Pressure relief valves

The pressure relief valve is an important type of safety valve. A pressure relief valve protects motors, pumps and actuators from becoming damaged from high pressure. When the pressure inside such equipment increases beyond a specified value, the excess pressure may result in a failure or damage called catastrophic failure occurs. To avoid such calamity, pressure relief devices are used at predetermined set pressures to protect the equipment by relieving the excess pressure.

The valve remain closed for normal operation. And no water passes through the valve.

When the pressure in the loop exceeds the limit the valve opens, and the relief the excess pressure. Thus protects the expensive machinery. Without a pressure relief valve excess pressure directly exposed to machinery damages them.


  • Direct acting

  • Pilot operated

Direct acting:

During normal pressure the valve held closed by a mechanical spring. The pressure exerted by the spring and the fluid will be the same, so the valve won’t open. The spring tension can be adjusted by rotating the adjustment knob. Adjustment knob is adjusted to specified position for a cracking pressure, where cracking pressure is pressure when the valve starts to open.

Beyond the cracking pressure the valve begins to open and the fluid starts to trickles through. the pressure at which the valve fully open is the full relief valve pressure. At this pressure valve is fully opened.

Pilot operated relief valve:

The difference between a pilot-operated safety relief valve and a direct acting pressure relief valve is that the pilot-operated valve uses process pressure to keep the valve closed instead of a spring. The first stage consist of main poppet and main spring. Now the second stage consist of pilot poppet, pilot spring, adjustment knob.

Until the pump line pressure exceeds the relieving pressure set on the control knob, the pilot poppet remain closed. And the main poppet. .

when the pump line pressure increase the pilot moves to open position. And the water flows out to the tank,
Know there is a pressure drop across the main poppet.

Due to the pressure drop across the main poppet, the main moves up, so the valve opens. Same thing happens when the pump line pressure drops the pressure difference cause the main poppet close.