Understanding the Regulator in FRLs

Filter, regulator and lubricator units (FRLs) are used to condition compressed air before its use in a machine or tool. In a past edition of Tech Talk (MRO, November 2002, p. 36), we looked at filters, why they are needed and how they are used in a compressed air system. This issue will look at the next component in the system — the airline regulator.

Regulators are used to provide a relatively constant downstream pressure, regardless of variations in the inlet pressure and system flow demands. Various levels of regulating precision and consistency of performance are available to meet more exacting demands, but the standard regulator used in FRLs will usually be adequate for most applications.

Regulators are generally fitted with a pressure gauge to monitor the down-stream pressure. They can be built into the top of a filter to provide a combined filter-regulator unit.

Pressure regulation is used to control the force of an air cylinder or the torque output of an air motor. Pressure regulation is not intended to control the speed of these devices. Flow controls, or governors, are more effective for speed control, especially when the load on these devices varies.

The use of a regulator on each machine connected to the plant compressed air system is essential for consistent operation of that machine. High flow demands from one machine or a particular area of the plant can cause fluctuating pressure drops in the entire distribution system The on and off cycling of the plant compressors also adds to the problem.

Regulators (Fig. 1) are actually two-way valves (or three-way when self-relieving) normally held open to flow by an adjustable spring (C). Downstream air pushing on a diaphragm or piston (B) located below the spring will deflect to close the poppet valve (A) to stop the flow of air when sufficient downstream pressure is reached.

Most pressure regulators used in FRLs have a relieving feature which allows the set pressure to be decreased with the flow dead-ended. This is generally accomplished by bleeding any excess pressure from the diaphragm chamber through a hole in the diaphragm seat (D) that is opened when the diaphragm lifts clear of the fully-closed valve stem. The air bleeds into the spring bonnet and finally to atmosphere through a bleed hole (F). Any increase in downstream pressure above the set pressure will also be bled to atmosphere in a similar fashion.

The relief flow-back through the regulator is very small and should not be considered as part of the function flow pattern of a circuit. For example, if the return stroke of a double-acting cylinder needs to be at a lower pressure, the obvious position for a pressure regulator is in the airline between the valve and the cylinder port. However, the relief capacity of the regulator will restrict the exhaust flow of the cylinder on the forward stroke. A solution is to put a quick exhaust valve in the line between the regulator and the cylinder to allow the exhaust air to bypass the relief system in the regulator and exit straight to the atmosphere.

A regulator and quick-exhaust combination can also be used to replace a return spring in a cylinder or to create a cylinder balancing device (Fig. 2). In both cases, the “air spring,” unlike a mechanical spring, will have a constant pressure over its entire stroke length — a very valuable feature in both applications.

Precision regulators (Fig. 3) offer highly accurate regulation, consistent over the entire flow range. They are used for air gauging, instrumentation and other sensitive lab applications. They are also very reliable in low-flow or “dead-ended” situations.

Inaccuracies in low-flow applications are generally caused by the fully closed poppet valve in the regulator sticking slightly before it opens. To prevent this, manufacturers build a tiny leak into the diaphragm area of the regulator so that the air flow never really stops and the poppet valve never fully closes.

Another feature normally found in higher flow precision regulators is the use of a secondary “pilot” regulator to control an “air spring” in the main regulator. The air spring replaces the mechanical spring in the main regulator and, because it exerts a constant force on the diaphragm over the entire stroke, maximum opening of the poppet valve is allowed, thus creating large and immediate flow responses to maintain constant downstream pressures.

The secondary plot can be an integral part of the main regulator or can be mounted remotely to allow the main regulator to be put in an inaccessible location in the ceiling or under a bench

Compressed air regulators are indeed an important part of any air system and as you can see their uses extend far beyond the common application of FRLs. In the next issue of MRO, we will dissect the airline lubricator — the last unit in the all-important FRL chain.

Ted Grove is a widely experienced fluid power trainer and is corporate training manager of Wainbee Limited of Mississauga, Ont. He can answer your specific questions regarding fluid power systems at 905-568-1700 or tgrove@wainbee.ca.