The ASME Boiler and Pressure Vessel Code Section VIII sets out requirements for standard pressure vessels and the relief valves protecting them.
Traditionally, direct spring safety valves are the predominate choice for Section VIII applications. The reason for this preference is that direct spring safety relief valve designs are inexpensive and mechanically simple. There is also a confidence in the use of the design because of their use in ASME Section I applications.
The evolution of design and materials of construction has now made pilot operated safety relief valves (POPRV) a viable Section VIII solution. The use of POPRV’s can result in a lower cost of ownership and will provide an improvement to the unit operation that the POPRV is protecting.
ASME VIII requires a pressure protection installation as follows:
The evolution of design and materials of construction has now made pilot operated safety relief valves (POPRV) a viable Section VIII solution. The use of POPRV’s can result in a lower cost of ownership and will provide an improvement to the unit operation that the POPRV is protecting.
ASME VIII requires a pressure protection installation as follows:
ASME Section VIII dictates that a pressure safety relief valve only needs to provide bubble-tight shutoff at 90% of setpoint. As the system pressure reaches 95% of the set point, there is enough system force to begin to overcome spring force and cause the valve plug to lift from the seat causing leakage. This event is called simmering and not only results in the loss of process media, but the impact force between the plug and seat, as simmering is occurring, will affect the metallurgical integrity of the valve’s internals.
The diagram below shows the difference in lift performance between a direct spring and piloted operated safety relief valve.
Pilot operated pressure relief valves use process pressure, instead of a spring, to keep its primary disc closed at pressures below setpoint.
The benefit of a POPRV is that it is possible to operate a system closer to the valve’s set pressure and not have leakage or unwanted opening cycles. This increased operating pressure can optimize the equipment design and allow for the maximum throughput for the process. Since process pressure is being sensed by a small pilot orifice, a POPRV is smaller and lighter than a direct spring PRV design.The diagram below shows the difference in lift performance between a direct spring and piloted operated safety relief valve.
ASME code dictates that any PRV must be shown to be fail-safe if any essential part of the valve is compromised. For example, if the piston seal is damaged and cannot hold pressure, the main valve will fail open. Code also dictates that the pilot must be self-actuated and use the process itself and not an external source to operate. The pilot will use the process pressure to either snap open or modulate the main valve during a relief cycle.
The type and design of the pilot to be used on a POPRV is dependent upon the process media. If the process media is compressible, a full-lift/snap action pilot can be used. A snap-action pilot meets Section VIII criteria with its inherent design which prohibits the process fluid from circulating through the pilot during the relieving cycle.
This is accomplished by using a second pilot seal call the blowdown seat, which will seal off when the pilot relief seat opens. This design keeps the pressure static in the pilot sense line from the process being protected to the pilot valve.
This is accomplished by using a second pilot seal call the blowdown seat, which will seal off when the pilot relief seat opens. This design keeps the pressure static in the pilot sense line from the process being protected to the pilot valve.
This pilot design is referred as a “non-flowing” design. The main valve recloses when the pilot valve senses a reduced process pressure; the relief seat closes and the blowdown seat opens simultaneously thus reloading the dome pressure.
The performance of a snap-action, non-flowing pilot is shown below:
In applications with incompressible media (eg. water), pilot design alterations are necessary to ensure stability during main valve lift. Pilot design cannot allow for the immediate full evacuation of dome pressure to occur at the set point.
Modulating pilots incorporate a design that allow for stable operation with incompressible media, by only opening at the specific amount until the main valve obtains the needed lift to deliver only the required capacity of the overpressure contingency. The benefit of the modulating pilot is that the process media will be conserved during an upset condition, minimize interaction with system control valves and reduce noise levels.
The performance of a snap-action, non-flowing pilot is shown below:
The other advantage of POPRV’s is that whether a snap-acting or modulating pilot is used, the presence of superimposed back pressure does not affect the opening pressure when the valve is in service. This is unlike direct spring safety relief valves, which require expensive and fragile bellows to protect against backpressure.
The modern POPRV can be used confidently in ASME Section VIII applications. POPRV’s provide a leak-free system operation very close to the PRV set pressure. A non-flowing pilot design assures that the POPRV will relief consistently within code tolerances even in “dirty” service applications, thus lower cost of ownership. Since process pressure is used to provide sealing force, a lighter unit weight and smaller size results in a lower cost of installation. POPRV’s provide advanced, reliable, and efficient overpressure protection, utilizing a product technology designed for a wide range of ASME Section VIII applications.
Learn More About Anderson Greenwood Safety Relief Valves
The modern POPRV can be used confidently in ASME Section VIII applications. POPRV’s provide a leak-free system operation very close to the PRV set pressure. A non-flowing pilot design assures that the POPRV will relief consistently within code tolerances even in “dirty” service applications, thus lower cost of ownership. Since process pressure is used to provide sealing force, a lighter unit weight and smaller size results in a lower cost of installation. POPRV’s provide advanced, reliable, and efficient overpressure protection, utilizing a product technology designed for a wide range of ASME Section VIII applications.
Learn More About Anderson Greenwood Safety Relief Valves