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Risk of tank over pressurisation & cargo tank lid / Hatch securing methods

Improper closing and sealing of cargo tank hatches can be a major cause of cargo contamination. A properly closed and sealed tank hatch/opening will prevent sea water ingress and maintain a positive pressure Nitrogen blanket in the ullage space. Cargo tank hatches must be properly maintained, with the correct type of tank packing fitted and in good condition (no visible damage/deformation, clean and free from cargo odours), any doubt as to the suitability or condition of the hatch closing arrangements must be reported to the Management Office.

chemical tanker navigation at sea
Non-Flanged Hatches/Openings

If the hatch cover is fitted with a stong bar, ensure that the securing end is correctly hooked. As the packing is about to make contact with the coaming, visually inspect contact points to insure that the packing is exactly centred on the top of the coaming edge. When hatch cover is properly aligned, correct pressure must be applied to ensure an even tightness between the hatch coaming and the lid.

Modern Chemical tanker underway

Fig:Modern Chemical tanker underway

For hatches with more than one securing dog, the dogs must be tightened sequentially in a criss-cross pattern (12 & 6 o`clock, 9 & 3 o`clock)

When the hatch is properly secured, inspect the actual sealing

Flanged Tank Washing Openings

For hatches with bolts and studs, the bolts must be tightened sequentially in a crisscross pattern (12 & 6 o`clock, 9 & 3 o`clock)

Whenever there is a doubt regarding the tightness of the tank packing, silicon sealer may be applied (silicon may, in some cases, damage packing and should not be used on packing that provide proper sealing in itself). The silicon sealer should be applied on the edge of the hatch coaming before closing down the hatch cover (ensure the hatch is clean and free of rust particles). Whatever action is necessary to prevent water ingress to the tank lid must be taken
When replacing packing on tank hatches, the instruction given by hatch and packing manufacturer must be followed and the edge of the tank hatches/openings must be clean and free from corrosion. Carry out tank Vapour tightness test as per USCG 40 CFR.61.304(f)

Risk of tank over pressurisation

Tank over pressurisation accident the resulting damage could be significant, taking a vessel out of service for several months. In an incident a chemical tanker was loading ethanol in the USA and due to winter conditions the ambient temperature was below zero degrees Celsius. At this time the vessel involved in an accident of tank over pressurisation. The resulting conclusion : It is clear that there was a failure of the primary and secondary means of venting . Maintenance and set points :-
P/V Valves - to be inspected regularly and thoroughly overhauled and decarbonised at least every six months, or more frequently, if specified by the manufacturers’ instructions. In addition. annual testing of each PV valve is to be carried out in the workshop to ensure that both sides of the valve open at the specified pressure and vacuum. All such maintenance is to be recorded in the ships PMP.

With regards to this particular incident it is suspected that the PV valves may have been frozen due to ambient temperatures. Vessels trading in cold weather areas it is essential to check that PV valves and any deck seal heating systems are fully operational.

Loaded Passage

Particular attention to carriage instructions during the loaded voyage is essential in order to maintain product quality integrity which may be affected by variations of temperature, inhibitor levels, moisture and/ or the oxygen content of the containment tank vapours.

Certain chemical products are shipped with specific requirements for the minimum and/or maximum allowable temperatures of both the commodity in question and those parcels stowed in adjacent cargo tanks to ensure that each does not affect the other.

Chemicals sensitive to degradation by oxygen are usually required to be loaded into tanks which have previously been purged with nitrogen to a specified reduced oxygen level. The inert atmosphere must be restored on completion of loading and, by routine monitoring, maintained inert throughout the voyage.

Chemically unstable products, notably those which polymerise over the passage of time are normally shipped with an inhibitor added to prevent the reaction. The inhibitor system may require specific tank atmosphere conditions and during a long voyage it may be necessary to add extra inhibitor if routine cargo tests indicate a decline in inhibitor activity. It is also essential to monitor the temperature of such cargoes.

Related Info:

Types of Chemical tankers at Sea

Available shipbuilding materials for modern chemical tankers

Pre-cleaning /washing of cargo tanks

Final cleaning of cargo tanks prior loading

Tank cleaning and posoning hazards

Testing of tanks and cargoes

Practical tank cleaning methods for various noxious liquid cargo

Special tank cleaning method

Determining proper tank cleaning by acid wash method

Supervision of all tank cleaning and gas freeing operations

Disposal of tank washings, slops and dirty ballast - safe method

Chemical Tanker Guide !
Shipboard safe practice relating to seagoing chemical tankers

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