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Wednesday, October 17, 2018


A recent enquiry has prompted INTERTANKO to advise on the reasons and problems surrounding the transfer of VOC gases/vapours from one cargo tank to another particularly onboard product tankers when carrying multigrade cargoes of petroleum products.

Although every attempt is made during the loading, transportation and discharge of petroleum products to maintain a complete segregation between the variety of grades carried, it is often forgotten that there remains a common connection between all the cargo tanks – the Inert Gas pipeline system.

If the cargo parcels carried are of diverse volatility then this common system will allow the transfer of vapours from one tank to another unless the tanks carrying the more volatile cargoes are isolated from the common Inert Gas pipeline system. Isolating these tanks, however, by shutting the IG inlet valve to the respective cargo tanks - assuming that they are gas tight – will prevent such transfers taking place but does expose the vessel to vapour spaces above cargo volumes whose over/under pressure is uncontrolled and not monitored by the centralised pressure gauges. This could, of course, lead to unsafe circumstances for the vessel.

How does this transfer occur? A volatile hydrocarbon liquid will seek to achieve an equilibrium vapour pressure with its associated vapour phase given the temperature of the liquid phase and the extent of the vapour space available. If the associated parcels of cargoes onboard are of low or non existent volatility then the volatile hydrocarbon will utilise the whole of the available vapour phase in all tanks to achieve this equilibrium vapour pressure given the common connections with other cargo tanks through the Inert Gas pipeline connections.

Notwithstanding the presence of Inert Gas pressure in the cargo tanks the VOC vapour will travel to the other cargo tanks at lower total pressures until the equilibrium is reached for the total vapour phase in the system due to the combining of pressures – Dalton’s Law of Partial Pressures.

Once VOC vapours have entered the vapour system for alternative cargo tanks it is capable of “condensing” into the liquid cargo when the equilibrium vapour pressure varies with a change in the liquid phase cargo temperature of the volatile cargo/es. Thus VOC in a “liquid phase” can be absorbed into the low or non volatile liquids stored in alternative cargo tanks.

How does this contaminate a cargo with the very small proportions of VOC “liquid”? The flash point of petroleum products is an important parameter for many product cargoes. However, flash point does not reflect the average mixture of the diverse hydrocarbon compounds in the product but reflects the product’s ability to evolve a flammable vapour mixture given the constraints of the test method procedure and the heating of the sample under test. Thus, a small amount of “condensed VOC” can cause an immediate drop in the flash point of a non volatile hydrocarbon product thereby rendering it “off specification” with respect to its minimum specification criterion for the flash point parameter. Very little can be done to avoid this physical problem, from an operational perspective, without taking safety risks onboard.

Contact: Timothy Gunner