Pilot-operated pressure relief valves

Because the PRV piston has larger area on the pilot valve side than its area on the process side, the PRV piston will be closed even tighter as process pressure approaching its set pressure. This is quite a different feature comparing to the spring-loaded PRV. 

Some pilot-operated PRV uses diaphragm instead of piston to control the PRV open or close. This kind pilot PRV is usually used for low pressure service (3 inch water column to 50 psig). 

One unique advantage of the pilot-operated PRV is that the lift of the piston does not depend on the back pressure of the system and the performance of the valve is not affected by the built-up back pressure. 

Process engineering and design using Visual Basic 

The pilot can vent to the atmosphere or to the valve outlet if atmosphere venting is not allowed. In case the pilot is vented to the atmosphere, the performance of the valve will be unaffected by the back pressure of the valve. In case the vent is connected to the valve outlet with variable pressure, a balanced-type pilot will be required for proper performance of the valve. 

The performance of pilot-operated PRVs depends on the pressure balancing line. If this line is blocked, pressure balancing between the top and bottom of the piston will not be possible, and the valve will lift at comparatively low operating pressure. This is the major disadvantage of pilot-operated valves. This type of valve can be recommended only if the operating fluid is clean. The pilot-operated valve is not very popular for hydrocarbon service. 

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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. 

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 ... 

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. 

Typical pilot-operated pressure relief valve installation... 

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. 

Pressure relief device (PRD) is a safety device to protect equipment or piping from damage due to overpressure or vacuum, but in this chapter, we wiU discuss FRD to provcfit overpressure only. Commonly used pressure relief devices are spring-loaded and pilot-operated pressure relief valve (PRV), rupture disk, or pin-actuated pressure relief device. For equipment, it is installed cither directly on the equipment or at its inlet/outlet piping. When the equipment or piping pressure reaches the pressure relief de vice set pressure, it will open to relieve fluid inside the equipment or piping to avoid overpressure. The relief fluid can be vapor, liquid, or two phase mixture. The relieved fluid is either sent to floic system to be disposed or relieved to atmosphere. 

Among the relief devices mentioned above, spring-ioaded and pilot-operated pressure relief valve (PRV) are the ones used most fiequently, PRV is a reclosing pressure relief device, since after it is opened and relieved, it will be reclosed, when equipment or piping pressure is dropped below PRV set pressure. Once it is dosed, h will continue to protect the equipment or piping from overpressure- Rupture disk or pin-actuated relief device is the non-reclosing pressure relief device, since it cannot be reused after it is ruptured or actuated - its rupture disk or pin needs to be replaced. 

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