Modulating pilot-operated valve

The liquid capacity is >30% total (volumetric capacity) If a spring valve is required, it should be a valve with liquid trim. Preferably, however, a modulating pilot-operated valve can be used. The modulating pilot ensures stability of operation whatever the phase and cannot be oversized as it adapts its flow to the need of the system. 

Figure 4.7 Modulating pilot-operated valve (flowing type). (Courtesy American Petroleum Institute, 1220 L Street, NW, Washington, D.C.)...

An SRV may release the process fluid directly to the atmosphere or release it via a pipe to a flare, scrubber, header or some other equipment. An SRV is always actuated by the upstream pressure and is usually characterized by what is described as a pop action upon opening in the case of compressible fluids. It is important to recognize that one should not expect a gentle release of gas proportional to valve lift, regardless of the design of the valve one possible exception is when a modulating pilot-operated SRV is used. 

Long blowdown required due to inlet pressure losses above 3% On gas service, a pop or modulating action POSRV can be used. On some pilot valves, blowdowns between 3% and 25% can be achieved. In case of very high pressure losses, pilot-operated valves with a remote sensor should be considered. 

Modulating pilot valves shall be exclusively used for two-phase flow conditions. All pilot-operated valves shall be equipped with on-site test connections and equipped with a supply filter with minimum 60 pm element. Pilot-operated valves can be of semi- or full nozzle design. 

Suitable for liquid service where good modulation is required Disadvantages of pilot-operated valves ... 

Modulating action A gradual opening and closing characteristic of some SRVs, particularly some pilot-operated types, in which the main valve opens in proportion to the relief demand at that time. This proportionality is not necessarily linear. 

One can also consider replacing the chattering valve with a pilot-operated safety valve with modulating action. This option would require a new capac-... 

However, a conservative approach, which has been successful for many years in my personal experience, is to limit the liquid phase to about one-third by volume. Anything above that will need at least a liquid trim for a spring-operated valve or a modulating pilot as a solution to this problem. Oil/gas separators are a typical application where this rule of thumb can be applied. 

Operating pressures above 95% of set Preferably use modulating POSRVs or soft-seated valves. The choice for the more expensive modulating pilots depends on system pressure fluctuations. The more fluctuation dose to or over set pressure, the more a modulating valve should be considered. 

Normal operation Spring-operated valves can be used but need to be equipped with a trim suitable for liquid service so they can obtain nominal flow at 10% overpressure. When quick opening (and closing) is required, the operation will be unstable and will possibly cause water hammer. Alternatively, a modulating pilot valve can be used, preferably equipped with a filter. The volume of the filter slows the pilot, which may otherwise react too fast, creating instability and water hammer. 

Comment Pilot-operated rehef valves are available in both pop-action and modulating-achon designs.These valves are suitable for applications where it is desired to maintain system operating pressure very close to the valve s opening pressure (Fig. 5.193). 

Modulating versus pop action pilot valve operation... 

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