Hydraulic pressure regulators maintain a specific pressure range and are commonly known as unloading valves or unloading relief valves. In hydraulic power systems, these regulators both unload the fluid being pumped and regulate a set operating pressure. All positive displacement fixed-volume pumps for hydraulic circuits require an unloading valve to protect the system from excess pressure. Fixed-volume pumps move a set amount of fluid when they turn, regardless of pressure. If the system is not using that fluid it must be relieved to prevent system overpressure. When a pump is unloading through an open-center circuit or actuators are in motion, fluid movement is not a problem. When actuators or motors stop with a valve still sending fluid to those devices, a relief valve is required