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The role of valve actuation in a VOC emission control system

25 March 2018

Minimising the emissions of VOCs (Volatile Organic Compounds) from oil tankers during the fluctuating ambient conditions experienced during sea voyages is vitally from both environmental and commercial points of view.

The controlled release is often undertaken when the gas pressure approaches a pre-set point. However, it is not always clear at what pressure a manually controlled release should be stopped. Without this information, excess vapours can be released, causing air pollution and a loss of cargo.

To meet this challenge the VOCON Valve and Reporting System controls the vapour pressure in oil cargo tanks to minimise and fully control VOC emissions. Designed to comply with the latest international rules and regulations it is equipped with the most advanced reporting system available.

At the centre of the system, a venting control valve operated by a Rotork CMA electric process valve actuator is installed on the bypass line between the inert gas main pipeline and the mast riser. In automatic mode the actuator modulates the valve position in response to a control signal from a pressure transmitter to control the vapour pressure in all the cargo tanks. This critical duty reduces VOC loss by maintaining a constant pressure in the cargo tanks during the voyage.

The compact actuator selected for this duty is environmentally sealed to IP67 and certified for use in Zone 1 hazardous areas. Its ambient operating temperature range of -20 to +65ÂșC is said to facilitate long-term reliability and maintenance-free operation in the exposed environments experienced on the decks of oil tankers. Accepting an industry-standard 4-20mA control signal, resolution is 0.2%, delivering the accurate, repeatable and backlash-free positional control demanded by the VOCON application.


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