The 106-PR-S and 206-PR-S pressure reducing valves with downstream surge control are based on the 106-PG or 206-PG main valve.
Pressure reducing pilot valve senses the downstream pressure through a connection at the valve outlet. Under flowing conditions, the pilot reacts to small changes in pressure to control the valve position by modulating the pressure above the diaphragm. The downstream pressure is maintained relatively steady at the pilot set-point.
The surge pilot senses the downstream pressure. If the pressure rises above the pressure reducing pilot setting and reaches the surge pilot setting, the surge pilot opens in order to close the main valve rapidly.
For more information on the 106 / 206-PR-S pressure reducing valve: