A shutdown signal from a plant or platform ESD to the wellhead control system is usually caused by a possible dangerous or damaging condition existing downstream of the wellhead.
It is important that the wellhead shutdown system be designed with a manual reset pilot relay. See Figure 1700-1. The manual reset pilot relay has a knob on one end as well as the pressure pilot on the other end of a three-way valve. The three-way valve will switch and shut down the surface safety valve either when the knob is pushed or the pilot is depressured.
This means that when the shutdown signal is initiated, the system will trip and stay shut down. Clearing of the possibly dangerous or damaging condition will not automatically reset the system and open the surface safety valve. The only way the SSV or the SCSSV can be opened is for the production operator to manually reset this shutdown relay by pulling the knob. Pulling the knob sets a pin that latches the three-way valve until pressure is applied to the pilot. The pin will disengage when the pilot is pressured, thus activating this shutdown relay.
This relay should not have a lockout feature that disengages the manual reset, which would allow the surface safety valve to automatically cycle shut and open. The design of this safety relay should require that it can only be latched in the open position while no instrument pressure is applied to the pilot diaphragm.
Indicators are available to enable the operator to tell at a distance that a wellhead surface safety valve is shut down. On panels controlling multiple wells, indicators for the valve signals are very helpful to the operator. These indicators can be integral to the relay knob, or they can be a separate panel-mounted device.
It is important to determine that the possibly dangerous or damaging condition has been corrected and to make sure that the operator is present at the wellhead to monitor the well while it is placed back into operation.