CHEMICAL TANKERS

SOLAS makes it mandatory for chemical tankers built on or after 1 st July, 1 986 to comply with provisions of International Bulk chemical code (IBC Code) - chapter VII, Para B Reg - 8. Whereas ships built earlier are required to comply with requirements of Bulk chemicaf Code (BCH code).

CARGOES CARRIED IN CHEMICAL TANKERS ARE :

a)        Petrochemicals

b)      Alcohols and Carbohydrates.

c)       Vegetable, animal oils and fats.

d)      Acids and inorganic chemicals.

CLASSIFICATIONS OF CHEMICAL TANKERS

a)             Type 1 for highly hazardous substances with cargo tanks located inboard B/5 or
11.5 M, which ever is less,   

b)             Type 2'for less hazardous substancs with cargo tanks 760 mm inboard and

c)              Type 3 for least hazardous substances with no special requirement for ship sides
or bottom plating".

CONSTRUCTIONAL FEATURES

Purpose built Parcel chemical-carriers are built with double skin and double bottoms and cofferdams between to cargo tanks to segregate incompatiable cargoes carried at different temperatures. Double bottom and double skin tanks are used for segregated ballast.

A multiplicity of cargo tanks and sizes, some dedicated to a particular product, but each having its own cargo pumping, piping and venting system, thus giving maximum segregation and protection against contamination and reactions with other cargoes. This also enables flexibility of cargo handling system, concurrent discharge, loading and tank cleaning operations.

CARGO TANK COATINGS

i)     Stainless steel for versatile compatibility with chemical cargoes.

ii)     Rubber lining for highly corrosive substances like phosphoric and hydrochloric acid dedicated for specific trades, (not suitable for oils, fats and solvents)

iii)   Zinc coated mild steel for corrosive cargo.

Coatings protects the steel from the products carried or ballast water, protects from contamination and also improves cleaning of cargo tanks.

CHEMICAL HANDLING SYSTEM

There is a need to reduce the possibility of cargo contamination on safely as well as commercial ground, alongwith the pumping requirements of a wide range of cargoes.

Submerged or deep well pumps are installed in each single tank i.e. concept of one tank, one pump, one line together with elimination of pump room is the typical characteristic of modern parcel tanker.

The-system gurantees the absolute segregation between cargoes and a greater flexibility for handling cargoes from and to different sources.

The efficient stripping system (ESS) made mandatory with the entry into force of MARPOL Annex-ll will enhance the already remarkable performance of submersible/ • deep well pumps in drying cargo tanks.

Double hull and some double bottom tanks are used for segregated ballast and selected double bottom tanks for trim and heel correction during cargo transfer and tank cleaning operation.

TANK CLEANING AND PREPARATION

The majority of tank cleaning is done using tank washing machines where water is the main washing agent.

Present regulation require that the capacity of each single machine's nozzle should not be greater than 1 7.5 m³/hr with a maximum throughput per tank of 170 rrrVhr, to reduce static electricity and the potential of water slug formation.

Sea water either cold or hot can be used, however, presence of chlorids may initiate local corrosion and pitting on stainless steel and affect the quality of some 'chloride' sensitive cargoes. It may be better to use fresh water instead or at least to give tanks a final rinse with it.

EFFECT OF WASHING WITH WATER IN PRE - CLEANING PHASE

a)             Products with good solubility in water viz. Phosphoric acid and sulphuric acid,
alcohols (methanol, ethanol, butanol). The more viscous water soluble will require hot
water washing.

b)             Some vegetable and animal oils (fatty acids) oxidise and dry in presence of air and
heat. Pre-cleaning should be carried out with cold water only,  to prevent clingage
hardening on bulkheads.

c)              Vegetable and animal oils (non drying) like coconut oil, palm oil, whale oil should
be preferably pre-washed with hot water at about 75°C.

d)             Polymerising Products should be pre-washed only with cold water e.g. styrene:
monomer, vinyl acetate, Acrylonitrile)           

e)             Lub. oil are normally washed with hot water.                              

Final cleaning is often necessary when dealing with vegetable oil, animal oil, polymerise products heavy oils, lub oils and lub. oil additives by adding washing soda (caustic soda) and solvent or detergent.

Cleaning of tank by steaming with a low flash chemical solvent may be effective for removal of very last traces of previous cargo.

This operation should only be done when the tank is inerted and maintained in such condition.            

All chemicals used for cleaning purpose (low and high flash solvents, detergents or synthetic soaps) may generate electrostatic charge if allowed to free fall or when sprayed. 

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