Fig. 1. Simple, direct-acting relief valve has no adjusting screw and therefore opens at a fixed, pre-set pressure as controlled by setting of compression spring.

Fig. 2. Adjustable, direct-acting relief valve blocks flow through the valve until force of system pressure on the poppet overcomes the adjustable spring force and downstream pressure.

Fig. 3. Pilot-operated relief valve has orifice through piston, which is held closed by force of light spring and system pressure acting on larger piston area at spring end.

Fig. 4. Comparison of action of relief valves at cracking and full-flow pressure.

Fig. 5. Direct-acting, pressure reducing valve is held open by spring force. Increasing pressure at outlet port moves the spool to the right, closing the valve.

Pressure-control valves are found in virtually every hydraulic system, and they assist in a variety of functions, from keeping system pressures safely below a desired upper limit to maintaining a set pressure in part of a circuit. Types include relief, reducing, sequence, counterbalance, and unloading. All of these are normally closed valves, except for reducing valves, which are normally open. For most of these valves, a restriction is necessary to produce the required pressure control. One exception is the externally piloted unloading valve, which depends on an external signal for its actuation.

Relief valves

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 can be divided in two categories: direct-acting and pilot-operated.

Direct-acting - A direct-acting valve may consist of a poppet or ball, held exposed to system pressure on one side and opposed by a spring of preset force on the other. In a fixed, non-adjustable, normally closed relief valve, Figure 1, the force exerted by the compression spring exceeds the force exerted by system pressure acting on the ball or poppet. The spring holds the ball or poppet tightly seated. A reservoir port on the spring side of the valve returns leakage fluid to reservoir.

When system pressure begins to exceed the setting of the valve spring, the fluid unseats the ball or poppet, allowing a controlled amount of fluid to bypass to reservoir, maintaining system pressure at the valve setting. The spring re-seats the ball or poppet when enough fluid is released (bypassed) to drop system pressure below the setting of the valve spring.

Because the usefulness of a fixed relief valve is limited to the single setting of its spring, most relief valves are adjustable. This is commonly achieved with an adjusting screw acting on the spring, Figure 2. By turning the screw in or out, the operator compresses or decompresses the spring respectively. The valve can be set to open at any pressure within a desired range. Aside from the adjustable feature, this valve works just like the fixed valve in Figure 1.

Poppet design - Spring-loaded poppet valves are generally used for small flows. They don't leak below cracking pressure and respond rapidly, making them ideal for relieving shock pressures. They often are used as safety valves to prevent damage to components from high surge pressures, or to relieve pressure caused by thermal expansion in locked cylinders. The differential between cracking and full open pressure on spring-loaded poppet relief valves is high. For this reason they are not recommended for precise pressure control.

Reverse flow and guided piston designs - Relief valves are also made to relieve flow in either direction. Fluid pressure at the other port acts on a shoulder on the plunger to open the valve.

Another type of direct-acting relief valve has a guided piston. In this valve a sliding piston, instead of a poppet, connects the pressure and reservoir ports. System pressure acts on the piston and moves it against a spring force. As the piston moves, it uncovers a reservoir port in the valve body.

These valves have a fast response but may be prone to chatter. They can be damped to eliminate chatter, but this also slows their reaction time. They are reliable and can operate with good repetitive accuracy if flow does not vary widely. Valves with hardened-steel pistons and sleeves have a very long service life. They may leak slightly below cracking pressure unless the pistons are sealed.

Guided-piston relief valves generally are used for pressures below 800 psi, although they can be made with heavier springs for higher pressures. The heavier springs give the valve a greater differential and consequently increase the size of the valve.

Differential-piston design - A variation of the guided-piston relief valve is the differential-piston relief valve. Here, the pressure acts on an annular area (the difference between two piston areas). This annular area is smaller than the valve's seat area. This permits the use of a lighter spring than would be needed if pressure acted on the entire seat area. These valves have a lower pressure differential than poppet or guided-piston relief valves.

Pilot-operated reliefs - 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 B rises 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 and as long as 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.

Should the valve close fully, leakage past the spool could cause pressure build-up 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 reservoir. (Valves with built-in relieving capability also are available to eliminate the need for this orifice.)

Constant and fixed pressure reduction - Constant-pressure-reducing valves supply a preset pressure, regardless of main circuit pressure, as long as pressure in the main circuit is higher than that in the secondary. These valves balance secondary-circuit pressure against the force exerted by an adjustable spring which tries to open the valve. When pressure in the secondary circuit drops, spring force opens the valve enough to increase pressure and keep a constant reduced pressure in the secondary circuit.

Fixed pressure reducing valves supply a fixed amount of pressure reduction regardless of the pressure in the main circuit. For instance, assume a valve is set to provide reduction of 250 psi. If main system pressure is 2,750 psi, reduced pressure will be 2,500 psi; if main pressure is 2,000 psi, reduced pressure will be 1,750 psi.

This valve operates by balancing the force exerted by the pressure in the main circuit against the sum of the forces exerted by secondary circuit pressure and the spring. Because the pressurized areas on both sides of the poppet are equal, the fixed reduction is that exerted by the spring.