Relief pilot-operated

Pressure Relief Valves Definitions for pressure relief valves, relief valves, pilot-operated pressure relief valves and safety v alves, are found in the ASVIE Boiler and Pressure Assel Code, Section Division 1, Rules for Construction of Pressure Assels, Paragraphs UC-125 and UC-126, The pressure-relief valve is an automatic pressure relieving device designed to open when normal conditions are exceeded and to close again when normal conditions are restored. Within this class there are relief valves, pilot operated pressure relief valves, and safety valves. 

Pilot-Operated Pressure Relief Valve - A pilot-operated pressure relief valve is one that has the major flow device combined with and controlled by a self-actuated auxiliary pressure reliefs valve. This type of valve does not utilize an external source of energy. 

Remote Depressuring - A pilot operated valve is sufficiently positive in action to be used as a depressuring device. By using a hand valve, a control valve or a solenoid valve to exhaust the piston chamber, one can open the pilot-operated valve and close it at pressures below its set point from any remote location, without affecting its operation as a pressure relief valve. 

Sizing of Pilot-Operated Pressure Relief Valves... 

Sizing methods for pilot-operated pressure relief valves are in accordance with the accepted formulas described above, utilizing the appropriate discharge coefficients and effective orifice areas as recommended by the valve manufacturer. The following points should be noted ... 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position... 

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

A disadvantage of pilot operated valves is that, if there is no pressure in the vessel, back-pressure could cause the disc to lift. This could occur if t was shut-in and depressured for maintenance, the relief val ve wa led in a header, and another valve in the header was opened,... 

The set points for pilot-operated and balanced-bellows relief valves are unaffected by back-pressure, so they are able to tolerate higher backpressure than conventional valves. For pilot-operated and balanced-bellows relief valves, the capacity is reduced as the back-pressure goes above a certain limit. 

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

The percent absolute back-pressure for conventional and pilot-operated safety relief valves is ... 

Calculate orifice size for pilot-operated relief valve. 

Taxonomy No. 4.3.3.1 Equipment Description PRESSURE - SAFETY RELIEF VALVES-PILOT OPERATED ... 

When properly designed, this type of valve arrangement conforms to the ASME code. It is a pilot operated pressure relief valve in which the major relieving device is combined with and is controlled by a self-activating auxiliary pressure relief valve. See Figures 7-5A and B. 

Figure 7-5A. Pilot operated safety relief valve. By permission, Anderson, Greenwood and Co.

WThen the pressure rise in a system is gradual and not explosive in nature, a safety or safety relief valve is the proper detdce, but when it is critical to completely depressure a system or the rate of pressure increase might be expected to be rapid, then a rupture disk is the proper device. Properly designed a pilot operated valve may be selected after checking its performance with the manufacturer. 

Typical pilot-operated pressure relief valve installation... 

Radioactive decay contributed to further heat buildup. This caused water in the primary circuit to boil. The pilot-operated relief valve on the... 

Backpressure has no effect on the set pressure or flow capacity of pilot-operated pressure relief valves except when the flow is subcritical (ratio of absolute backpressure to absolute relieving pressure exceeds 55%). In this case, the flow correction factor Kb (see Appendix B) must be applied. If the ratio of absolute backpressure to absolute relieving pressure is less than 55%, no correction factor is required, Kb = 1. 

Reclosing relief devices are mainly safety valves, relief valves or SRVs (either weight, spring or pilot operated), whereas the non-redosing rdief devices are bursting disc or buckling pin devices. 

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. 

Flowing pilot A pilot design that continuously discharges the fluid through the pilot throughout the relieving cycle of the pilot-operated safety relief valve (POSRV). 

Pilot-operated safety relief valve (POSRV) A self-actuated SRV comprising a main valve and a pilot. The operations of the main valve are controlled by the pilot, which responds to the pressure of the fluid. The main valve assures the quantity (capacity) of the valve, while the pilot assures the quality (accuracy) of the operation(Figure 3.14). 

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