As described above these valves direct the regulated power fluid to the desired side of the preventer, valve or connector operating piston and vent the fluid from the other side of the piston to the sea. The annular preventers typically use large 1 V2" SPM valves in order to provide sufficient fluid flow, the ram preventers use 1" valves and the other functions such as failsafe valves and connectors use %" valves. Figure 65 shows an example SPM valve.
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The valve is a poppet type in which a sliding piston seals at the top and bottom of its travel on nylon seats. In the normally closed position a spring attached to the top of the piston shaft keeps the piston on the bottom seat and prevents the power fluid from passing through the valve to the exit port. Power fluid pressure, which is permanently present, also assists in keeping the valve closed by acting on a small piston area on the spindle. In this position fluid from the valve's associated operating piston is vented through the sliding piston at ambient conditions.
When pilot pressure is applied to the valve the sliding piston moves up and seals against the upper seat which blocks the vent ports and allows regulated power fluid to flow through the bottom section of the valve to function the BOP. Note that the pilot fluid therefore operates in a closed system whilst the hydraulic power or control fluid is an 'open' circuit with all used fluid being vented to the sea.
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