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Isolation method of cargo tanks and piping systems for chemical tankers

When a single parcel of cargo is carried in several tanks served by a common pipeline system, containment within each tank depends upon the tightness of the inlet valve. Due to the pressure differential on either side during sequential loading into or discharge from such tanks, the tightness of a single valve should not be relied upon to prevent the entry or escape of cargo.

It is advisable to achieve two valve separation between completed tanks and pump pressure in the pipeline. Regular checks should be made on completed tanks to ensure that the level of cargo does not change.

More thorough segregation may be necessary when different parcels of cargo are carried in tanks served by common systems. In particular when carrying toxic cargoes, it is a requirement that cargo piping and pumps, as well as tank venting systems, are separated from those containing other products, to prevent any leakage causing toxic contamination of non-toxic products and subsequent exposure of personnel unaware of the contamination.

Modern Chemical tanker hellespont credo underway
Fig : Modern Chemical tanker hellespont credo underway

Ships with common pipeline systems can achieve the separation required when carrying toxic cargoes by inserting two blank flanges (spectacle plates) with a bleed drain in between, or by removing a spoolpiece and using blank flanges strong enough to withstand the pressure expected. The engineering principle of using two stops, with a provision for detecting if one of the stops does not hold tight, ensures that a leak can be spotted immediately.

The requirement is met on many chemical tankers by having separate pumps, pipelines and vents so that segregation is achieved by design.

When blanks or a removable spool piece have been used, attention should be paid to restoring the system when the extra separation is no longer needed.

Related Info:

How to prevent cargo pipeline leakage

Pre-cleaning /washing of cargo tanks

Final cleaning of cargo tanks prior loading

Tank cleaning recommended methods

Testing of tanks and cargoes

Personal safety for working in cargo tanks and other enclosed spaces

Pump room hazards & safety precautions

Practical tank cleaning methods for various noxious liquid cargo

Ship shore safety checklist while alongside a terminal

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