It automatically releases the air vapours from the tank during the filling operation thus preventing the tank from bulging out due to over pressure and breaths in air in the event of tank being emptied thus preventing the tank from collapsing due to creation of vacuum. It has two seats i.e. Pressure Seat & Vacuum Seat. The pressure seat opens when the pressure in the tank exceeds the atmosphere pressure, thus allowing escape of air & vapour from the tank during filling operation and conversely when the tank is being emptied, the pressure seat closes and the vacuum seat opens, thus allowing air from atmosphere to breath in. So this Valve always struggles to maintain atmosphere pressure within the tank. They are available in C.I./G.M. Body with flanged ends to BS:10 Table-E/F and tested to 300 Lbs. Hyd.
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