All hydraulic power units will have a pressure relief valve somewhere in the circuit as safety protection to stop the pressure getting too high, if something else in the circuit fails.
Pressure relief valves can provide protection against high external loads acting on actuators that would increase the hydraulic pressure above the equipment's safe working limits. Where check valves are used in cylinder lines then pressure relief valves can provide protection against thermal expansion of the fluid, which can cause excessive pressures.
Pressure relief valves are occasionally used to control system or load pressure levels but because this generates heat and is an inefficient use of energy, they are far more common as safety protection devices.
One area where relief valves are used to control pressure levels is within pilot pressure control systems. Here the pilot flows across the relief valves are small but they act on other main spool or logic elements that control the main flow, without such inefficiency.
Another place where you may need a very good relief valve is controlling the pressure on a test rig.
By adding an electrical solenoid relief valves become electrical unloader valves e.g. as soon as the system registers a fault, or is turned off, the power is removed from the solenoid and the relief valves opens at zero bar.
In their simplest form a pressure relief valve can be a ball bearing held against an orifice by a spring. As the pressure increases the ball is pushed back against the setting of the spring.
Better valves will have hardened seats and shaped poppets to give better pressure vs flow characteristics and more consistent operation throughout their life.
High flow valves tend to have two stages with a small pilot relief valve that opens a larger, second main stage.
Relief valves open when the pressure across them exceeds a pre-set limit. However, if the downstream pressure is acting with the spring then this will change and therefore the relief valve setting will change. Often multistage relief valves will have external pilot drains or feeds to ensure the signals are stable and reliable.
See pressure control valve in our symbols sections for more details.
All relief valves are not the same. Simple ball and spring valves are cheap to produce but will not perform so well. This may not be a problem if it's purely a safety relief valve that should never operate, however, controlling the pressure against different flow rates on a test rig will need a much better standard of valve.
Relief valves options include, direct acting, pilot operated 2 stage, electrical unload, proportional, integrated or line mounted versions.
Look at pressure vs flow curves in the supplier's datasheet and select the valve appropriate for your operating conditions. In our example PQ curve you can see that valve A has a low set or cracking pressure but then the pressure rises as the flow increases. As the flow drops there is a large hysteresis gap between the curves and the reset pressure is below the cracking pressure. Valve B shows much better performance with a flat curve and less hysteresis.
Consider the robustness and of the environment with particular regard to how the adjusters will survive in bad weather.
Does the valves operate regularly or only in emergencies. It may need a hard seat to protect against changes in the setting level if it operates continuously.
To protect against shock loads you may need a very fast opening valve, probably direct operated. However, some very fast valves can be unstable if you need quiet, constant pressure level.
Do you need leak free sealing or low leakage operation. Direct operated valves can provide this but pilot valves tend to have small leakage paths, which do prevent trapped in pressure but add to the contamination risk.
Check that valve leakage will not cause uncontrolled lowering of the load.
Consider the effect of drain or return line pressures on the valve's opening pressure.
Will the valve open quickly enough? Pressure transducers may show over pressure peaks during shock loads.
Relief valves are generally set 15 bar above the working pressure of system.
Most relief valves work against a spring and have a small Allen key adjuster with locking nut at the end. Always switch the system off before you adjust the setting.
Hydraulic safety relief valves do not need to be removed and recalibrated ever year. Risks are more likely to come from contamination getting into the open pipework that setting drift. Check the valve in situ if safe to do so.
The relief valve seats may take a few operations before they bed in from new. It's good to operate them a few time before first setting. Remember equipment often comes with the relief valves backed off before commissioning.
Design calcs TBC
Experiment with a pilot operated relief valve