Liquefied Gas Carrier

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Reactivity of liquefied gas cargo

A liquefied gas cargo may react in a number of ways: with water to form hydrates, with itself, with air, with another cargo or with other materials.

Reaction with Water – Hydrate Formation
Some hydrocarbon cargoes will combine with water under certain conditions to produce a substance known as a hydrate resembling crushed ice or slush. The water for hydrate formation can come from purge vapours with an incorrect dew point, water in the cargo system or water dissolved in the cargo. Care should be taken to ensure that the dew point of any purge vapour or inert gas used is suitable for the cargo concerned, and that water is excluded from the cargo system.

Hydrates can cause pumps to seize and equipment to malfunction. Care should therefore be taken to prevent hydrate formation.

Certain cargoes, notably LPGs, may contain traces of water when loaded. It may be permissible in such cases to prevent hydrate formation by adding small quantities of a suitable anti-freeze (e.g. methanol, ethanol) at strategic points in the system. It is emphasized that nothing whatsoever should be added to any cargo without the shipper’s permission. For LPG mixtures a small dose of anti-freeze may be permissible, but for chemical cargoes such as ethylene the addition of even one liter per two hundred tons could make the cargo commercially valueless. In the case of inhibited cargoes the anti-freeze could adversely affect the inhibitor.

If the use of anti-freeze is permitted it should be introduced at places where expansion occurs because the resultant lowering of temperature and pressure promotes hydrate formation.

Anti-freeze additives are often flammable and toxic, and care should be taken in their storage and use.

Inhibitor: A substance used to prevent or retard cargo deterioration or a potentially hazardous chemical self-reaction, e.g. polymerization.

Product carrier in rough sea condition

Fig: Product carrier in rough sea condition


Some cargoes may react with themselves. The most common form of self-reaction is polymerization which may be initiated by the presence of small quantities of other cargoes or by certain metals. Polymerization normally produces heat which may accelerate the reaction.

The IMO Codes require cargoes which may self-react either to be carried under an inert gas blanket, or to be inhibited before shipment. In the latter case a certificate must be given to the ship, stating:

(1) The quantity and name of the inhibitor added;

(2) The date it was added and how long it is expected to remain effective;

(3) The action to be taken should the voyage exceed the effective lifetime of the inhibitor;

(4) Any temperature limitations affecting the inhibitor.

Normally there should be no need to add any inhibitor to the cargo during the voyage. If it should become necessary, however, any such additions should be made in accordance with the shipper’s instructions.

The inhibitor may not boil off with the cargo and it is possible for reliquefaction systems to contain uninhibited cargo. The system should therefore be drained or purged with inhibited cargo when shut down.

Many inhibitors are much more soluble in water than in the cargo, so to avoid a reduction in inhibitor concentration, care should be taken to exclude water from the system. Similarly the inhibitor may be very soluble in anti-freeze additives if these form a separate phase and the shipper’s instructions on the use of anti-freeze should be observed. If the ship is anchored in still conditions the cargo should be circulated daily to ensure a uniform concentration of inhibitor.

Certain cargoes which can self-react (e.g. ethylene oxide, propylene oxide), but which cannot be inhibited, have to be carried under inert gas. Care should be taken to ensure that a positive pressure is maintained in the inerted atmosphere at all times and that the oxygen concentration never exceeds 0.2% by volume.

Reaction with Air

Some cargoes can react with air to form unstable oxygen compounds which could cause an explosion. The IMO Codes require these cargoes to be either inhibited or carried under nitrogen or other inert gas.

Reaction with Other Cargoes

Certain cargoes can react dangerously with one another. They should be prevented from mixing by using separate piping and vent systems and separate refrigeration equipment for each cargo. Care should be taken to ensure that this positive segregation is maintained.

To establish whether or not two cargoes will react dangerously, the data sheet for each cargo should be consulted. This issue is also covered in various national regulations, which should be observed.

Reaction with Other Materials

The data sheets list materials which should not be allowed to come into contact with the cargo. The materials used in the cargo systems must be compatible with the cargoes to be carried and care should be taken to ensure that no incompatible materials are used or introduced during maintenance (e.g. gaskets).

Reaction can occur between cargo and purge vapours of poor quality: for instance, inert gas with high CO2 content can cause carbamate formation with ammonia. Reaction can also occur between compressor lubricating oils and some cargoes, resulting in blockage and damage.

Methane is an asphyxiant in high concentrations because it dilutes the amount of Oxygen in the air below that necessary to maintain life. Due to its inactivity, Methane is not a significant air pollutant although it is not a significant greenhouse gas and, due to it insolubility, inactivity, and volatility, it is not considered a water pollutant.

Related Information:

  1. How to prevent corrosion by the gas carrier

  2. propylene oxide mixtures procedure

  3. Vapour characteristics of liquefied gases

  4. Low temperature effects of liquefied gases

  5. Flammable vapours in cargo tanks and how to monitor safety limitsProcedure for safe cargo handling in liquefied gas carriers

Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Cargo Transfer between Vessels (STS Operation)

Procedure for segregation of Liquefied Gas Cargoes

Procedure for Stripping Liquefied Gas Cargoes

Procedure for Changing Liquefied Gas Cargoes

Displacing Atmosphere with Inert Gas (Inerting)

Procedure for Water washing after Ammonia Cargoes

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer

Procedure for commissioning the cargo systemCargo emergency shutdown requirement for liquefied gas carrier Gas analyzing equipment

External links :

  1. "Polskielng " LNG terminal to off-take natural gas & increasing the energy security of Poland.

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