Relief Valves

What is a Relief Valve?

A relief valve is a Valve which is installed on a pump or pipework to relieve excessive pressure in a positive displacement pump which could cause damage to a pump or system. Relief valves (also known as bypass valve or safety valve) are known as an Internal Relief Valve if installed on the pump itself, and if installed on the system they are known as an External Relief Valve.

Such valves are used on positive displacement pumps as the pumps are designed to prevent recirculation of fluid back to the inlet, meaning the displacement is always forward. However if a valve is closed on the outlet of the pump, or a blockage occurs the pump will continue to build pressure in the outlet pipework and within itself. This can cause pressures to reach dangerously high levels, and the pressure will vent at the weakest part of the system, causing a leak, damage or pump failure.

Such valves work in different ways depending on where they are installed. If installed on a pump the fluid is recirculated within the pump head, preventing the buildup of pressure, however the fluid begins to heat up, and is something which should only be done temporarily as it can lead to pump failure. External relief valves should be used in conjunction with internal relief valves to relieve any excessive pressure in the pipework either back to a vented tank or to dump contents in an emergency to atmosphere.

A relief valve design consists of a housing with inlet and outlet connections, a spring within a chamber which keeps a plate in place sealing the discharge. When the valve reaches the set relief pressure, the pressure plate causes the spring to compress revealing the discharge, causing the pressurised contents to vent through the discharge. A set screw or nut is used to set the relief pressure, at the top of the housing and when screwed clockwise, compresses the spring increasing the amount of pressure required to compress the spring and open the valve.

Internal relief valve set pressure is usually set at 10% above the operating pressure of the pump, and external relief valves should be set at 10% higher than the relief valve on the pump to prevent nuisance tripping. Valves are typically calibrated during manufacture and can be set on site with the use of pressure gauges during commissioning. Valves are rated in ranges such as from 0-10 bar, or 10-15 bar so should a valve pressure rating require recalibrating, checks should be made to ensure the setting is inline with the rating of the valve. Equipment should be checked at regular intervals to ensure succesful operation during an emergency.

Relief valves are not always required if the pressure capability of the pump is low, but also if used in conjunction with a product which can solidify when cooled if it enters the relief valve such as Chocolate or Bitumen. If fluid sets within a relief valve this will prevent it from operating as normal as any material which hardens will form a plug and can also cause contamination as it can be very difficult to clean thoroughly.

On some pumps a pressure relief valve is used in conjunction with a pressure regulator which regulates the pump to deliver the required pressure for the system. An analogy often used to describe a pressure regulator is akin to brakes in a car, with and the relief valve being the seat belt. A relief valve should only be relied on for use in emergencies to prevent damage to the pump or system and not as frequent protection for the system.