Always use a relief valve with fixed-displacement hydraulic pumps. Pressure-compensated pump circuits also may use a relief valve for certain applications.
Think of a relief valve in a hydraulic system as a fuse or circuit breaker in an electric circuit. An electric circuit never blows a fuse unless it overloads. When an electric circuit overloads, it is inoperable until reset. Usually the person responsible for resetting the fuse looks for the reason it blew and fixes the problem before restarting the machine. Many hydraulic circuits allow the relief valve to dump some or all pump flow to tank all or part of the time. The extra power to produce that unused flow is expensive. Also, heat generation from excess flow requires larger heat exchangers that are expensive to buy and operate.
Protecting the pump and the system from excess pressure is the only valid function for a relief valve. At no time should the relief valve be used to pass excess pressure fluid to tank. When excess pump flow goes to tank, it generates heat. The relief valve in a well-designed hydraulic circuit never relieves oil to tank — unless there is a circuit or control malfunction.
Figure 18-1 pictures the symbol for a direct-acting relief valve. A direct-acting relief valve responds quickly when pressure tries to go above the valve’s setting. It can be use it on circuits with pressure-compensated pumps to reduce pressure spikes. On a hydraulic circuit with a fixed-displacement pump, a direct-acting relief valve opens partially early and thus wastes energy. When the system must operate near maximum pressure without any fluid bypass, use a pilot-operated relief valve.
Figures 18-2 and -3 show the simple and complete symbols for a pilot-operated (or compound ) relief valve. This type relief valve has two sections. The pilot operator on top is a small, poppet-type direct-acting relief valve. The main flow section of the valve is a poppet- or piston-type, normally closed 2-way valve. Through internal porting, a small direct-acting relief poppet controls a large poppet or piston. A pilot-operated relief valve responds more slowly, but does not even partially open until system pressure reaches approximately 95% of set pressure. Pilot-operated relief valves are suitable for remote operation, they open to unload pumps at pressures below 50 psi, and they act as large 2-way valves in some circuits.
Examples of relief-valve circuits
Always locate the relief valve as close as possible to the outlet of a fixed-displacement pump. A pilot-operated relief works best because it does not pass any fluid until system pressure is very near the valve’s set pressure.
Figure 18-4 shows a typical fixed-displacement pump circuit. Except in the event of a control circuit malfunction or if it is used to hold the cylinder at pressure, the relief valve never opens. Heat generation is minimal and the circuit usually can run without a heat exchanger.
Figure 18-5 shows a pressure-compensated pump with a direct-acting relief valve to protect it against overpressure. Pressure spikes often occur in pressure-compensated pump circuits with high flow or fast cycling. When the pump must compensate rapidly or often from full flow to no flow, the resulting overpressure drastically shortens pump life.
In Figure 18-5, the pump would be at low pressure and full flow when cylinder <I>CYL3 </I> extends rapidly. When the cylinder stops, fluid requirement is zero, but pump flow is still 40 gpm. As pressure builds, the pump finally starts compensating at about 1900 or 1950 psi. It is still producing 40 gpm — with no place for the oil to go. Without a relief valve in the circuit, system pressure spikes during each cycle can reach four to ten times the compensator setting. Pressure spikes damage the pump and piping after a few hours of operation. The faster the cycle, the more quickly shock damage from pressure spikes causes problems.
A relief valve, installed in Figure 18-5, reduces pressure spikes to protect the system. When the pump shifts to no flow, excess flow goes to tank through the relief valve. When the pump reaches compensator pressure, the relief valve closes. (For another and better way to reduce pressure spikes and protect a pressure-compensated pump from rapid cycling, see Chapter 1, Figures 17-19.)
Set the relief valve in a pressure-compensated pump circuit at 150 to 200 psi higher than the pump compensator. With relief pressure below compensator setting, pump flow goes to tank and makes heat. With relief pressure set at compensator pressure, the relief valve starts dumping when the pump starts compensating. When the relief valve passes fluid, the pump sees a pressure drop, and starts flowing again. The resultant pressure drop allows the relief valve to close and the dump/flow cycle starts again. After a few hours of this erratic operation, the pump fails.
Adding a solenoid valve to the vent port of a pilot-operated relief valve makes an effective unloading valve. Figure 18-6 shows a fixed-displacement pump supplying three cylinders. There is no power to the solenoid on the relief valve with the cylinders idle, so pump flow goes to tank at low pressure. Energizing a solenoid on the relief valve and one cylinder’s directional valve causes an action. Energizing both solenoids at the same time sends pump flow to the cylinder until reaching maximum relief pressure. A solenoid relief valve always has a slight delay before blocking flow to tank after energizing the solenoid. The delay is in milliseconds so it usually is only noticeable on very fast cycles.
The circuit in Figure 18-7 uses a solenoid-operated relief valve to unload the high-volume pump in a hi-lo circuit. Instead of waiting for pressure to build before the high-volume pump dumps to tank, the solenoid relief dumps oil on demand. The demand signal could come from a pressure switch, a limit switch, or an electric eye that senses cylinder position (then slows it before it contacts the work).