What is in this article?:
- BOOK 2, CHAPTER 16: Reducing valves
- Two-pressure circuit with a pressure-reducing valve
- Remotely operating a pressure-reducing valve
- Modular pressure-reducing-relieving valves
When one branch of a fluid power circuit must operate at a lower pressure, use a reducing valve to provide it. Reducing valves control their outlet or downstream pressure only.
Air line regulators, Figure 16-1, reduce pressure for a pneumatic circuit. Because air in the supply line to a machine is at maximum pressure, energy can be saved by reducing pressure whenever possible. With a compressor setting between 115 and125 psi and a machine requirement of 70 psi, approximately 40% of the input energy would be lost without a properly adjusted regulator. The air-driven machine will work at the higher pressure, but it consumes more compressor horsepower than necessary.
Another use for air line regulators is on retraction strokes of air cylinders. Reducing pressure on a cylinder’s retraction stroke saves air and thus consumes less compressor horsepower.
In multiple actuator circuits, it is often impossible to size all actuators to operate at maximum system pressure. For example: when a cylinder needs 5000 lb of force and one standard bore produces only 4712 lb at maximum pressure, the designer must go to the next larger standard bore. However, the next larger bore produces 7363 lb of force, which can cause machine or part damage. Instead, install a pressure-reducing valve in the branch circuit with the over-sized cylinder, as in Figure 16-2, to lower that branch’s pressure to generate the required cylinder force.
A standard reducing valve is normally open. When downstream pressure goes higher than its setting, the valve closes, blocking flow. If pressure downstream tries to increase — say due to resistance from an opposing cylinder — a reducing valve also blocks reverse flow. Escalating pressure in the downstream line continues until something bursts or gets mechanically damaged.
Figure 16-3 shows the symbol for a reducing-relieving valve. A reducing-relieving valve sets maximum outlet pressure, then relieves fluid to tank when outlet pressure tries to go higher. The overpressure could be due to outside forces or possibly high temperature in some environments. A reducing-relieving valve has an integral relief valve with a full-flow line to tank. When pressure in the downstream circuit rises 3 to 5% above reduced pressure, trapped fluid relieves to tank. Adjusting the reduced pressure automatically sets the maximum relief pressure.
Hydraulic reducing valves always have a drain line open to tank for control oil flow. Drain oil flows when reducing valve outlet is lower than its inlet. This generates a small amount of heat in the system. Blocking the drain line forces the valve wide open and lets outlet pressure rise to system pressure.
Multiple pressures in one circuit
Figure 16-4 has a schematic diagram for two cylinders that need different pressures. One option a novice designer might use is to add a second relief valve. However, second relief valve B reduces pressure in the whole circuit. System pressure cannot go above 400 psi — making high-pressure relief valve A useless.
In Figure 16-5, reducing valve C replaces relief valve B. Now each cylinder operates at a different pressure. Note that there is no bypass check valve on reducing valve C. When the system does require reverse flow through the reducing valve, the bypass check valve can be omitted. However, for a circuit with reverse flow always use a bypass check.
With the reducing valve installed in the line that feeds the directional control valve, pressure at both ends of the cylinder is reduced. Also, when the pump is at pressure, reducing valve drain line flow is constant. Drain flow amounts to 20 to 70 in.3 minimum, and produces heat. With several reducing valves in a system, drain line flow might require a larger pump and a heat exchanger.
Figures 16-6 and 16-7 show the preferred location for a reducing valve. In Figure 16-6, the circuit is at rest. There is no drain flow with the reducing valve in the line between the directional valve and the actuator. This arrangement eliminates oil heating and provides extra flow to other actuators. When both ends of the cylinder need pressure reduction and/or different pressures, use the arrangement in Figure 16-7. The components cost more up front but the energy it saves often pays for the extra reducing valve.
A reducing valve is normally open from inlet to outlet, but closes when reaching the outlet pressure setting. When an actuator at reduced pressure reverses suddenly, the reducing valve does not have time to open. Oil forced out of the cylinder that tries to go back through the reducing valve keeps pressure on the outlet, holding it shut. A small pilot-drain flow in this blocked reverse flow condition allows very slow reverse cylinder movement. A reducing valve with a bypass check may try to stay closed but will not block flow, so the cylinder reverses easily.