or group of atoms.

Oxygen analyzer/meter

An instrument for determining the percentage of oxygen in a sample of the atmosphere drawn from a tank, pipe or compartment.

Oxygen-deficient atmosphere

An atmosphere containing less than 21 % oxygen by volume.

Packaged cargo

Petroleum or other cargo in drums, packages or other containers.

Padding

Filling and maintaining the cargo tank and associated piping system with an inert gas, other gas or vapour, or liquid, which separates the cargo from air.

Partial pressure

The pressure exerted by a constituent in a gaseous vapour mixture as if the other constituents were not present. Generally this pressure cannot be measured directly but is obtained by analysis of the gas or vapour and calculation by use of Dalton’s Law.

Peroxide

A compound that is formed by the chemical combination of cargo liquid or vapour with atmospheric oxygen or oxygen from another source. These compounds may in some cases be highly reactive or unstable and constitute a potential hazard.

Petroleum

Crude oil and liquid hydrocarbon products derived from it.

Petroleum gas

A gas evolved from petroleum. The main constituents of petroleum gases are hydrocarbons, but they may also contain other substances, such as hydrogen sulphide or lead alkyls, as minor constituents.

pH

This can be used as an arbitrary indication of the acidity of a solution. Its practical range is 0 to 14 pH 7 represents absolute neutrality. A value of 1 represents high acidity (e. g. dilute hydrochloric acid) and 13 represents high alkalinity (e. g. a caustic soda solution).

Poison

A very toxic substance which, when absorbed into the human body by ingestion, skin absorption, or inhalation, produces a serious or fatal effect. Notwithstanding the above corrosive liquids, such as acids (which, due solely to their corrosive nature, can be fatal if ingested), should not be classed as poisons.

Poly

A prefix, meaning «many».

Polymerization

The phenomenon whereby the molecules of a particular compound can be made to link together into a larger unit containing anything from two to thousands of molecules, the new unit being called a polymer. A compound may thereby change from a free-flowing liquid to a viscous one or even to a solid. A great deal of heat may be evolved when this occurs. Polymerization may occur automatically with no outside influence, or it may occur if the compound is heated, or if a catalyst or impurity is added. Polymerization may, under some circumstances, be dangerous.

Pour point

The lowest temperature at which petroleum oil will remain fluid.

Pressure/vacuum valve (sometime referred to as PN valve, breather valve)

A dual-purpose valve commonly incorporated in the cargo tank venting system of tankers, the operation of which, when appropriately set, automatically prevents excessive pressure or vacuum in the tank or tanks concerned. On a tanker, such a valve may be either manually jacked open or by-passed when the vent system must handle large gas flows during loading or gas-freeing.

Pressure surge

A sudden increase in the pressure of the liquid in a pipeline, brought about by an abrupt change in flow velocity.

Pyrophoric iron sulphide

Iron sulphide that is capable of a rapid exothermic oxidation, with incandescence, when exposed to air which is capable of igniting flammable hydrocarbon gas/air mixtures.

Primary barrier

The inner structure designed to contain the cargo when the containment system includes a secondary barrier which will contain the cargo for a time should the primary barrier fail.

Purging

The introduction of nitrogen or suitable inert gas or suitable cargo vapour to displace an existing atmosphere from a containment system.

The introduction of inert gas into a tank that is already in the inert condition, with the object of:

1. further reducing the existing content;
2. or reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if air subsequently introduced into the tank.

Reducing agent

An element or compound that is capable of removing oxygen, or adding hydrogen, or one that is capable of giving electrons to an atom or group of atoms.

Reid vapour pressure (RVP)

The vapour pressure of a liquid determined in a standard manner in the Reid apparatus at a temperature of 100 °F (37,8 °C) and with a ratio of gas to liquid volume of 4:1.

Relative liquid density

The mass of a liquid at a given temperature compared with the mass of an equal volume of fresh water at the same temperature or at a different given temperature.

Relative vapour density

The mass of a vapour compared with the mass of an equal volume of air, both at standard conditions of temperature and pressure.

Respiratory tract

The air passages from nose to lungs inclusive.

Responsible officer (or person)

A person appointed by the employer or the master of the ship and empowered to take all decisions relating to his specific task, having the necessary knowledge and experience for that purpose.

Responsible terminal representative or Terminal representative

The shore supervisor in charge of all operators and operations at the terminal associated with the handling of products, or his responsible delegate.

Restricted gauging system (also known as «restricted ullage system»)

A system employing a device which penetrates the tank and which, when in use, permits a small quantity of cargo vapour or liquid to be exposed to the atmosphere. When not in use, the device is completely closed. The design ensure that no dangerous escape of tank contents (liquid or spray) can take place in opening the device.

Resuscitator

Equipment to assist or restore the breathing of a man overcome by gas or lack of oxygen.

Rollover

The phenomenon where the stability of two stratified layers of liquid is disturbed by a change in their relative density resulting in a spontaneous rapid mixing of the layers, accompanied, in the case of liquefied gases, by an increased evolution of vapour.

Sacrificial anode

The preferential corrosion of an active metal for the sake of protecting a more noble(less reactive) metal. For example, a zinc anode immersed in an electrolyte (seawater) will, by galvanic action, preferentially corrode and thereby protect the adjacent steelwork of a ship’s hull.

Safety relief valve

A valve fitted on a pressure vessel to relieve over-pressure.

Saturated vapour pressure

The pressure at which a vapour is in equilibrium with its liquid at a specified temperature.

Secondary barrier

The liquid-resisting outer element of a cargo containment system designed to afford temporary containment of a leakage of liquid cargo through the primary barrier and to prevent the lowering of the temperature of the ship’s structure to an unsafe level.

Scale

Deposit or incrustation which may form on metal as a result of electrolytic or chemical action.

Self-reaction

The tendency of a chemical to react with itself, usually resulting in polymerization or decomposition. Self-reaction may be promoted by contamination with small amounts of other materials.

Self-stowing mooring winch

A mooring winch fitted with a drum on which a wire or rope is made fast and automatically stowed.

Shore Authority

The body responsible for the operation of a shore installation or shore equipment associated with the handling of chemical cargoes.

SI (System international) units

An internationally accepted coherent system of units, modeled on the metric system, consisting of base units of length (metre), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (Kelvin), luminous intensity (candela) and amount of substance (mole).

SIGTTO

Society of International Gas Tanker and Terminal Operators Limited.

Slip tube

A device used to determine the liquid-vapour interface during the ullaging of semi-pressurized and fully pressurized tanks. See «Restricted gauging».

Sloshing

Wave formation which may arise at the liquid surface in a cargo tank from the effects of ship motions.

SOLAS

International Convention for the Safety of Life at Sea, 1974.

Solubility

The solubility of a substance in water, at a specified temperature, is the maximum weight of substance which will dissolve in a given weight of water, in the presence of undissolved substance. The value is usually expressed as the number of grams of substance dissolving in 100 grams of water. In the case of liquid dissolving in liquid, the term «miscibility» is often used instead of «solubility». Ethanol dissolves in water at ordinary temperatures in all proportions, and is said to be completely miscible. A hydrocarbon and water, on the other hand, are immiscible. Aniline and water are partially miscible.

Sour crude oil

A crude oil containing appreciable amounts of hydrogen sulphide or mercaptans.

Span gas

A vapour sample of known composition and concentration that is used to calibrate gas-detection equipment.

Specific gravity

The ratio of the weight of a substance at a temperature t1, to the weight of an equal volume of fresh water at a temperature t2, where t′ does not necessarily equal t2. Temperature will affect volume; therefore the temperature at which the comparison was made is stated on each data sheet, after the ratio.

e. g., SG = 0,982 at 20 °C/15 °C.

«20 °C» referring to the temperature of the substance and «15 °C» referring to the temperature of the water.

Specific heat

The ratio of the thermal capacity of a substance to that of water. For a gas, the specific heat at constant pressure is greater than that at constant volume.

Spontaneous combustion

Ignition of a combustion material is termed «spontaneous» if the inherent characteristics of the material cause a heat-producing (exothermic) chemical action, and thus ignition, without exposure to external fire, spark or abnormal heat.

Static accumulator oil

An oil with an electrical conductivity less than 100 picosiemens/metre (pS/m), so that it is capable of retaining a significant electrostatic charge.

Static electricity

The electricity produced on dissimilar materials through physical contact and separation.

Static non-accumulator oil

An oil with an electrical conductivity greater than 100 picosiemens/metre (pS/m), which renders it incapable of retaining a significant electrostatic charge.

Stern discharge line

A cargo pipeline over the deck to a point terminating at or near the stern of the tanker.

Stripping

The final operation in pumping bulk liquid from a tank or pipeline.

Sublimation

The conversion of a solid direct into a vapour without melting, e. g. naphthalene. The significance of sublimation is that there may be sufficient vapour above the solid for combustion. In such a case that flashpoint may be lower than the freezing point.

Submerged pump

A type of centrifugal cargo pump commonly installed on gas carriers and in terminals in the bottom of a cargo tank, i. e. with drive motor, impeller and bearings totally submerged when the tank contains bulk liquid.

Supercooling

This takes place if a liquid drops in temperature below its freezing point without freezing.

Surge pressure

A phenomenon generated in a pipeline system when there is any change in the rate of flow of liquid in the line. Surge pressures can be dangerously high if the change of flow rate is too rapid, and the resultant shock waves can damage pumping equipment and cause rupture of pipelines and associated equipment.

Systemic toxic effect

The effect of a substance or its vapour on those parts of the human body with which it is not in contact. This presupposes that absorption has taken place. It is possible for chemicals to be absorbed through skin, lungs or stomach, producing later manifestations which are not a result of the original direct contact.

Tank cover

The structure intended to protect the cargo containment system against damage where it protrudes through the weather deck and/or to ensure the continuity and integrity of the deck structure.

Tank dome

The upward extension of a portion of a cargo tank. For below deck cargo containment systems the tank dome protrudes through the weather deck, or through a tank cover.

Tank vent system

The piping system and associated valves, installed to prevent over-pressure and excessive vacuum in cargo tanks.

Tanker

A ship designed to carry liquid petroleum cargo in bulk, including a combination carrier when being used for this purpose.

Tension which (automated or self-tensioning mooring system)

A mooring winch fitted with a device which may be set to maintain the tension on a mooring line automatically.

Terminal

A place where tankers are berthed or moored for the purpose of loading or discharging petroleum cargo.

Terminal representative

The person designated by the terminal to take responsibility for an operation or duty.

Threshold limit value (TLV)

Concentration of gases in air to which it is believed personnel may be exposed 8 hours per day or 40 hours per week throughout their working life without adverse effects. The basic TLV is a Time-Weighted Average (TWA) and may be supplemented by a TLV-STEL (Short-Term Exposure Limit) or TLV-C (Ceiling exposure limit, which should not be exceeded even instantaneously).

Topping off

The operation of completing the loading of a tank to a required ullage.

Topping up

The introduction of inert gas into a tank which is already in the inert condition, with the object of raising the tank pressure to prevent any ingress of air.

Torch

See «Flashlight».

Toxic

Poisonous to human life.

Toxic liquid

A liquid which, if ingested or absorbed through the skin, causes bodily harm that may be severe.

Toxic vapour

A vapour which, if inhaled, causes bodily harm that may be severe.

True vapour pressure (TVP)

The true vapour pressure of a liquid is the absolute pressure exerted by the gas produced by evaporation from a liquid when gas and liquid are in equilibrium at the prevailing temperature and the gas/liquid ratio is effectively zero.

Ullage

The depth of the space above the liquid in a tank.

Upper flammable limit (UFL)

The concentration of a hydrocarbon gas in air above which there is insufficient air to support and propagate combustion. Sometimes referred to as «upper explosive limit (UEL)».

Vapour

A gas below its critical temperature.

Vapour density

The relative weight of the vapour compared with the weight of an equal volume of air at standard conditions of temperature and pressure. Thus vapour density of 2,9 means that the vapour is 2,9 times heavier than an equal volume of air, under the same physical conditions.

Vapour pressure

The pressure exerted by the vapour above the liquid, at a given temperature. It is expressed as absolute pressure.

Vapour seal system

Special fitted equipment which enables the measuring and sampling of cargoes contained in inerted tanks without reducing the inert gas pressure.

Venting

The process of air/vapour release to and from cargo tanks.

Void space

An enclosed space in the cargo area that is external to a cargo containment system and which is not a hold space, ballast space, fuel or oil tank, cargo pump or compressor room or any space in normal use by personnel.

Volatile petroleum

Petroleum having a flashpoint below 60 °C (140 °F), as determined by the closed-cup method of testing.

Volatile liquid

A liquid which evaporates readily at ambient temperatures.

Volatile organic compound (VOC)

Any volatile compound of carbon which participates in atmospheric photochemical reactions. For regulatory purpose this may exclude:

• carbon dioxide;
• carbon monoxide;
• carbonic acid;
• metallic carbides or carbonates;
• and ammonia carbonate,

depending on regulatory body.

Volatility

The tendency for a liquid to vaporize.

Water fog

A suspension in the atmosphere of very fine droplets of water, usually delivered at a high pressure through a fog nozzle for use in fire fighting.

Water spray

A suspension in the atmosphere of water divided into coarse drops by delivery through a special nozzle for use in fire fighting.

Work permit

A document issued by a responsible person permitting specific work to be done during a specified period in a defined area.

Author

Olga Nesvetailova

Freelancer

Literature

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