Valve simmer

The application of pilot-operated PR valves is limited to clean services, where they are an acceptable alternative to the bellows valve if a balanced characteristic is required. Their particular advantage is freedom from "simmer", which may permit a closer operating pressure/set pressure margin than normal. 

Simmer the audible or visible escape of fluid between the seat/disk of a pressure-relieving valve at an inlet static pressure below the popping pressure, but at no measurable capacity of flow. For compressible fluid service. 

Separator, wall wiper, 265 Physical arrangements, 265 Set pressures, safety relief valves, 425 Simmer pressure, 412 Sizes, air-borne solids, 227 Dispersed, 226... 

Chatter, simmer or flutter Abnormal, rapid reciprocating motion of the movable parts of a PRV in which the disc makes rapid contacts with the seat. This results in audible and/or visible escape of compressible fluid between the seat and the disc at an inlet static pressure around the set pressure and at no measurable capacity, damaging the valve rapidly. 

Have too long a simmer Dual ring control, valves (ASME I) may fail to achieve full-lift at allowable overpressure limit if the nozzle ring is set lower than that recommended, a situation which could lead to a catastrophic event as it could take too long for the valve to react on the overpressure condition. 

The addition of a skirt, shown in Figure 5.2, creates a secondary area on the seat assembly. This provides a larger area for the inlet pressure to act on upwards as the valve begins to open, and redirects the flow downwards both boost the valve to open more quickly with less simmer (seat leakage as the SRV approaches set pressure) and overpressure... 

The main requirement for thermal relief valves in cryogenic conditions is to reduce their freezing risks at any cost (Figure 11.2). Therefore, we must select valves with low simmer, a rapid pop/snap opening and high seat tightness. We need to reduce unnecessary product loss, so again low simmer and preferably a short blowdown is required. 

The highest risk is when the cryogenic medium (which is usually ultra dry) comes in contact with the moisture present in the atmosphere, for instance, due to a simmering or leaking valve. The cold medium will freeze up the moisture around the seat and disc and in a short time it will become a solid block of ice around the seat area, which will prevent the valve from operating properly, or at all, creating a very dangerous situation. 

Limited-volume testing often fails to result in a distinct pop. (A pop is defined as the rapid opening of the SRV with its associated audible report.) If a valve only simmers instead of an actual pop, internal parts can be misaligned on some SRVs, resulting in leakage [23] (see Fig. 12.20). 

---