Risk Of Cargo Contamination During Loading At Chemical Tankers

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  • Handling noxious liquid chemicals in bulk got numerous difficulties.
  • On oil tankers it is normal to have a common cargo tank ventilation system.
  • Many cargo samples are taken during the loading of chemicals.

Handling noxious liquid chemicals in bulk got numerous difficulties. Outlined below are some problems associated with various cargo systems found in tankers, says an article published on chemical tanker guide website. 

Problems associated with various cargo systems

There is a high risk of contamination when a common pump is used for several chemical tanks, if the system is not properly drained between different cargoes.

Modern tankers are often equipped with individual submersible cargo pumps but if these are connected to common or shared lines there is still a risk of contamination.

There will normally be a drain cock near or on the cargo pump itself, this drain cock should be opened to verify that the line has been properly drained before pumping a different cargo. In addition most of these pumps are hydraulically driven, damage oil seals can lead to hydraulic oil leakage into the cargo tank contaminating sensitive cargoes.

On oil tankers it is normal to have a common cargo tank ventilation system. The vapours from one cargo tanks can easily enter a different tank in the system. This may result in cargo contamination or change the flash point of the product. Petroleum products are classified into volatile and non-volatile cargoes. 

A cargo with a flash point below 60°C is a volatile product and a cargo with a flash point above 60°C is a non-volatile cargo (different rules apply to the handling of volatile and non-volatile cargoes).

Diesel oil has a flash point of around 63°C and vapours from a volatile cargo can easily change the flash point to below 60°C, causing the cargo to be re-classified. Many lube oils and lube additives are heated during transport. Steam coils are normally used for this purpose. 

A leaking heating coil can lead to water entering the cargo tank and consequently contaminating the cargo. Laboratory tests should be carried out on cargoes contaminated by water. It should be established whether the water is fresh or salt water. 

Contamination caused by fresh water is most likely to be caused by a leaking heating coil whereas salt water contamination would probably be caused by a leaking tank hatch, in which case the hatch packing should be checked.

Chemical tankers are normally constructed with completely independent cargo systems. Each cargo tank will have an independent pumping and venting system.

Cargo contamination in chemical tankers are often caused by poor cleaning of cargo tanks or pipe lines. Unfortunately many chemicals are extremely sensitive to contamination, just a few parts per million of a previous cargo can contaminate an entire shipment.

Many cargo samples are taken during the loading of chemicals. When a cargo is contaminated it must be established whether the cargo was effected prior to loading; or during its transportation on board. Cargo transfer hoses may also contribute to cargo contamination, therefore hoses should be properly cleaned in between different cargoes.

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Source: chemical tanker guide 

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