Flow and pressure controls, Part IV: Pressure control in circuit branches - Hydraulic Systems Trends

Diesel Progress North American Edition, July, 2003 by Russ Henke

In this discussion on flow and pressure controls, we will take a look at pressure control in circuit branches, system pressure drop considerations, the force-balance concept and the pressure compensator.

Pressure reducing valves are normally open, two-way valves that sense system pressure downstream from the valve inlet. One type maintains fixed reduced pressure in a circuit branch regardless of the pressure in the balance of the system, while the other maintains a fixed pressure differential to provide varying reduced pressure with change in system pressure. Like relief valves, pressure reducing valves are either direct acting or pilot operated.

To ensure proper functioning, pressure reducing valves should be drained to tank. This prevents downstream pressure build up (backpressure) due to leakage in normal return lines.

Another application suitable for some types of reducing valves is shown in Fig. 1. A reducing valve, G, supplies a secondary branch at a pressure level below that in the primary circuit.

A sequence valve is a normally closed, internally piloted valve which remains closed until pressure in the primary circuit reaches the preset level of the valve. Then the sequence valve, H, (Fig. 2) opens to provide output flow into the secondary circuit. This type of control is used to switch flow to a secondary circuit after the actuator in the primary circuit has reached the end of its stroke and pressure rises. This design eliminates the need for a directional control valve to sequence flows.

The sequence valve, H, will close again when the directional control valve in Branch 1 (not shown) is shifted to retract cylinder A. To prevent premature operation, design operating pressure to cylinder A in Branch 1 must be below the setting of sequence valve, H.

Fig. 3 illustrates a typical application of a counterbalance valve, J, a normally closed, two-way valve with internal pilot, internal drain and a built-in, free-flow check valve for reverse flow. It is used to prevent free fall of a load held up by a cylinder or fluid motor or to provide controlled resistance in a line. Overcenter valves are counterbalance valves with external piloting to accommodate load reversals on actuators. Brake valves are special counterbalance valves used with hydraulic motors.

A pressure switch is an electrical device operated by a pressure-sensitive element which can actuate a solenoid- controlled valve when a preset pressure is reached. This valve may unload a pump, switch to a secondary circuit, or reverse an actuator.

Though not a pressure control technique in the strictest sense, pump unloading is of considerable importance in some circuit designs because it re- duces power consumption during idle periods in the cycle. By eliminating the need to bypass oil over a relief valve, an unloaded pump reduces the amount of heat a system generates.

There are additional pressure control techniques. Control valve unloading is shown in Fig. 4 and shows a circuit using a two-way, normally closed directional control valve, K, which vents system relief valve, L, to unload the pump. The solenoid in K is actuated by a limit switch or other signaling device. In this application, the normally closed directional control valves must be centered to unload the pump.

An accumulator maintains a preset system pressure when the closed-center directional control valve is in neutral. A pressure switch energizes the solenoid of two-way valve which shifts to bypass pump flow to tank.

A low pressure relief valve, P (Fig. 5), connected in parallel with the cylinder's head end, is set just high enough to retract the cylinder. When the cylinder is fully retracted, low pressure relief valve, P, unloads the pump. The machine operator doesn't need to hold a spring-centered directional control valve shifted until the cylinder is frilly retracted.

If three-position, two-way, normally open valves are arranged in series with each main directional control valve (Fig. 6) the system relief valve will be vented when all directional control valves are in neutral. However, if any one of the main directional control valves is shifted, the vent line will be blocked. The number of venting valves that can be put in series is limited by the backpressure across each valve.

Pressure Drop

Pressure drop is a reduction in pressure between two consecutive points in a fluid power system. This happens because energy in the system is required to do work to maintain fluid flow against a resistance. This resistance can be internal fluid friction, orifice-like restrictions in the flow paths or external load resistance.

A pressure drop between a pump and motor represents a loss in energy which manifests itself as heat. The pressure drop across the motor reflects the energy being transferred to the external load. Motor efficiency is an indication of internal losses which reduce the actual energy of the motor and the energy available for transfer.

The pressure drop across a given valve varies as the ratio of the specific gravity of the fluid. If, for instance, we know the pressure differential, [[DELTA].sub.p1], for a fluid with a specific gravity [S.sub.g1], then we can approximate the value of [[DELTA].sub.p2] for a second fluid from the expression: [[DELTA].sub.p2] = [[DELTA].sub.p1] ([S.sub.g2]/[S.sub.g1]). Similarly, the pressure drop varies as the square of flow rate: [[DELTA].sub.P2] = [[DELTA].sub.p1] [([Q.sub.2]/[Q.sub.1]).sup.2].