A relief valve functions to bleed pressure off to return (by creating an open flow path to return) when a prescribed pressure level is reached. Relief valves are simple devices (similar to check valves) where a piston or poppet is held closed by a spring. Figure 1 shows a schematic of a pressure relief valve. Also shown in Figure 1 is a variable delivery pump and the flow path to the system components (valves, actuators, etc.). The spring is set such that the poppet will open, providing a flow path to return, when pressure reaches a prescribed level. Every hydraulic systems requires at least one pressure relief valve to protect the system against thermal expansion and excessive pressure transients. The relief valve setting provides burst protection for all components in a system.
Figure 1 Relief Valve Schematic
The cracking pressure for the relief valve is
where F0 is the spring preload, Ff is the friction and As is the relief valve poppet area. Below the cracking pressure the valve has no effect on the system. Once the relief valve is open, system pressure will need to drop below the cracking pressure to close (re-seat) the valve. This is a hysteresis characteristic and is primarily due to the friction in the poppet.
When selecting a relief valve, ensure the relief valve is able to flow maximum pump flow under a failed pump compensator condition. Otherwise, line pressure will build up to a level for potential rupture in a tube or component will occur. Rupture will lead to leakage in undesirable areas. In addition, when a pump compensator has failed (pump becomes a fixed delivery pump) and full pump flow is going through the relief valve, significant heat is generated by the system that is undergoing flow. Therefore, a thermal analysis of system temperatures and analysis of critical aircraft zones should be performed to ensure no components or structure would be heated beyond acceptable limits. This analysis will include all components installed in the vehicle that could be affected by overheating in a hydraulic system.
When selecting a pressure relief valve, the following parameters should be considered
Pressure Rating – make sure valve is rated for your system pressure
Relief Setting – should be in the range you desire. Note that the relief valve setting will be above normal system pressure by approximately 5-10%.
Relief Pressure Range – what is the minimum and maximum range required for the valve to open
Reseat Pressure - what is the re-seat pressure (the pressure where the valve will close, once open)
Flow Rate – what is the flow rate when the valve is fully open? Is the opening sufficient to keep up with maximum pump flow, should a pump compensator fail?
Temperature Rating – valve should be rated for fluid temperatures and applicable environmental temperatures
Valve Materials – should be sufficient to pass proof and burst testing, not be susceptible to corrosion and other environmental considerations, and not cause any problems under temperature extremes
Seals/Clearances
– since relief valves don’t see many operating cycles seals are
not as critical as in modulating valves. However, the method of
sealing pressure should be examined. For relief valves with ball or
conical poppets, damage may be possible when re-seating against sharp
edges (see Figure 2). Wear around the poppet seats will lead to
leakage. For relief valves with elastomeric seals, see Seals
- Hydraulic Components for
discussion on hydraulic seals.
Failure Modes – the main failure mode is a jam of the poppet in the closed position, which would be a latent failure. It may also be possible for the adjustable device on the valve spring to fall out of adjustment leading to lower or higher regulation settings.
Hysteresis – what is the difference between the relief pressure setting and reseat? This difference should be large enough to prevent any chattering of the valve. Chattering may occur in a situation where the valve opens and system pressure drops quickly due to flow through the relief valve and the valve re-seats. Then the pressure builds up and the valve re-opens, repeating the cycle at a very high frequency.
Pressure Relief Valve Qualification
See Qualification - Hydraulic Components for discussion on control valve qualification and required certification testing. Note that endurance and fatigue testing will be much less for a relief valve than other valves due to the low cycles expected to see in service.