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Double block and bleed
A three valve configuration common in shut off applications. Two main shut off valves (block valves) operate on the main process line to stop flow. Then a third bleed valve to a vent can be opened to relieve pressure of remove the process fluid from the region between the two block valves. Typically considered as a 1oo2 voting shut off system provided the bleed valve opening is not critical to achieving the safe state.