Most fluid power systems are designed to operate within a preset pressure range. This range is a function of the forces the actuators in the system must generate to do the required work. Without controlling or limiting these forces, the fluid power components (and expensive equipment) could be damaged. Relief valves avoid this hazard. They are the safeguards which limit maximum pressure in a system by diverting excess oil when pressures get too high.
Cracking pressure and pressure override —The pressure at which a relief valve first opens to allow fluid to flow through is known as cracking pressure. When the valve is bypassing its full rated flow, it is in a state of full-flow pressure. The difference between full-flow and cracking pressure is sometimes known as pressure differential, also known as pressure override.
In some cases, this pressure override is not objectionable. However, it can be a disadvantage if it wastes power (because of the fluid lost through the valve before reaching the maximum setting). This can further permit maximum system pressure to exceed the ratings of other components. (To minimize override, use a pilot-operated relief valve.)
Relief valves are either direct-acting or pilot-operated.
Bypass Valves Act as Relief Valves
In some cases, a so-called bypass valve acts as a relief valve by being used to return all or part of the fluid discharged by a pump or gas compressor back to either a storage reservoir or the inlet of the pump or gas compressor. This is done to protect the pump or gas compressor and any associated equipment from excessive pressure. The bypass valve and bypass path can be internal (an integral part of the pump or compressor) or external (installed as a component in the fluid path).