Pressure/Vacuum Valve

A pressure/vacuum valve is a self-actuating safety fitting mounted on each cargo tank vent outlet that limits the range of internal tank pressure during loading, discharge, and temperature changes due to insolation or cooling. The pressure-relief side opens when tank pressure exceeds the set point — typically between 1,400 and 2,000 mm water gauge on crude tankers — venting cargo vapours safely to atmosphere. The vacuum-relief side opens when the tank pressure falls below about 350 mm water gauge vacuum, admitting air or inert gas to prevent the tank structure from being deformed inward. Since both a catastrophic over-pressure and an unexpected vacuum have caused major tanker structural failures, the proper functioning of PV valves is mandatory under SOLAS II-2 and IMO MSC/Circ.677. Engineers inspect valves for correct set pressure, freedom of movement, and condition of the pallet seats and weight before each voyage and after cargo operations in ice conditions where freezing can cause the valve to stick open or shut. Tanker companies include PV valve inspection in their shipboard safety management procedures, and classification society surveyors verify valve condition and test records during annual and special surveys.