The SSV is normally located as a wing valve on a high- or low-pressure wellhead Christmas tree. This name is derived from the tree-like appearance of the valves and fittings branching out from it. A manual valve must be installed between the SSV and the well to allow for maintenance. See Figure 1700-7.
The type of valve used in the flowline for a shutdown valve is usually a reverse gate valve. This valve is well suited for this application due to its self-closing feature.
The valve consists of a gate assembly that operates at ninety degrees to the pathway through the valve. The valve stem and gate rise to effect closure. This stem action is opposite the stem action of a normal gate valve.
The internals of the valve are designed so that the body pressure generates a force on the gate and stem in the upward direction, always tending to drive the valve shut.
A diaphragm- or piston-type of actuator is used with a reverse gate valve. The valve is opened by applying pressure above the diaphragm, which drives the stem down.
To close the valve, the pressure is removed from the diaphragm. The flowline pressure drives the gate stem upward, closing the valve. A spring, located below the actuator diaphragm, will also close the valve when equal pressure is present on both sides of the valve.
The actuator must be sized above the maximum anticipated operating pressure or for the maximum allowable working pressure (MAWP) of the valve itself. A safety factor of about 25% for wear and friction losses in the future should be added. Diaphragm actuators are presently used up to 15,000-psig design pressures.
Manual overrides for an SSV can be provided on land but are not allowed offshore.
Three types are available:
• Lockout cap
Lockout caps should be furnished with a fusible insert so that the valve will close in case of a fire.
A diaphragm-actuated valve is shown in Figure 1700-8. All surface safety valves should have a firesafe seal on the shaft and an external relief valve on the actuator housing. A fusible link has been installed on the pressure line to the actuator in some areas.
Many wellhead SSVs require a quick bleed or quick exhaust valve on long actuator supply lines to ensure that the valve closes quickly enough. See Figure 1700-9. SSVs should be provided with some means to visually indicate to the operator whether the valve is open or closed.