Cargo compatibility and reactivity for ships carrying dangerous chemicals in bulk
Transporting of dangerous and noxious liquid chemicals in bulk involved various risk factors. Chemical cargoes can be very dangerous, most of them being flammable and/or toxic,
some of them extremely so.
Between some chemicals violent reactions may occur if the chemicals are mixed in certain proportions. The result
may possibly be an eruption and tank rupture. Such an occurrance must be prevented.
Water may also have to be considered in this respect.
Fig: Chemical carrying at sea
Leakages through bulkheads occur at times in any tanker. Normally, however, such leakages are only minor seep
ages. They will not cause any violent reaction due to the great disproportion in mixture from dangerous
proportions. But legislation as expressed in the IMO Chemicals Bulk Code ref (25), and in the US Coast Guard
Rules ref (18) and Appendix 3 expressly prohibits the placement of inter-reactive cargoes on both sides of a
bulkhead. There must be an empty tank, a cofferdam or a tank with a cargo neutral to both products in between. This
requirement causes some headaches in cargo planning. "Diagonal contact'' between tanks is normally considered as
sufficient separation between reactive cargoes.
More important, however, is the complete separation of piping systems so that one product cannot inadvertently be
pumped into another. To this effect strategic pipe bends may have to be removed and blind flanges fitted on each
pipe end. Modern chemical tankers will have blind flange valves fitted . Such a
blind flange valve must have a double separation between the products with a drain in the interspace. A single blind
flange is not acceptable. Remember also to separate drain lines or slop connections to avoid the possibility of cargo
mixing.
The cargo inter- reactions may be of type:
a) Chemical reaction: Strong ( inorganic) acid plus alikali (or water) causes heat, e g sulphuric acid plus caustic
soda or water. Therefore sulphuric acid may not be carried in tanks bordering the side shell or filled ballast tanks.
Similarly sulphuric acid may react with a number of hydrocarbons except parraffines (petroleum oils). Amines (
aniline, diethylamine) may react with esters ( butyl, acetate, ethyl acetate). Caustic soda will react violently with
acrolein, acrylonitrile and allyl alcohol.
b) Oxidation : An Oxygen-rich compound like propylene oxide may react with an amine (e g diethylamine) or an
aldehyde (e g acetaldehyde). An ether (e g ethyl ether) may react with oxygen and from a peroxide which is an
explosive hazard. The ether should be inhibited and carried in an inerted (N 2 ) tank.
c) Auto- reaction :
Certain hydrocarbons compounds have a tendency to polymerize with time, accelerated by heat, light, sometimes air
or other matter such as rust. Polymerization means that several molecules of the same kind binding together to
bigger molecules. The compound tends to become more viscous or eventually solidify. Heat is liberated , which
accelerates further polymerization.
Chemically most cargoes are monomers, which means that they, before any polymerization, consist of single
molecules.
Toxic vs edible products
Toxic products must never get mixed into edible products for human or cattle feed! In this case minor seepages
between tanks might prove disastrous.
IMPORTANT: Edible products should never be loaded
with bulkhead to bulkhead contact with toxic cargoes! The piping systerns should be entirely segregated or provided
with double blind flanges.
Chemicaltankerguide.com is merely an informational site about various aspects
of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage,
Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and
Chemical Processing. If you are
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