A remote emergency shutdown from the platform ESD system must shut down the SCSSV, but only after a nominal 2-minute time delay.
This delay will ensure that the SSV at the wellhead is closed first to allow it to absorb the wear and tear of opening and closing against a differential pressure at flowing conditions. The SSV is much easier and cheaper to repair than the SCSSV.
The time delay, which is adjustable, is accomplished by adding a needle valve and volume bottle in the pneumatic ESD signal line going to a pilot relay that controls the air or gas to the hydraulic pump and dump valve. The orifice in the needle valve restricts the air or gas bleed to the piston of a pilot relay. After about 2 minutes, the three-way valve in the relay will shift and depressure the system. This action will open the hydraulic dump valve, which quickly allows the hydraulic pressure on the SCSSV to be relieved to the supply tank. Refer to Figure 1700-13.