For applications requiring valves that must relieve large flows with small pressure differential, pilot-operated relief valves are often used, Figure 3. The pilot-operated relief valve operates in two stages. A pilot stage, which consists of a small, spring-biased relief valve (generally built into the main relief valve), acts as a trigger to control the main relief valve. However, the pilot may also be located remotely and connected to the main valve with pipe or tubing.

The main relief valve is normally closed when the pressure of the inlet is below the setting of the main valve spring. Orifice B in the main valve, Figure 3, permits system pressure fluid to act on a larger area on the spring side of the poppet so that the sum of this force and that of the main spring keep the poppet seated. At this time, the pilot valve is also closed. Pressure in passage B is the same as system pressure and is less than the setting of the pilot valve spring.

As system pressure rises, the pressure in passage Brises as well, and, when it reaches the setting of the pilot valve, the pilot valve opens. Oil is released behind the main valve through passage B through the drain port. The resulting pressure drop across orifice A in the main relief valve opens it and excess oil flows to tank, preventing any further rise in inlet pressure. The valves close again when inlet oil pressure drops below the valve setting. Pilot-operated relief valves have less pressure override than direct-acting relief valves, such as in Figure 2.

Because these valves do not start opening until the system reaches 90% of full pressure, the efficiency of the system is protected because less oil is released. These valves are best suited for high-pressure, high-volume applications. Although their operation is slower than that of direct-acting relief valves, pilot-operated relief valves maintain a system at a more constant pressure while relieving. Figure 4 plots the operating characteristics of direct-acting and pilot-operated relief valves.

Pressure-reducing valves

The most practical components for maintaining secondary, lower pressure in a hydraulic system are pressure-reducing valves. Pressure-reducing valves are normally open, 2-way valves that close when subjected to sufficient downstream pressure. There are two types: direct acting and pilot operated.

Direct acting —A pressure-reducing valve limits the maximum pressure available in the secondary circuit regardless of pressure changes in the main circuit. This assumes the work load generates no back flow into the reducing valve port in which case the valve will close, Figure 5. The pressure-sensing signal comes from the downstream side (secondary circuit). This valve, in effect, operates in reverse fashion from a relief valve (which senses pressure from the inlet and is normally closed).

As pressure rises in the secondary circuit, Figure 5, hydraulic force acts on area A of the valve, closing it partly. Spring force opposes the hydraulic force, so that only enough oil flows past the valve to supply the secondary circuit at the desired pressure. The spring setting is adjustable.

When outlet pressure reaches that of the valve setting, the valve closes except for a small quantity of oil that bleeds from the low-pressure side of the valve, usually through an orifice in the spool, through the spring chamber, to reservoir.

If the valve closes fully, leakage past the spool could cause pressure buildup in the secondary circuit. To avoid this, a bleed passage to reservoir keeps it slightly open, preventing a rise in downstream pressure above the valve setting. The drain passage returns leakage flow to tank. (Valves with built-in relieving capability also are available to eliminate the need for this orifice.)