The pneumatic wellhead control system must be pressurized in order to operate. The interface with the process or platform shutdown system is accomplished by using a pilot relay. Pilot relay is short for a “pilot operated, three-way valve or relay.”
The relay consists of a three-way block and bleed valve and a pneumatic piston or a pressure pilot on the other end. The relay is mounted inside the wellhead control panel. This pilot relay will automatically reset when the process shutdown system has been repressured or brought back to a normal situation. With a signal applied from the remote process or platform shutdown system, the three-way pilot valve allows free passage of the pneumatic holding pressure. When the remote signal is removed, the relay will switch, block in the supply pressure, and vent all the downstream holding pressure. This series of actions will initiate a wellhead shutdown by venting the pneumatic signal from a manually reset pilot relay that controls the surface safety valve (SSV).