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Properties of ammonia & how to remove ammonia vapour from cargo tanks ?

Liquid ammonia is a colourless alkaline liquid with a pungent odour. The vapours of ammonia are flammable and burn with a yellow flame forming water vapour and nitrogen, however, the vapour in air requires a high concentration (16-25 per cent) to be flammable, has a high ignition energy requirement (600 times that for propane) and burns with low combustion energy.

For these reasons the IMO Codes, while requiring full attention to the avoidance of ignition sources, do not require flammable gas detection in the hold or interbarrier spaces of carrying ships. Nevertheless, ammonia must always be regarded as a flammable cargo.
Gas carrier sea passage

Ammonia is also toxic and highly reactive. It can form explosive compounds with mercury, chlorine, iodine, bromine, calcium, silver oxide and silver hypochlorite. Ammonia vapour is extremely soluble in water and will be absorbed rapidly and exothermically to produce a strongly alkaline solution of ammonium hydroxide.

One volume of water will absorb approximately 200 volumes of ammonia vapour. For this reason it is extremely undesirable to introduce water into a tank containing ammonia vapour as this can result in vacuum condition rapidly developing within the tank.

Since ammonia is alkaline, ammonia vapour/air mixtures may cause stress corrosion. Because of its highly reactive nature copper alloys, aluminium alloys, galvanised surfaces, phenolic resins, polyvinyl chloride, polyesters and viton rubbers are unsuitable for ammonia service. Mild steel, stainless steel, neoprene rubber and polythene are, however, suitable.

Ammonia vapour is normally removed from the tanks at sea by introducing large amounts of air and ventilating to atmosphere. However, the removal of all traces of ammonia by ventilation alone is a lengthy process.

If desirable, remaining traces of ammonia may be removed by water washing or water sweeping. Ammonia is extremely soluble (one volume of water dissolves up to 1,000 volumes of ammonia vapour), and the introduction of water into tanks containing high concentrations of ammonia may immediately cause dangerous vacuum conditions unless unrestricted access of air is provided. Ship’s inert gas containing CO2 should never be used for purging after ammonia cargoes as carbamates will be formed which may block the cargo pipe lines.

If water washing, the following precautions should be taken:

(1) Personnel should wear breathing apparatus and protective clothing as necessary.

(2) Low ammonia concentrations must be achieved before water washing. All tank manhole covers should be opened up to provide unrestricted access of air and prevent dangerous vacuum conditions which may cause the tank to collapse.

(3) Fresh water should be used, because sea water will leave deposits which are difficult to remove and will considerably increase rust formation on steel surfaces.

(4) Tanks should not be washed if they contain submersible pumps unsuitable for water immersion. Even if water is not harmful to the cargo pumps, separate portable pumps should preferably be used for removal of washing water from the tanks. If the use of cargo pumps is proposed, the possibility of over-loading pump motors when pumping water (SG=1) should be considered; pump manufacturers’ instructions should be consulted.

(5) After pumping out as much water as possible, any remaining water on the tank bottom should be wiped up and the tanks dried out by ventilation before being closed. For water washing to be successful it is essential to dry the tank and cargo lines with dry air from the inert gas system. In conditions of high relative humidity ventilation with warm air may be necessary. If cargo pumps have been used for pumping washing water, they should be carefully dried out by ventilation and treated with anti-freeze. Excess anti-freeze collecting at the tank bottom should be wiped up, as it may not be acceptable for the next cargo.

(6) The discharge overboard of ammonia washings may be prohibited in certain areas, and care should therefore be taken. The requirements of the control of pollution in Annex II of the MARPOL 73/78 Convention should be observed. If discharged overboard, ammonia-contaminated washings should not be allowed to enter the ship’s seawater intakes because ammonia is corrosive to copper-based alloys in the seawater system.

Related Information:

  1. Procedure for segregation of liquefied gas cargoes

  2. Safety guideline for handling LPG & ammonia cargo

  3. Inerting of cargo tanks prior loading LNG cargo

  4. Risk of overfilling of cargo tank during loading

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