Pressure-reducing and reducing//relieving valves
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There are times in multi-actuator hydraulic circuits when system pressure is too high for some actuators while others need maximum force. One suggested remedy is the circuit in Figure 14-12. Cylinder 1 needs 2000 psi to maintain force, while Cylinder 2 can damage the product when pressure exceeds 800 psI. Adding Relief Valve 2 (set at 800 psi) takes care of Cylinder 2’s overpressure, but limits the entire circuit to 800 psi. Pressure in a circuit with more than one relief valve will never be higher than the setting of the lowest valve. The correct way to have two or more pressures in a single circuit is to incorporate reducing valves. (Figure 14-14 diagrams a circuit using a reducing valve to give two pressures.)
The cutaway and symbol in Figure 14-13 depicts a pilot-operated reducing valve that allows flow from the inlet to the reduced-pressure outlet until pressure reaches the setting on the direct-acting relief valve in the pilot section. Unlike the other four pressure controls (relief, unloading, sequence, and counterbalance valves), a reducing valve is normally open and blocks flow at set pressure.
The normally closed direct-acting relief valve in the pilot section traps fluid from the reduced-pressure outlet port through the control orifice on top of the spool when pressure is below its setting. The spool stays in the normally open position because pressure on both ends balances it hydraulically while the light spring keeps it pushed down. As pressure at the reduced-pressure outlet port continues to increase, it finally starts to open the direct-acting relief valve in the pilot section. Some fluid then flows to tank through the drain port. When flow through the direct-acting relief valve is more that the control orifice can handle, pressure on top of the spool drops and pressure on the bottom of the spool pushes it closed. The spool never closes completely because there is flow through the drain port anytime pressure at the outlet is lower than at the inlet. Drain port flow amounts to about 60 to 90 cim. This flow is all wasted energy and it can cause a system to overheat if more reducing valves are installed than necessary. When pressure drops below the direct-acting relief valve’s setting in the pilot section, the valve closes and forces the spool to the open position.
A reducing valve is normally open so it appears reverse flow should not be a problem. However, when the valve is working, it is almost closed -- and it can be held closed by back flow when the actuator starts to return. Anytime a reducing valve must pass reverse flow, select a valve with an integral bypass check valve to eliminate the possibility of blocked return flow.
It also is very important to have a free-flow drain port with very low (or even no) backpressure. Backpressure in the drain port adds to the setting of the direct-acting relief valve and can cause erratic results when drain pressure fluctuates. (Our next e-book, Fluid Power Circuits Explained, discusses how this drain port can be used advantageously in a dual-pressure circuit. This book will be launched in the next few months.)
The modified circuit in Fig, 14-14 allows two pressures without lowering system pressure (as happened in Figure 14-13). A pressure-reducing valve in place of Relief Valve 2 makes it possible to set pressure for Cylinder 2 without affecting pressure at Cylinder 1. This reducing valve never has reverse flow so a bypass check valve is not required.
When it is working, a reducing valve is nearly closed and will pass very little reverse drain flow unless it has a bypass check valve. Even then, reverse flow must be at a pressure greater than that at the inlet. If this much pressure cannot be tolerated, use the reducing/relieving valve depicted in Figure 14-15.
Reducing/relieving valves function exactly like reducing valves -- until an external force starts to increase pressure at the reduced-pressure outlet above the pressure set by the pilot section. When outlet pressure is 4 to 6% above set pressure, the spool moves up until the outlet is connected to tank. Any fluid at pressure above set pressure returns to tank, so outlet pressure does not continue to climb. Tank flow comes only from the reduced-pressure outlet, not from the pump through the inlet. When excess pressure at the outlet drops, the reducing/relieving valve continues to perform its reducing function.
Note that the left cutaway view has an internal drain for the pilot section. This saves connecting a separate drain line for pilot flow. However, when backpressure in the tank line is high or may fluctuate due to other return functions, it adds to the pilot-section setting and can elevate pressure at the reduced-pressure outlet above allowable rates. When tank-line backpressure may be high or when pressure fluctuations cannot be tolerated, use a valve with an external drain. When reverse flow is necessary, specify a model with an integral bypass check valve for piping convenience.
Figure 14-16 shows most of the modular valve configurations for sequence, counterbalance, and reducing valves. Modular valves simplify piping and eliminate many connections that can generate backpressure or add potential leakage points.