PRV – Simmer; 

Simmer for Pressure Relief Valve (PRV) refers to the behavior of the PRV as close to launch. That is, while the pressure in the system ( Inlet Pressure) is approaching the Set Pressure but not yet Opening / Popping Pressure.

See: What is PRV pop test? (Popping test)

Simmer will create a small opening between Disc and Seat, which we may see Fluid inserted. Come or hear the sound of the fluid inserted through the opening and the PRV occurring Simmer may cause a discrepancy between the Disc and Seat of the PRV.

If excessive simmer, it makes the surface of the Disc wearing to Metal Seated (caused PRV fail by leakages) . PRV generally starts the Simmer at a pressure of 90% of Set Pressure, We can adjust Simmer by adding or subtracting Blowdown.