The most common pressure valve used in hydraulic systems is the relieve valve. Nearly every hydraulic system has one, and they can also be one of the simplest valves in construction. Some relief valves are merely a ball and spring, while most are with a spring with adjustable preload pushing down upon either a poppet or spool. Most other pressure valves, such as counterbalance valves and sequence valves, are a modification of the relief valve.
I don’t need to spend much time describing their operation, so I won’t. There is a spring pushing on a ball or poppet, and when hydraulic pressure increases force upon the ball or poppet to a point where that force is higher than the spring force, the valve starts to open. It’s important to know that a relief valve will only “start to open” when its cracking pressure is reached by the fluid pressure.
As a relief valve starts to open, it dumps the oil molecules that would have been normally stuffing themselves into the downstream portion of the circuit, creating more pressure. Whatever fluid not being used to create useful work, either because load pressure or back pressure is too high, is diverted and dumped back to tank. This dumped fluid, I should also mention, is a hundred percent heat. A relief valve doesn’t have to be diametrically opposed at either fully open or fully closed; only the fluid required to be dumped off to maintain force against the relief spring is enough, which could be a tiny fraction of pump flow.
There are considerations when applying a relief valve, as well as considerations for troubleshooting one. You want to set the pressure of a relief valve some range above your maximum working pressure, some using the 10% higher rule, some using the 300 PSI higher rule, but as long as that pressure doesn’t tax the power capacity of the prime mover or components in the system. Also, it is ideal to set the relief valve while putting through it the same flow potential as would be seen in service duty. It won’t do any good to set your relief valve on the test bench with 1 GPM of flow when it will see 30 GPM on the machine.
When troubleshooting, the relief valve is often the first place to look to diagnose a failure. A simple heat gun will show you if fluid is going over the relief valve. Some like to touch the relief valve to see if it’s hot, but that can be ineffective since a machine might have a normal operating temp of 70° C (≈150° F), which feels no less hot to the touch than 100°C (≈210°F)…both would cause you to pull your hand away quickly.
The point of observing relief valve temperature is to see if the valve is either stuck open or some operation of the machine is creating full system pressure, forcing fluid over the relief valve. This pressure could be the result of a plugged flow control, a stuck actuator, a partially shifted directional valve, or many other reasons creating the scenario of the relief valve being the path of least resistance