Pilot Operated PSV
R020 Series Pilot Operated Safety Relief Valves
A Pilot Operated Safety Relief Valve (POSRV) combines the main relieving device or main valve with a self-actuated auxiliary pressure relief valve (pilot). This type of valve consists of the main valve, which typically encloses a floating unbalanced piston assembly and an external pilot.
The piston is designed with a larger area on the top than on the bottom. Until the set pressure is reached, both the top and bottom areas are exposed to the same inlet operating pressure. The larger top area generates a force that holds the piston tightly against the main valve nozzle. As the operating pressure increases, the net seating force also increases, making the valve tighter. This feature allows most pilot-operated valves to function effectively even at higher expected operating pressures.
At the set pressure, the pilot releases pressure from the top of the piston, resulting in an upward net force that causes the piston to lift, allowing process flow through the main valve. Once the overpressure incident is resolved, the pilot closes the vent from the top of the piston, reestablishing pressure and causing the piston to reseat.
A disc, which normally closes the main valve inlet, is integral with a flexible diaphragm. The external pilot senses process pressure, vents the top of the diaphragm at the set pressure, and reloads the diaphragm once the process pressure decreases.
Unlike conventional or balanced spring-loaded valves, the lift of the main valve piston or diaphragm is not affected by built-up backpressure. This allows for even higher pressures in the relief discharge manifolds.
The pilot operating the main valve can be either a pop-action or modulating-action pilot. The pop-action pilot causes the main valve to lift fully at the set pressure without overpressure, while the modulating pilot opens the main valve enough to satisfy the required relieving capacity.
Pilots may be either flowing or non-flowing types. The flowing type allows process fluid to continuously flow through the pilot when the main valve is open, while the non-flowing type does not. The non-flowing pilot type is generally recommended for most services to reduce the possibility of hydrate formation or solids in the loading fluid affecting the pilot's performance. Pilot-operated pressure relief valves are available for use in both liquid and vapor services. Some pilot-operated valves are unaffected by the state of the fluid and are recommended for two-phase flow applications.
For pilot-operated PSVs, the valve lift is not affected by backpressure. For compressible fluids at critical flow conditions, a backpressure correction factor of 1.0 should be used for pilot-operated PSVs. Neither the set pressure nor the capacity of pilot-operated PSVs is typically affected by backpressure for compressible fluids at critical flow conditions. The capacity of these balanced valves begins to decrease when the backpressure exceeds 30% to 50% of the set pressure due to subsonic flow and/or physical responses to the high backpressure.
A backflow preventer is required when there is a possibility of developing a pressure on the discharge side of the valve that exceeds the inlet pressure. Higher discharge pressure can create sufficient upward force on the diaphragm or piston to open the valve and cause flow reversal. The backflow preventer allows the discharge pressure to exert a net downward force on the diaphragm or piston to keep the valve closed. Proper operation of the backflow preventer is crucial to prevent flow reversal in the valve.
Types of Pilot Operated Safety Relief Valves (PSVs)
Pop-action Pilot-operated Valve (Flowing-type)
Pop-action Pilot-operated Valve (Non-flowing-type)
Modulating Pilot-operated Valve (Flowing-type)
Pilot-operated Relief Valve with a Non-flowing Modulating Pilot Valve
Low-pressure Pilot-operated Valve (Diaphragm-type)
When to Consider Pilot Operated PSVs
Variable and High Backpressure: Consider pilot-operated PSVs if the superimposed backpressure is variable and exceeds 10% of the set pressure, or if the built-up backpressure is too high for conventional and bellow-type pressure relief valves.
High Backpressure: Use pilot-operated relief valves when the backpressure exceeds 40-55% of the set pressure.
Close Operating Pressure: Choose pilot-operated relief valves when the maximum operating pressure is less than 10% below the design/set pressure or if the required orifice for a single valve installation exceeds the maximum size (8T10) of API 526, or when the set pressure is closer than 10% to the operating pressure.
Pressure Loss: Consider using pilot-operated relief valves when the pressure loss from the protected equipment to the valve exceeds 3% of the proposed set pressure.
Automatic Depressurizing: Pilot-operated relief valves may also be used for systems requiring automatic depressurizing in addition to overpressure protection and for short blow down without increasing the simmer range.
Alternative Unavailability: Pilot-operated valves should only be used if no alternative is available, but they should not be used for waxy crude duty.
Characteristics of Pilot-operated PSVs
Vented or Balanced Pilots: Pilot-operated PSVs can have pilots vented to the atmosphere or balanced to maintain set pressure in the presence of variable superimposed backpressure.