Figure 2.4 Solubility of water in butadiene

2.3 THE CHEMICAL GASES

The chemical gases commonly transported in liquefied gas carriers are ammonia, vinyl chloride, ethylene oxide and propylene oxide. Apart from the latter two examples, since these gases do not belong to one particular family, their chemical properties vary considerably.

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, ammonia vapour in air requires a high concentration (14-28 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 Gas Codes, while requiring full attention to the avoidance of ignition sources, do not require flammable gas detection in the hold or interbarrier spaces. Nevertheless, ammonia must always be regarded as a flammable cargo.

Ammonia is 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 strong alkaline solution of ammonium hydroxide. One volume of water will absorb approximately 200 volumes of ammonia vapour. For this reason it is unsafe to introduce water into a tank containing ammonia vapour, as this can result in a vacuum condition rapidly developing within the tank. (See also 7.9.5)

Since ammonia is alkaline, ammonia vapour/air mixtures may cause stress corrosion on cargo tank shells. The factors contributing to stress corrosion cracking are the material of construction, residual stress within structures (from tank fabrication) and the nature of the cargo (including its temperature, pressure and impurities). Stress corrosion cracking occurs as a result of a chemical reaction and thus will happen faster at higher temperatures.

Stress corrosion cracking is identified as cracking in a containment vessel where (typically) fine cracks may be formed in many directions. Cracks caused by stress corrosion cracking are usually fine and brittle in nature (see Reference 2.30).

The risk of stress corrosion cracking occurring can be reduced by the following measures:

· The provision of refrigerated storage at a temperature of below -30°C

· During construction, by using steels having a low yield strength

· During construction, by having tank welds stress-relieved by thermal methods

· Adding 0.2 per cent water to the ammonia

· Developing procedures to minimise the ammonia being contaminated with air

Because of ammonia's 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.

Vinyl chloride is a colourless liquid with a characteristic sweet odour. It is highly reactive, though not with water, and may polymerise in the presence of oxygen, heat and light. Its vapours are highly toxic and flammable. Aluminium alloys, copper, silver, mercury and magnesium are unsuitable for vinyl chloride service. Steels are, however, chemically compatible.

Ethylene oxide and propylene oxide are colourless liquids with an ether-like odour. They are flammable, toxic and highly reactive. Both polymerise; ethylene oxide does so more readily than propylene oxide, particularly in the presence of air or impurities. Both gases may react dangerously with ammonia. Cast iron, mercury, aluminium alloys, copper and alloys of copper, silver and its alloys, magnesium and some stainless steels are unsuitable for the handling of ethylene oxide. Mild steel and certain other stainless steels are suitable as tank shell construction materials for both ethylene and propylene oxides.

Chlorine is a much less frequently carried cargo and restricted to special ships. It is a yellow liquid which evolves a green vapour. It has a pungent and irritating odour and is highly toxic. It is non-flammable but it can support combustion of other flammable materials in much the same way as oxygen. It is soluble in water forming a highly corrosive acidic solution and can form dangerous reactions with all the other liquefied gases. In the moist condition, because of its corrosivity, it is difficult to contain. Dry chlorine is compatible with mild steel, stainless steel, monel and copper. Chlorine is very soluble in caustic soda solution which can be used to absorb chlorine vapour.

2.4 CHEMICAL PROPERTIES

The chemical properties and compatibilities of many liquefied gases are summarised in Tables 2.2, 2.3(a) and 2.3(b). For details of chemical reactivity and of suitable materials of construction for the various liquefied gases, reference should be made to the data sheets contained in Reference 1.1 and Reference 2.1 in Appendix 1.

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