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Procedure for carriage of inhibited flammable chemical products in cargo tanks

Why chemical inhibitor is required ? In certain conditions of heat, pressure and in the presence of Oxygen, some chemical cargo types can become viscous and possibly solid and dense in nature. This self reaction can cause some cargoes , especially in the presence of high temperatures and Oxygen, to begin an exothermic reaction, becoming self heating and rapidly expanding which may result in possibly disastrous consequences for the vessel.

As a precaution against this, a chemical inhibitor may be added to prevent the cargo from bonding with itself, however, one aspect of inhibitors is that they sometimes require Oxygen to activate them and this means that the tank cannot be inerted. When such a situation exists, the management Office must be contacted. See IBC code regarding carriage of inhibited flammable products in cargo tanks of more than 3000m3 and using inerting.

There are many inhibitor types, most of which are toxic and need to be handled with care. Usually the inhibitor is added by the Terminal personnel during the loading programme.

Chemical tanker deck area
Fig : Chemical tanker cargo pipelines


Shippers of inhibited cargoes must advise the vessel (and present an inhibitor certificate onboard prior to loading) of the quantity of inhibitor added, the hazards of the inhibitor, the time validity of the inhibitor, the temperature parameters within which the inhibitor will work and the emergency actions should these be exceeded. Masters are to check that the Inhibitor validity is sufficient for the voyage length.

The vapour of the cargo will not necessarily contain inhibitor as the two liquids will have differing evaporation properties. Therefore, it is possible for some solid polymer build-up to occur in the tank vents / screens, these must be verified as clear during voyage and prior to commencing discharge in order to prevent the possibility of damage from under pressure being created in the tanks during the discharge.

The temperature of inhibited cargoes must be checked and recorded daily in order to be able to note any abnormal rise that may indicate either inhibitor failure and/or polymerisation. Notice of any rise or excessive temperatures should be notified immediately to the Management Office with a request for the action to be taken.

Inhibited cargoes often need the presence of some oxygen in the tank atmosphere in order to permit the inhibitor to work properly. The minimum level of oxygen is usually stated on the inhibitor certificate but, as a general rule, a cargo containing an inhibitor that needs oxygen should not be carried in an inerted tank.

If nitrogen is bubbled through an inhibited cargo (such as when compressed nitrogen is used to clear the cargo hose after loading) the nitrogen will deplete the oxygen dissolved in the liquid, thereby requiring the inhibitor to take oxygen from the atmosphere. It is possible that excessive nitrogen used for blowing through might linger in the ullage space





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Carriage of inhibited flammable chemical products in cargo tanks
In certain conditions of heat, pressure and in the presence of Oxygen, some chemical cargo types can become viscous and possibly solid and dense in nature. This self reaction can cause some cargoes , especially in the presence of high temperatures and Oxygen, to begin an exothermic reaction, becoming self heating and rapidly expanding which may result in possibly disastrous consequences for the vessel.

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Practical example of solving tank cleaning problems

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