Machine Design

Sorting out pressure-compensated flow control

Fluid-power-system designers often regulate flow with pressure regulators. Unfortunately, some applications require constant volumetric flow rate regardless of pressure fluctuations. When this is the case, pressure-compensated flow-control (PCFC) regulators are a good solution.

The devices are based on the principle that flow is proportional to pressure drop. By maintaining a fixed differential pressure across a restriction, the flow remains constant. In practice, the regulator requires two restrictions, one variable (a reducing valve) and one fixed (a reference restriction).

A piston modulates the reducing valve, which experiences the same differential upstream and downstream pressures that act on the reference restriction. This arrangement allows the reducing valve to vary the inlet pressure of the reference restriction and maintain its differential pressure at a constant value. The reference restriction is a needle valve set to the desired flow rate.

Modulating the upstream pressure of the needle valve regulates flow. If supply pressure to the regulator varies, the reducing valve opens or closes to maintain constant upstream pressure to the needle valve. When downstream pressure varies, the reducing valve adjusts pressure upstream of the needle valve.

Downstream pressure fluctuations can have undesirable effects when regulating the flow of a compressible fluid. PCFC flow regulators respond by modulating upstream pressure to the needle valve. This causes the upstream density of the compressible fluid to vary. The regulator attempts to maintain a constant volumetric flow regardless of fluid density and, thus, the mass flow fluctuates. If an application requires precise mass-flow regulation, install a backpressure regulator downstream of the flow regulator to maintain constant downstream pressure.

PCFC flow regulators have other operational limitations. The devices require adequate supply pressure to generate the differential pressure across the needle valve. In addition, because the reducing valve is not balanced, changes in differential pressure will vary its balance point and result in small changes in the output flow rate of the PCFC.

Several guidelines help ensure proper use of PCFC flow regulators. First, as with all regulating and metering devices, it is good practice to install a filter upstream of the device. Also, if the application has severe swings in pressure differential, or very stringent flow requirements, it may be necessary to add pressure regulation upstream or downstream of the flow control.

Keep in mind that PCFC Series regulators operate with best accuracy when the flow is set with inlet/outlet pressure differentials in the middle of the expected range. Also, verify that the application is suitable for the flow control. If the regulator seems unresponsive, there may be inadequate supply pressure or excessive downstream pressure. Finally, be aware of mass-flow fluctuations when regulating the flow of compressible fluids.

This information supplied by Beswick Engineering Co., Inc., Greenland, N.H.

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