Pressure-vacuum valve (P/V valve)
A valve which keeps the tank over pressure or under pressure within approved, limits. P/V valve provides for the flow of the small volumes of vapour, air or inert gas mixtures caused by thermal variations in a cargo tank. The vacuum valve opens if the pressure in the tank falls below a pre-set level – to draw air into the tank through a flame trap. In the event of over-pressure, two release valves open to ensure that venting is upwards and at high velocity, to avoid concentrations of gas on deck.