Almost all hydraulic systems have so-called "positive d […]
Almost all hydraulic systems have so-called "positive displacement pumps". This means that every time the pump rotates, the hydraulic fluid is pushed out by sturdy mechanical parts, and the fluid has nowhere to go. Since the liquid cannot be compressed like air, this means that if the outlet is blocked and the fluid cannot flow back to the reservoir through the normal circuit, a part of the pump will rupture.
Without a relief valve, and without a positive displacement pump like a power steering pump, the pressure will immediately increase until something happens. The seal will blow out, the hose will rupture, or part of the diverter will break.
However, the use of a pressure relief valve can avoid all of these situations. The fluid only flows out of the pump and returns directly to the tank through the pressure relief valve. It sets the maximum pressure of the system, which is determined in advance and is safe for the weakest components in the system. Therefore, most hydraulic steering gears are equipped with safety valves.