Most plant air systems produce pressures between 90 and 125 psi, while most air circuits are designed to operate at 75 to 85 psi. Other systems may operate at pressures as low as 15 to 20 psi. To accommodate these ranges, some method is needed to reduce the system pressure without wasting energy. A relief valve that would release plant air to atmosphere and try to lower the whole system it is not a good solution. The air line regulator shown in Figure 14-10 reduces outlet pressure by shutting off flow when downstream pressure tries to go above the regulator’s setting. There is very little energy loss because air merely expands from its elevated pressure to meet the lower pressure requirement. In other words, an air compressor operating at 120 psi only has to run about a third as often when regulated or reduced to 40 psi.

This points out the main reason why an air line regulator should be set just high enough to do the job at hand. Without a regulator, not only does it cost more to operate a machine, but the machine tries to run a repetitive cycle with fluctuating pressure, therefore different forces and speeds.

The cutaway views and symbols in Figure 14-10 show two common direct-acting air line regulators. They are normally available in sizes from 1/8-in. through 2-in. pipe thread. (Larger sizes are built but they usually are pilot-operated from a small direct-acting regulator.) Air flows freely from inlet to outlet until the outlet pressure reaches the set pressure. The adjustable spring holds the shut-off valve off its seat by extending the diaphragm during free flow. As pressure at the outlet continues to build, it passes through the pilot passage to the underside of the diaphragm. At set pressure, the diaphragm pushes the adjustable spring back, allowing the shut-off valve to seat. The light spring pushes the shut-off valve closed. Pressure at the outlet now is stable at its reduced setting -- as long as the inlet pressure is equal to or higher than the outlet. Any pressure drop at the outlet reduces pressure under the diaphragm and the adjustable spring again pushes the shut-off valve open to let more air in.

If there is a possibility of the reduced pressure line seeing excess pressure for any reason, use the relieving-type regulator shown on the right in Figure 14-10. This valve closes a hole through the diaphragm’s center section with the shut-off valve’s stem. After reaching set pressure, the shut-off valve cannot move up. Any extra pressure buildup under the diaphragm raises its center section off the shut-off valve’s stem and allows air to flow to atmosphere through the vent hole. This feature should not be used as a relief valve function where pressure increases during every cycle -- it is only for occasional overpressure situations.

Every pneumatically powered machine should have a regulator set for the lowest pressure that will produce good products. Costly overpressure should be eliminated in every case. Use an air line regulator anytime a job can be done at a pressure lower than plant air supply.

Another application for air line regulators that can save compressor output is reducing pressure on the return stroke of actuators that can use low power to retract. Many cylinders need high force to extend and do work, but the retract portion of the cycle needs very low force. An air line regulator positioned as shown in Figure 14-11 can save air during part of every cycle on many cylinder operations in most circuits.

A 5-way spool valve, piped with a dual-pressure inlet as shown, can give normal cycle time while conserving plant compressed air. Return pressure is set on the regulator supplying the cylinder rod end at the lowest possible pressure that maintains cycle integrity. A reduction as small as 20 psi below working pressure can pay for the regulator in a short time. Shifting the 5-way valve starts the cylinder extending. (There will be a brief lunge as the lower-pressure air in the rod end compresses to hold back against the higher pressure in the cap end.) To control cycle time, adjust cylinder speed with the rod-end meter-out flow control. When the 5-way valve shifts again to return the cylinder, the meter-out flow control on the cap end must be adjusted for a faster rate because return power is limited.