![]() Fig-1-19Īn accumulator absorbs excess pump flow with minimal pressure override or shock. To avoid these problems, use the correct procedure when setting pressures on a relief valve used to reduce pressure spikes. In another example, if the relief valve setting is lower than the pump compensator, all pump flow goes to tank at relief pressure, generating excess heat. This oscillation cycle repeats rapidly, causing damage to the pump and possible line failure due to shock. The drop in pressure allows the relief valve to close, so downstream pressure builds up again. A flow path is created when the relief valve begins to open, so downstream pressure drops, causing the pump to go back on stroke. As the pump nears its pressure-compensator setting and starts to compensate, the relief valve starts to relieve. ![]() For example, if the relief valve setting is at or near the pump-compensator setting, the pump can start oscillating on-off flow. Other problems can occur with relief valves. This reduced spike is better, but still is not as good as what an accumulator could provide. ![]() Once the relief reaches maximum pressure, it starts to open, but by the time it actually relieves, the pressure may be 11/2 to 3 times its set pressure. A relief valve remains closed until pressure reaches 90 to 98% of its setting. This relief valve should reduce the pressure spike, but it does not lower it as much as it appears. Fig-1-18Ī common fix for this pressure spike is to add a relief valve near the pump outlet, set 150 to 200 psi higher than the pump compensator (as shown in Figure 1-16). This pressure spike can cause premature failure of the pump, plumbing, and actuators. All pump flow during shifting time has no place to go, so this excess flow generates a pressure spike of five to ten times the compensator setting. When pressure reaches compensator setting, the pump starts to shift to no flow. There has been zero flow needed for some time, but the pump does not know this until pressure is near maximum. The pump will continue at full flow until pressure reaches 80-98% of the compensator setting. ![]() The pressure-compensated pump is still flowing at the maximum rate and pressure starts to climb. On the other end of the cycle, if the pump is at full flow and all valves center or all the actuators hit the end of stroke, the flow requirement suddenly drops to zero. The cylinder takes off quickly and smoothly, and the pump has time to respond to the flow need. This pressure drop causes the pump to go on stroke, but now pressure drop is minimal. As a cylinder starts to cycle, as seen in Figure 1-18, fluid flows directly to the actuator from the accumulator and pressure starts to drop. However, there is a ready supply of oil at pressure available. With an accumulator installed, as shown in Figure 1-17, the pump is still at no-flow when the circuit is at rest. Pump shifting times vary, but no matter how fast they shift, the actuator’s initial response will be slowed down. When the pump sees a pressure drop, its internal mechanism starts shifting as fast as possible to start fluid flowing. After any directional valve shifts to start an actuator’s movement, pressure in the circuit starts to drop. Accumulators used for fast response and over-pressure control of pressure-compensated pumpsīecause most pressure-compensated pump circuits have closed-center or two-position directional valves (such as the one shown in Figure 1-16), they stay at full-pressure, no-flow until a valve shifts. ![]()
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