Pressure Relief Valve Working and Their types

Pressure relief valve is one of the most important types of safety valve used in hydraulic system for controlling the rise of pressure within a hydraulic line. In normal condition valve is closed but if the pressure in hydraulic line exceed the design pressure limit, then valve opens to relieve the pressure.

Without a relief valve, pressure can continue to grow until hydraulic component fail and pressure is release.

One port of pressure relief valve is connected with pump delivery line or pressure line and other port is connected with hydraulic reservoir.

Types Of Pressure Relief Valve

Direct Acting Pressure Relief Valve – It consist of poppet, spring, adjustment knob
Pilot Operated Pressure Relief Valve – It consist of main poppet, main spring, pilot poppet, pilot spring & adjustment knob.

Direct Acting Pressure Relief Valve Working Principle

Direct acting pressure relief valve are normally closed by direct force created by mechanical spring which was adjusted with the help of pressure adjusting screw. The spring force holding the valve closed is opposed by system hydraulic pressure.

If pump line pressure is not enough to overcome the spring force, in that situation poppet will be remain seated over its seat and will not allow the flow of fluid through it and therefore relief valve will be closed and will not allow flow through it.

When pump line pressure increases and overcomes the spring force, in that situation poppet will leave the seat and will permit the flow of fluid through relief valve.

Pilot Operated Pressure Relief Valve Working Principle

Pilot operated pressure relief valve will have one small pilot poppet type of relief valve normally closed by force created by pilot spring and one main poppet relief valve which are also normally closed by force created by main spring. The pilot spring was adjusted with the help of pressure adjusting screw.

If pump line pressure is less than the pressure created by pilot spring, in that situation pilot poppet will be remain seated over its seat. Since the pilot poppet is closed, the pressure in main spring is same as the pump line pressure. Due to balanced pressure the main poppet remains in closed position and will not allow the flow of fluid through it.

When pump line pressure increases higher than the pilot spring pressure, in that situation pilot poppet are in open position and allow small amount of fluid to flow through the pilot line to reservoir. This will create a pressure difference in main spring and pump line pressure. Since pump line pressure is higher than main spring pressure, the main poppet leaves their seats by moving upward and allow the fluid to flow from pump line to reservoir directly.

Note:

The pressure at which valve first permit the flow through it is called as cracking pressure
The pressure at which valve allows to flow the full rated capacity is called as valve full flow pressure.

Frequently Ask Questions on Pressure Relief Valve

Q1. What is cracking pressure vs. full-open pressure?

Cracking Pressure: The pressure at which the PRV first starts to open.
Full-Open Pressure: The pressure at which the valve is fully open and bypassing maximum flow.
Difference is called pressure override.

Q2. What is pressure override in a PRV?

Pressure override is the increase in pressure above the set pressure required to fully open the valve.
Too much override causes: Heat generation, Wasted power, Unstable pressure
Pilot-operated PRVs have lowest override.

Q3. How do you adjust a pressure relief valve?

Steps:
1. Loosen locknut.
2. Rotate screw clockwise → increase pressure.
3. Rotate screw anticlockwise → decrease pressure.
4. Tighten locknut after setting.
5. Keep pump running while adjusting.
6. Use a calibrated gauge.
7. Never exceed pump or actuator rating.

Q4. What happens if PRV setting is too high

Hose burst, Seal damage, Pump overload, Component failure, Overheating.
Always set PRV according to manufacturer data.