Pneumatic motion control — using compressed air to power actuators, air motors, grippers, and the like — is a long-proven, time-tested technology. Pneumatics applications are found in most any industry, and performance requirements and design parameters can vary widely.
In life sciences, for example, compressed air is used in patient care devices like respirators. They require high precision and controllability, and often light weight and small size. At first glance, a rail-car braking system may appear far removed from a respirator, but it also requires reliability and controllability despite extreme temperatures and dirty environments.
Pneumatics in robotic manufacturing for automotive assembly and metal stamping requires repeatable, tight-tolerance motion control. Food and beverage packaging demands high-speed automation. Controls on commercial vehicles must be compact and accurate, with quick response times. And industrial automation and processes like energy production require long-lasting, powerful systems.
These examples illustrate both the versatility and variety of pneumatic systems. And designers naturally focus on actions and outputs when developing a compressed-air circuit, considering factors such as function, size, energy consumption, power, and capacity.
Regardless of application requirements, proper air preparation is a must if the system is going to perform as required for the longest possible service life. Here’s a look at what designers should consider to select the right filters, regulators, and lubricators to keep systems running efficiently and hassle free.
First, keep in mind that air produced by a compressor is typically hot, wet, and dirty. The first step in good air preparation is to filter out contaminants that interfere with proper operation and shorten equipment life.
Moisture vapor in the air exiting a compressor outlet will condense to liquid as the air cools. This can reduce efficiency and capacity. Water is most efficiently removed at the lowest possible air temperature and highest pressure. Thus, air should be cooled with an aftercooler near the compressor outlet, along with dripleg drains, automatic drain valves, or filters to remove water upstream of any pressure-reducing valves. A properly sized general-purpose filter will remove liquid water.
Removing liquid water does not remove water vapor from the air. Applications ranging from paint spraying to locomotive braking may require that all vapor be removed. To do this, air dryers are needed. The three principle types are refrigerant, regenerative adsorbent, and deliquescent absorbent dryers, and they vary in terms of drying capabilities and operating costs. (For more information on air dryers from our Fluid Power Basics section, click here.) Also note that all dryers will be compromised if contaminated by liquid water, oil, or water/ oil emulsions, so they should always be used in tandem with filters and air coolers.
Solid particle contaminants can be introduced through the ambient air or by corrosion or wear in the compressor. Most standard air-line filters remove coarse particles 40 μm and larger. Fine particle filtration (10 to 25 μm) is required for high-speed pneumatic tools and process-control instrumentation. Filtration ≤10 μm is essential for air bearings and miniature pneumatic motors. And for tasks such as paint spraying, breathing air, and food-safe applications, particle removal below 1 μm is essential. High-efficiency coalescing filters are required to remove such fine particles from the air stream.
Generally, it is inadvisable to provide finer filtration than is absolutely necessary because fine filter elements trap more dirt and become blocked more rapidly. But when needed, use standard air-line filters as prefilters to avoid overburdening high-efficiency elements with coarse particles.
Oil, another contaminant in compressed air, can exist in three forms — an oil/water emulsion, aerosol, or oil vapor. Standard air-line filters can remove emulsions.
Aerosols — small particles between 0.01 and 1 μm in size — can only be removed by special coalescing filters. These are typically rated by the amount of air they can process at a given cleanliness level, normally a maximum remaining oil content of 0.01 ppm in the exit air. Flow that exceeds the rating will not only increase pressure drop across the unit (and, therefore, energy costs) but, more importantly, remaining oil content will increase. Again, protect coalescing filters from particulate and water contamination with air-line filters mounted immediately upstream.
Oil vapor is typically present in such minute quantities that it can be ignored except in sensitive applications like food and beverage processing, pharmaceuticals, and breathing air. Passing air through an adsorbing bed of activated carbon, after flowing through standard and coalescing filters, removes oil vapor.
Once all contaminants have been considered, the degree of cleanliness for each machine or part of a plant can be determined. Using the proper filter in the right location minimizes energy and maintenance costs. Always determine the volume of air involved in each stage as undersized, inappropriate filters are a prime cause of high energy costs.
All pneumatic equipment has an optimum operating pressure. Exceeding this pressure doesn’t mean higher productivity; it just causes excessive wear while wasting compressed air. To use compressed air most efficiently it is necessary to reduce the pressure of air leaving the compressor to the precise level an application requires.
Systems tend to operate at two pressure levels — compressed air stored in a receiver at higher pressure (optimizing filter performance and energy efficiency); and air used by actuators and other devices, usually at a pressure 10 to 20% lower. This arrangement ensures the compressor is not constantly running.
Regulators (pressure-reducing valves) control pressure. They have two important performance characteristics: regulation (maintaining consistent outlet pressure regardless of inlet pressure) and flow (maintaining consistent outlet pressure regardless of flow rate). The precision of regulation and flow required will dictate the type (and cost) of the regulator.
Most regulators fall into four categories: general purpose, pilot-operated, precision, and special purpose. Most general-purpose regulators are diaphragm types, though piston versions are used where equipment demands higher flow capacity for a given size. Relieving regulators can be adjusted to lower downstream pressure without actuating downstream equipment. To adjust a non-relieving regulator to deliver lower pressure, downstream equipment must be cycled or a 3/2 shut-off valve used to expel excess air.
Pilot-operated regulators control outlet pressure by means of an air pressure signal produced by a precision regulator. This means, for example, that the regulator can be mounted in large distribution mains but controlled remotely from the shop floor. Engineers usually use this type of control where a continuous process requires a large, steady air flow.
Precision regulators, or controllers, are normally used for instrumentation applications where fast response, exact repeatability, and control of outlet pressure are necessary. These units have a limited range but superior flow and regulation characteristics. Precision regulators also can relieve up to 80 to 90% of their flow for specialized applications such as tensioning belts, paper rolling, and balancing.
Special-purpose regulators can be based on any of the other types, with application-specific modifications. For example, they may be constructed of special materials like stainless steel, have high relief flows, or operate with a plunger instead of a handwheel.
Combination filter/regulators clean air and control pressure in a compact unit, saving space and costs. Specialized filter/regulators remove fine particles of oil, offering precise regulation.
The next important step in processing compressed air is introducing a lubricant, usually oil, to ensure operating equipment performs efficiently without excessive resistance or wear. (Note that oil carried over from a compressor is a contaminant that has lost its lubricating capabilities and should be filtered out.)
The most widely used lubricator is the aerosol lubricator which, incidentally, was invented by Carl Norgren in 1927. Two types of aerosol lubricators are oil-fog and micro-fog. Both use reservoirs or bowls filled with oil. The lubricated air or “fog” generated by oil-fog lubricators has relatively large oil particles that cannot rise or travel far before dropping out of the air stream. So install oil-fog lubricators near the equipment they are meant to service, and never below it.
Micro-fog lubricators atomize the oil in the bowl, creating light particles less than 2 μm in size. This fog can travel upwards and for long distances through complex feed lines. Micro-fog units can also ensure proportionate distribution through numerous lubrication outlets, making it ideal for multiple valve-control circuits. Only about 5 to 10% of the oil in a micro-fog system becomes an aerosol, so it works well in applications requiring only small amounts of lubricant. By adjusting the drip rate, oil delivery can be raised to approach that of an oil-fog lubricator.
The other type of lubricator is a positive-displacement injection pump. It does not continuously deliver lubricant like aerosol lubricators but, rather, injects the same amount of lubricant every cycle. This type of lubricator is often used on conveyor chains, for example. Several injectors can be manifolded together to lubricate at several different points at the same frequency.
Most valves and cylinders powered by pneumatics are prelubricated and, in most applications, do not require additional lubrication during their service life. Once they reach the end of their lubrication life, non-repairable cylinders must be replaced. Other components can be repaired, but technicians must apply new grease before they are put back into service. In addition, contaminated air will gradually compromise the original grease lubricant and shorten seal life.
Lubricated air prolongs prelubricated components’ life and performance. Applications that have high cycles, operate at high speeds, or use large diameter bearings generate heat that speeds the deterioration of internal lubrication. Laboratory tests show that cylinder life in these applications can be doubled by using lubricated air. Lubricated air is also essential for equipment that is not prelubricated; for example pneumatic hand tools, such as screwdrivers.
Lubricated air extends cylinder life, but it also washes out the original lubrication. So once lubricated air is introduced, it must always be used. The amount of oil for sufficient lubrication will vary with each pneumatic device. So always follow equipment manufacturers’ recommendations. From regular inspection and servicing, technicians can determine the optimum setting and adjust the amount delivered.
Compressed air is powerful, so pneumatic systems must have safety features designed in and maintained to protect equipment and personnel.
The first consideration is overpressure protection. Components in pneumatic systems often have a pressure rating lower than that generated by the compressor. If for some reason the regulators do not maintain safe working pressure (SWP), downstream components exposed to excess pressure can malfunction or fail.
The most common overpressure protection is a relief valve. This device holds system pressure at a constant level at or below the stated SWP. Commonly there is a 10% overpressure allowance. Relief valves should operate only when the system exceeds regulated pressure, so they need to be set to a pressure higher than that for the regulator. A common problem is a relief setting too close to the system operating pressure, causing the relief valve to vent air during normal operation.
The relief device must also be sized to match or exceed flow through the part of the systems it protects, without system pressure rising above the acceptable overpressure level.
Finally, consider start-up. Loading during start-up can cause unnecessary wear on moving parts, and sudden movement can injure personnel. Soft start (also called slow start) valves prevent such problems. They let air gradually pass from a compressor to the pneumatic system. Adjusting the valve controls the rate of pressure build-up. The valves usually have a spring-operated internal poppet design, which is usually set to open, or “snap,” when pressure reaches between 40 and 70% of full-line pressure. It is more economical to set these devices near the equipment they are intended to protect than to fit a larger valve to the whole distribution system.
For more information on air preparation, visit www.norgren.com.