FRLS

Image courtesy of Emerson

Compressors generate pressurized air, but that exiting air typically contains dirt and water. Before it can traveldownstream to valves and actuators, it must be filtered, regulated andsometimes lubricated. Otherwise, left untreated, it can damage products,cause premature component wear, attack seals and cause them to leak,and permit rust and corrosion in tools and piping — all leading to fasterbreakdowns and higher maintenance and operating costs.

An air line filter traps particle and liquid contamination in compressed air. It captures solid particles (dust, dirt, rust), and also separates liquids (like water and oil) entrained in the compressed air. Filters are installed in the line upstream of regulators, lubricators, directional control valves and air-driven devices such as cylinders and motors.

There are three types of filters: general purpose, coalescing and vapor removal. General purpose filters are used to remove water and particles, coalescing to remove oil, and vapor removal to evacuate oil vapor and odor.

Pressure regulators reduce and control fluid pressure in compressed air systems. Regulators are also frequently referred to as PRVs (pressure reducing valves). Optimally, a regulator maintains a constant output pressure regardless of variations in the input pressure and downstream flow requirements, so long as upstream pressure doesn’t drop below that of downstream. In practice, output pressure is influenced to some degree by variations in primary pressure and flow.

Pressure regulators are used to control pressure to air tools, impact wrenches, blow guns, air gauging equipment, air cylinders, air bearings, air motors, spraying devices, fluidic systems, air logic valves, aerosol lubrication systems and most other fluid power applications requiring subordinated pressure.

Regulators employ a control-spring acting upon a diaphragm to regulate pressure, and its spring rate determines the range of pressure adjustment. General purpose regulators are available in relieving or non-relieving types. Relieving regulators can be adjusted over a wide pressure range, and even when downstream flow is blocked at the reducing valve, relieving regulators will allow the excess downstream load- or head-induced pressure to be exhausted. Non-relieving regulators, when similarly adjusted, will not allow the downstream pressure to escape. The trapped air will need to be released by some other means; for example, by operating a downstream valve.

A lubricator adds controlled quantities of oil or other lubricant into a compressed air system to reduce the friction of moving components.

Most air tools, cylinders, valves, air motors and other air-driven equipment require lubrication to extend their useful life. The use of an air line lubricator solves the problems of too much or too little lubrication that arise with conventional lubrication methods, such as either grease gun or direct oil application. Once the lubricator is adjusted, an accurately metered quantity of atomized lubricant is supplied to the air operated equipment, and the only maintenance required is a periodic refill of the lubricator reservoir. Adding lubrication to a system also “washes away” compressor oils that travel through the system in vapor form. Mineral oils added to the system prevent synthetic compressor oil build-up on system components. When lubricators are not used in a system, a coalescing filter should be installed to remove compressor oil aerosols.

Image courtesy of Clippard

WHAT ARE SIZING CONSIDERATIONS FOR AN FRL?

Careful consideration to the sizing of the components feeding an end use is important. If not done correctly, the performance of the device can be negatively affected.

Too often, low-pressure problems will occur because a plant has selected a filter, regulator, lubricator (FRL) combination that is a standard size for all devices in the facility. This saves inventory costs because all the spare parts are the same. However, depending on the characteristics of the demands, there could be performance issues caused by excessive pressure drop across undersized components.

If the compressed air device has a “flow static” characteristic, then the pressure only needs to be held above the minimum required level at the end of an operation. For example, a cylinder is moved from one position to another to hold a piece in a clamp. In this case the pressure needs to be adequate only when the clamping action takes place, with minimal flow, not while the cylinder is moving positions.

A “flow static” application is something that requires the minimum pressure to be maintained at the point of use at the same time as peak flow. An example of this might be a cylinder with a constant load that might need to stroke from one position to another in one half a second.

For the flow static application, smaller components can be used — it doesn’t matter much what pressure drop occurs in the filter regulator, as long as the proper pressure occurs at the end of the stroke to apply the required force.

For the flow dynamic application, careful design must take place to ensure the combined total of all the pressure drops across the filters, regulator, lubricator, supply hoses and connectors does not allow the pressure to fall below the minimum required pressure at the point of use.

One common mistake is to assume an incorrect average flow per minute and wrongly size the components too small. Consider a flow dynamic load that strokes a cylinder with a volume of 0.1 ft 3 at a pressure of 60 psi, 4x per minute at 0.5 sec per stroke. This load would consume 0.5 ft 3 of free air per stroke. Operating the cylinder one way, 4x per min would consume about 2 ft 3 of air in one minute. If an FRL were sized for this flow, an excessive pressure drop would occur. It must be sized for the dynamic flow.

The dynamic flow of the cylinder would be the flow rate in the 0.5 sec operating time. Since the 0.5 ft 3 of free air flows in one half a second the dynamic flow during the stroke would be 1 ft 3 per second (0.5/0.5 sec) or 60 cfm. Thus, the flow requirement would be much higher, requiring larger components.

Like all compressed air components, each element of the FRL will have a pressure loss characteristic. You first need to know the characteristic of the end used (flow static or flow dynamic), the flow, and the minimum required pressure. Then you must know the minimum input pressure from your compressed air system. Then it is an exercise in mathematics in selecting the components that will result in proper end use pressure. Each component will have a pressure loss curve you can consult to find the pressure loss at your stated flow.