VOCstop and low smoking oil barrels

Recent reports from CRUCOSGA and the National Academy of Science estimate that 0.2% of a crude oil tanker?s cargo is lost through vapour emissions. Over the course of a year the total loss is estimated at more than 6 million tons.

Much of the loss is a result of crew manually venting cargo tanks to release pressure. The vessel?s command carries this out when pressure approaches the pre-set opening pressure of the vessel?s p/v valves (normally about 1,500mm WG). Yet the command does not know when or at what pressure a manually controlled release should be stopped. The consequence is unnecessary boil-off as the p/v valves remain open until the pressure in the ullage spaces is far below the true vapour pressure of the volatile gases in the crude oil.

As a result Intertanko has developed the VOCON procedure, which utilises existing onboard equipment to help the crew minimise unnecessary cargo boil-off. The procedure uses the different physical behaviour of unsaturated gas from the inert gas supply and saturated hydrocarbon gas to determine when the rate of pressure decrease becomes constant and therefore when manually controlled release should be stopped.

Danish valve manufacturer Pres-Vac Engineering is fully supportive of the procedure. However it argues that a venting system capable of maintaining a constant pressure of 1,200-1,400mm WG would reduce VOC emissions even further and could do so without manual interference or supervision. This is the essence of its new VOCstop system. To maximise its potential market and in recognition that many commands will still distrust automatic venting systems, the company has also developed the VOCONvalve, a dedicated central relief valve with remote control, for ?manual? use with the VOCON procedure.

The company reckons that venting arrangements on most tankers are inappropriate to the task they are fitted for and the environment in which they must work. It says that venting systems are an identified risk according to the definition of the ISM Code and that turning to non-controlled manual release of gas is not how the authors of the Code envisaged such a risk would be tackled.

It says the new ISO standard 15364 for marine p/v valves opens a window of opportunity for owners wanting specifications for venting equipment to solve the in-service reliability problems that are the reason for manual gas releases.

Its recommendation is to use non-oscillating HS-ISO type valves in the venting system. It says such valves secure crew confidence in the venting system through giving no false alarms (with the alarm able to be set a maximum 50-100m WG above the vent?s set pressure because of the absence of any pressure surges), by having visual disc position indicators, by enabling visual determination when cleaning and through allowing parts replacement on-deck without machining and without interrupting operations. It says the performance of non-hammering valves requires a large margin to be set between set-pressure and alarm point and therefore endangers the all-crucial crew confidence.

The company believes that maintaining a constant tank pressure is the best way to instil crew confidence in the equipment?s reliability and that the HS-ISO type valve, which conforms to its VOCstop definitions, is able to maintain constant tank pressure with no risk of false alarms.

It admits that some owners might question a higher in-transit pressure, but points out that others, such as Saudi Arabia?s SAMCO, have already opted for designs that allow them to increase the standard pre-set opening pressure of a VLCC?s P/V valves to 1,700-2,000mm WG. Pres-Vac says over-pressure incidents occur because of faulty valves not because of designs calling for too high working pressures. The maximum tank pressure on a VLCC with non-oscillating HS-ISO type valves with a set pressure of 1,700mm WG is less than with ordinary non-hammering vents with a set pressure of 1,400mm WG, it says.

The company believes that preventing excessive boil-off is fundamentally better than a system that re-generates or re-circulates the boil-off as well as being cheaper to install and maintain. So confident is it of the system?s ability to prevent unnecessary cargo boil-off that it has made an amazing promise. “We have told customers that if they could not save $100,000 a year they would get it for free,” says Eric S?rensen, sales engineer at Pres-Vac.