DieselShip

Chemical carriers

 

Chemical carriers are basically tanker type structures which are arranged to carry certain types of noxious substances. The structure will generally consist of central cargo tanks with the wing tanks and double bottoms used for dirty ballast. Restrictions will apply if the cargo being carried reacts with water and in these cases void spaces are required. Materials used for construction will depend upon the type of cargo being carried and these are grouped depending upon their properties.

Chemical carriers must be designed for the safe carriage of particular types of cargo and these are classed into 3 groups according to the potential hazard of that group.

In general a class A, ship could theoretically carry any class A, cargo, but particular cargoes react with certain materials used for tank pipe and pump construction. The carriage of such cargoes is therefore limited to those which will not react with the materials of construction. A ship designed for class A cargoes may also carry class Band C provided they will not react with the materials of construction. Similarly, class B vessels may carry class C cargoes.

Liquid sulphur must be maintained within a temperature range of 127-138^oC , and not exceeding 155^oC. Tanks should be insulated and the internal construction at the top of the tanks should not allow vapour pockets to form.

Hydrochloric acid presents difficulties ( it reacts with most common metals) and special arrangements are required to line steel tanks with a material which will not react with the acid but is flexible enough to distort with the tank. A suitable material is rubber. Tanks must be separated from the main hull so that the stresses in the hull are not transmitted to the tank

Cargo classes

Class	Hazards	Examples
A,	Low flash point toxic Skin hazard Reactive with water	acetic acid Sulphuric acid Liquid sulphur Reactive with water Phenol
	These are listed and have a high rate of reaction with moisture or water. Tanks must be segregated from the ships side and pumps must be located in the tanks or on deck. Cargo tank vent outlets are not to be closer than 15m to inlets to the accommodation. In general these cargos are unsuitable for carriage in mild steel tanks (generally acids and petrochemical substances)
B	Low flash point e.g. Carbon tetrachloride Toxic Skin hazard	Formaldehyde Benzene
	Are not as restrictive as the type A although they are toxic and many have a low flash point their problems with relation to water are not as great. Mild steel is suitable for tanks but aluminium and copper are not.
C	Low flash point	acetone Asphalt Caustic soda
	Are less dangerous and most are not toxic in general. Most engineering materials are suitable for tank construction. There is some risk of contamination through water but it is not great