9 . 0 O V E R P R E S S U R E R E L I E F API6D trunnion mounted ball valves with single piston effect seats (SEP) are designed to automatically release cavity overpressure into the pipeline automatically using “self relieving seats”. Since body cavity and pipeline are isolated in the fully open and fully closed position, pressure trapped in the body cavity may be different than pressure in the line. Examples of this situation are during pipeline de-pressurisation, or heating of body cavity causing a sudden pressure build-up. Overpressure in the body cavity will pop the seats off the ball surface, and excess pressure will be relieved into the pipeline. After the pop-off action, the seats will return to their normal position, in contact with the ball. The pop-off action is triggered by differential pressure between body cavity and pipeline. The body cavity pressure should not exceed in 1.33 times the valve pressure rating at the specified maximum operating temperature; however, pop-off action usually happens long before, with just a few bars of differential pressure. For standard API6D ball valves, both seats are self relieving. However, on request valves can be ordered with just upstream or just downstream seat as self relieving. Also “double piston effect” (DPE) seats can be specified. DPE seated valves must have an external relief safety valve fitted to the body which vents to atmosphere. Due to environmental requirements this must now be plumbed back into the upstream side of the valve or recycled elsewhere. Refer section 10.0 for more information. Australian Pipeline Valve - Installation, Operation and Maintenance Manual 20 SIDE ENTRY 2P TRUNNION BALL VALVES API6D STANDARD - FS9000 SERIES If there are hydrates in gas service, the drain fitting may freeze during high- pressure draining. Work slowly and ensure that the pressure trapped in the body cavity has been completely released, by opening and closing the drain fitting several times. Both seats are self relieving hence over-pressure can be relieved up or/and downstream. Custom ‘single piston’ seats can be specified on one side, and ‘double piston’ seats on the other side to allow pressure to only automatically relieve just upstream or just downstream.