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Load-sensing, fixed-volume pumps
The oldest load-sensing circuits for fixed-volume pumps are like those diagrammed in Figures 8-9 and 8-10. The pumps in these circuits never operate at a higher pressure than work resistance and never send fluid across the relief valve unless there is a malfunction in the hydraulics or control circuit.
Figure 8-28 diagrams a simple load-sensing pump circuit using standard valves. A pilot-operated relief valve with a 70-psi spring dumps pump flow to tank at 70 psi when the vent port is at 0 psi. A shuttle valve receives pressure feedback from the actuator and signals the pilot-operated relief valve’s vent port with the actual working pressure. As the actuator moves at a reduced speed, pump pressure stays 70 psi above actual load pressure, so excess flow that goes to tank wastes less energy.
Several manufacturers offer fixed-volume pumps with integral load-sensing valves. Hookup is simple for these pumps, and in some designs, bias pressure can be adjusted.
This setup is not as efficient as a pressure-compensated pump with load sensing, but it always provides an advantage in fixed-volume pump circuits. The results are best when maximum system pressure is high and actuator’s extension and retraction speeds are low.
Horsepower- and/or torque-limiting pumps
Horsepower or torque limiting is another control technique that only works on pumps that are capable of variable volume. Its main application is in the mobile-equipment field, where most hydraulic circuits are powered by gas or diesel engines. These engines usually must move the machine as well. To maximize actuator speed and force while minimizing horsepower drain, all actuators can be fast at low loads but still able to move heavy loads without pulling excess horsepower. Each manufacturer that supplies this setup may have a different way of doing it.
Figure 8-29 shows the schematic diagram of a circuit with a horsepower-limiting pump. A pressure-compensator adjustment still controls maximum output pressure but the preset limiting valve can reduce flow as pressure increases. Reducing flow as pressure increases keeps horsepower or torque from exceeding a preset limit. The horsepower/torque limiter is preset for a given pressure and flow. This system could be useful in an accumulator circuit to allow higher flow as pressure decreases while limiting horsepower draw as pressure climbs.
Typical circuit for pressure-compensated pumps
Most pressure-compensated pumps use a closed-center circuit such as the one in Figure 8-30. These circuits could have load sensing or other controls. They usually include multiple actuators. Closed-center circuits typically operate at maximum system pressure and output flow matches the circuit requirement. Flow controls keep actuators at operating speed because maximum flow may make them move too rapidly. Flow controls also make it possible for more than one actuator to move simultaneously without affecting their stroke times. Note that flow controls also increase heat generation because the moving or work force may not require full system pressure. Also, some actuators may require pressure-reducing valves to lower the maximum force so that it doesn’t cause damage.
Normally, pressure-compensated pumps do not need relief valves to protect their systems from overpressure. However, many circuits with pressure-compensated pumps include a relief valve just in case the pump hangs on flow. When a relief valve, for whatever reason, is used on a pressure-compensated pump, it is imperative that the relief valve is set 100 to 150 psi higher than the pump compensator. If the relief valve is set lower than the compensator, the circuit will operate as a fixed-volume setup and quickly overheat the fluid. If the relief valve is set at the same pressure as the compensator, the relief valve can start to dump as the compensator starts to reduce flow. Then pressure drop lets the relief valve shut and the compensator ask for more flow. This oscillating action can continue until the pump fatigues and fails.
Setting the relief valve and compensator is a four-step operation:
- Set the relief valve at maximum pressure.
- Set the pump compensator at a pressure 200 to 300 psi higher than the final relief valve pressure.
- Set the relief valve 100 to 150 psi higher than the final compensator setting.
- Set the pump compensator at system pressure.
Another reason often stated for using a relief valve in a pressure-compensated pump circuit is because of pressure spikes. When a pressure-compensated pump has to instantaneously shift from full flow to no flow, fluid leaving the pump while it is shifting to center has no place to go. Because pressure is resistance to flow and resistance is maximum at this point, pressure can climb very high. These full-flow-to-no-flow spikes can easily go up to five to seven times the pump compensator setting, depending on the pump volume.
Adding a relief valve to this scenario can reduce the spikes because a relief valve will respond much faster than a pressure-compensated pump. However, a pilot-operated relief valve still has some response time and will often spike two to three times its setting before opening fully.
A better way to protect the pump and circuit is to install a small accumulator at the pump outlet, pre-charged to approximately 80% of set pressure. Now, when the pump must react quickly, the excess fluid can go into the accumulator with very little pressure spike. An accumulator also helps actuator response time at cycle start because there is a ready supply of fluid even though the pump is at no flow.
Another consideration is pump priming when a pressure-compensated pump is mounted above fluid level. When a system first starts (and sometimes when it has not been operated recently), the inlet line holds no oil above tank level. Atmospheric air in this line above fluid level must be evacuated before atmospheric pressure can push fluid in. Because most pressure-compensated pumps operate against a closed-center circuit, there is no place for this trapped air to go. Hydraulic pumps may easily move 100 gpm of fluid at 3000 psi, but they are very poor air movers. At startup, the pump never primes and could be damaged from lack of lubrication -- especially if the case has not been filled. Usually the outlet line is opened at a union or some other fitting and the pump primes as soon as the trapped air can leave.
A better approach is to install the air-bleed valve shown in Figure 8-31. This valve is not required in most cases when the pump is along side or below the tank because filling the tank should also fill the inlet line. It is also seldom required with a fixed-volume pump in an open-center circuit because the pump outlet has a direct path to tank. However, when priming is a problem and there are no inlet line leaks or restrictions, then the air-bleeds valve may be required. The circuit in Figure 8-30 shows the correct location and piping for this valve.
The cutaway view in Figure 8-31 shows the internal configuration of a typical air-bleed valve. The poppet in this valve normally is held open by a light spring, so trapped air can flow easily through its flow orifices to tank. When the pump primes and oil tries to flow through these orifices, pressure builds and the poppet closes. The poppet stays closed as long as the pump is running.
Always pipe the air bleed valve as close to the pump outlet as possible. Any oil in the line must be pushed out before trapped air can be exhausted, so the closer the better. Always terminate the air bleed valve’s outlet below fluid level. If it terminates above fluid level, air can pass through the valve and let oil in the pump return to tank.