Modulating pilot-operated valve flowing

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

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. 

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. 

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. 

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