Several types of pressure-control valves are found in fluid power circuits. Some keep the whole system from excess pressure while others only protect a portion of the system. Others allow flow to an isolated circuit after reaching a preset pressure. Some bypass fluid at low or no pressure when activated.
This chapter only covers relief valves and unloading valves because they are closely associated with hydraulic pumps. The other pressure-control valves are part of the control circuit and will be dealt with after directional control valves.
Why relief valves?
All fixed-volume pump circuits require a relief valve to protect the system from excess pressure. Fixed-volume pumps must move fluid when they turn. When a pump is unloading through an open-center circuit or actuators are in motion, fluid movement is not a problem. It is when the actuators stall with the directional valve still shifted that a relief valve is essential.
Pressure compensated pump circuits could run successfully without relief valves because they only move fluid when pressure drops below their compensator setting. (Most designers still use a relief valve in these circuits for reasons explained later.)
In either case, a relief valve is similar to a fuse in an electrical system. When circuit amperage stays below the fuse amperage, all is well. When circuit amperage tries to exceed fuse amperage, the fuse blows and disables the circuit. Both devices protect the system from excess pressure by keeping it below a preset level.
The difference is that when an electrical fuse blows it must be reset or replaced by maintenance personnel before the machine can cycle again. This requirement alerts the electricians to a possible problem and usually causes them to look for the reason before restarting the machine. Without the protection of a fuse, the electrical circuit would finally overheat and start a fire.
In a hydraulic circuit, a relief valve opens and bypasses fluid when pressure exceeds its setting. The valve then closes again when pressure falls. This means a relief valve can bypass fluid anytime . . . or all the time . . . without intervention by maintenance. (It also means the system can run hot even with a heat exchanger installed.)
Many fixed-volume pump circuits depend on this bypassing capability during the cycle, and some even bypass fluid during idle time. A well-designed circuit never bypasses fluid unless there is a malfunction, such as a limit switch not closing or an operator overriding the controls. This eliminates most overheating problems and saves energy.
Relief valve operation
There are two different designs of relief valves in use: direct acting and pilot operated. Both types have advantages and work better in certain applications.
Some terms relating to relief valves and their function are:
- Overshoot: The actual pressure reading when a relief valve first opens to bypass fluid. (It can be up to twice the actual pressure setting.)
- Hysteresis: The difference in pressure between when a relief valve starts letting some flow pass (cracking pressure) and when full flow is passing.
- Stability: The fluctuation of pressure as a relief valve is bypassing at set pressure.
- Reseat pressure: The pressure at which a relief valve closes after it has been bypassing.
- Pressure override: The difference in the pressure reading from the time a relief valve first opens (cracking pressure) until it is passing all pump flow to tank.
Direct-acting relief valves
Figure 9-1 shows a cutaway view and the symbol for a direct-acting relief valve. The valve has a poppet that is pressed against its seat by an adjustable spring. An adjusting knob can be change the force on the spring to raise or lower maximum pressure. The poppet remains seated while pump flow goes to the circuit and pressure is lower than the relief valve setting. If pressure tries to go above spring setting, the poppet is forced off the seat just enough to pass excess pump flow to tank.
The symbol shows a single box with a flow arrow offset from the inlet P and outlet T flow lines. The dashed pilot line from the inlet line to the bottom of the box indicates inlet pressure can push against the flow arrow. On the opposite side of the box is a spring with a sloping arrow through it to show an opposing force on the flow arrow. When pressure at port P builds enough to overcome spring pressure, it forces the flow arrow up until there is a path from P to T. Although there is no pilot passage in the actual valve, the function is implied and thus is part of the symbol.
The main advantage of direct-acting relief valves over pilot operated relief valves is that they respond very rapidly to pressure buildup. Any relief valve does not know there is a problem until pressure is very near or at its setting. Then it must open to relieve excess flow as quickly as possible to keep pressure overshoot low. Because there is only one moving part in a direct-acting relief valve, it can open rapidly, thus minimizing pressure spikes. Figure 9-2 shows typical performance graphs from direct-acting and pilot-operated relief valves. Notice the difference in response time and pressure spikes as the valves open to send excess flow to tank.
The main disadvantage of direct-acting relief valves is that they open partially at about 150 psi below set pressure. Because the poppet is in direct contact with the spring that sets maximum pressure, when the poppet opens it forces the spring back and increases pressure. The amount depends on the spring’s length and stiffness. The plot in Figure 9-3 shows the flow/pressure relationship of a typical direct-acting relief valve. With a direct-acting relief valve setting of 1500 psi at 10 gpm, it is very possible that some fluid will start to pass when pressure is as low as 1350 to 1400 psi. Continued pressure increase allows more flow until all pump flow goes to tank at 1500 psi. If work is still being performed at 1450 psi, it will be at a reduced speed because some flow is going to tank. When this valve is set at 1500-psi cracking pressure, no flow will bypass until pressure reaches that level, but final pressure would be as high as 1650 psi. (Pilot-operated relief valves . . . discussed next . . . do not start to open until pressure is within 25 to 50 psi of their settings.)
Direct-acting relief valves often are quite noisy due to the high velocity of the fluid bypass and the instability inherent in their design.
Direct-acting relief valves are not normally used on industrial hydraulic systems, except for those with flows under 3 gpm, and as pilot control devices. Most industrial designs use long springs that gain little force per compression increment to keep pressure override low.
When a direct-acting relief valve is specified as preset, non-adjustable, always specify whether the valve is to be set for cracking pressure or full flow. If full flow is desired, a flow must be specified also.