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Procedure of cargo stripping on board liquefied gas carriers

Before changing cargoes or gas-freeing it is most important to remove all cargo liquid from tanks, piping, reliquefaction plant and any other part of the cargo system. Any remaining cargo liquid will continue to give off vapour and will frustrate subsequent purging or gas-freeing.

If previous and subsequent cargoes are similar in chemical properties – e.g. propane and butane – purging may not be required (subject to shippers’ considerations), but even in such cases it is normally required that no previous cargo liquid remains. Shippers’ instructions regarding purging requirements should always be sought. To achieve maximum drainage of liquid during discharge, the following advice should be followed:

(1) Careful trimming or listing of the ship can, depending on the design of a tank, assist drainage of liquid.

(2) If pump are used for discharge, the pump discharge valve should be throttled toward completion of discharge to maintain suction to minimum liquid level. Manufacturers’ instructions should be consulted as to the liquid level at which throttling should be started and the pump pressure that has to be maintained during later stages of pumping to obtain maximum stripping. Each pump should be kept under continuous control during stripping to obtain the best results without pumps running dry.

(3) Even with good operation of cargo pumps, some liquid will remain in the tanks at termination of pumping. In the case of ships whose cargo tanks can accept overpressure, further stripping of liquid may be achieved by increasing tank pressure sufficiently to press out the liquid through the piping system ashore.

Alternatively, all stripping may be collected in one of the tanks for subsequent discharge ashore. The use of cargo compressors, taking suction from other tanks, will ensure that all tanks and associated piping systems are left liquid-free. Proper stripping of tanks should be checked by the bottom sampling line or temperature sensors.

(4) In the case of ships with cargo tanks designed for pressures only slightly above atmospheric (fully refrigerated ships), stripping by pressure alone is not possible. On such ship (and on ships with pressure tanks, if pressure stripping is not successful) the remaining liquid should be boiled off by introducing hot vapour from the cargo compressors to the bottom of the tanks, through puddle heat coils (if fitted).

During such operations the tank pressure must be closely observed, to avoid exceeding the relief valve set pressure. When pressure has increased to a safe level below the relief valve pressure, the cycle is reversed by starting compressor suction from the tank, reliquefying the vapour in the condenser, and discharging the condensate to shore or retaining on board in a deck pressure vessel. Alternatively, if the ship is at sea the vapour may be vented instead of being reliquefied.

(5) Provided the temperature of the liquid remaining in the tanks is above the saturation temperature corresponding to atmospheric pressure, the liquid ma also be boiled off by using the compressor to draw off gases from the tank (maintaining the tank pressure at atmospheric pressure), instead of using hot gas.

This is known as the vacuum method, the quantity of liquid that can be removed by this method is limited, however, as the boiling will soon sub-cool the liquid and no further evaporation will take place. The presence of sub-cooled liquid still remaining may be difficult to establish, as there will be insufficient tank pressure to detect it using the bottom sampling line. It will take some time before the liquid picks up sufficient heat from the surrounding tank structure to start boiling again and raise the pressure in the tank. Evaporation of the remaining liquid by means of hot gas is therefore recommended rather than the vacuum method.

(6) Some ships are fitted with heating coils in the tank bottom to evaporate liquid residues. The heating medium is hot cargo vapour for internal coils or may be thermal heating oil for coils fitted externally to the tank. Vapour circulating coils should be purged either with inert gas, or with vapour from the subsequent cargo if it is compatible with the previous cargo. Similar precautions should be taken with cargo compressors.

(7) Liquid is removed from the piping system and equipment by blowing through with vapour. Hot gas from the compressors passed through the liquid lines will provide heat to evaporate liquid not removed by pressure displacement. In cold weather and in insulated pipelines, liquid butane, butadiene etc. may evaporate very slowly even at atmospheric pressure.

(8) It may be necessary to change compressor lubricating oils when changing cargoes (the compressor manufacturer’s instructions should be observed).

Related Information:

  1. Procedure for Water washing after Ammonia Cargoes

  2. Procedure for Changing Liquefied Gas Cargoes

  3. Use of cargo as fuel -Cargo conditioning, reliquefaction and boil-off control for LNG carriers

  4. Displacing with Vapour of the Next Cargo (Purging)

  5. Dispersal of Vented Cargo Vapours

  6. Details of various cargo handling equipment onboard

More info pages

Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

Cargo Containment Systems in Liquefied Gas Carriers

cargo emergency shutdown requirement

damage stability guideline for liquefied gas carriers

Various Cargo handling equipments onboard

Cargo hoses connection guideline

Documents accompanying a liquid gas cargo

How LNG transferred from shore to ships cargo tanks ?

Cargo operation guideline onboard a liquefied gas carrier

Cargo piping Systems in Liquefied Gas Carriers

cargo planning requirement

cargo and pumproom safety precautions

cargo stripping guideline

Emergency response for cargo system leaks

Emergency response for cargo tank rupture

Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier

Preparation for cargo transfer

cargo transfer between vessels- safety guideline

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