If three sensors are used to shut down the surface safety valve (e.g., high-pressure, low-pressure, and sand probe), then first out indication may be desirable. Most wellhead control panels in the Company have not required this feature in the past, but new installations are beginning to put them in.
First out indicators can help during the testing of pressure sensors, especially when multiple wellhead control panels are installed. The sensors are mounted on the flowline and pilot relays are installed in the wellhead control panel. Panel indicators are connected to the outlet port of each of the pilot relays. From written instructions, the operator can determine which sensor tripped by observing the number of tripped indicators. The knob on the sand probe could also be used as an indicator, but because it is mounted remotely, it may not be visible from the panel.
A better way to do the same thing is to install first out indicators (e.g., Amot Model 4400) which have internal porting and will only indicate the sensor that tripped. See Figure 1700-16.