Here are some best practices to supply safe, clean and dry air to a machine or piece of equipment.
Plant-supplied compressed air often travels a long and varying path from the air compressor to where it is connected to a machine or piece of equipment. Along this path there are many opportunities for contamination via dirt, dust, oil, water, and other impurities. Therefore, compressed air must be filtered at the supply point, and it must also be safely controlled, filtered, regulated, and sometimes lubricated—all in the proper order—at the machine level.
This article will discuss what a plant air preparation assembly should include and why, and will also show how to calculate required flow rates.
Air prep required
A steady flow of clean and dry air is a common requirement to operate and protect pneumatic components in machines, processes and equipment. Any pneumatic-actuated motion—such as clamping, gripping, lifting, positioning, and pushing—requires clean and dry air supplied at sufficient flow and pressure. To supply this clean and dry air, an air prep unit provides filtering, regulating and sometimes lubricating (FRL) of the compressed air.
Understanding some of the signs of an air prep problem can enforce the need for an FRL at the machine level. The presence of dirt, particulates and moisture can cause valves to stick and not switch position when energized or turned off. A similar problem is a valve leaking when turned off, with the same issues causing seal wear and damage. Contaminates can clog-up valves, spools, small pilot ports, mufflers, and flow controls.
Just as dirt, debris and moisture can affect external and seal surfaces of cylinders and actuators, they can also cause internal wear due to material build-up, scratches and other friction-related issues. In some cases, especially in humid regions or with improper filtering and drying at the compressor, the cylinders can fill with water. This can damage the internal surfaces of the cylinder, and it can change the stroke because the water will not compress.
All of these potential problems can cause a host of productivity and maintenance issues. Valve switching speed and cylinder movement may slow, reducing equipment cycle times. The resulting excess friction, wear and/or fluid build-up can lock cylinders, often requiring repair and replacement.
Using an air prep system on a machine eliminates most problems caused by dirt, particulates and moisture. The filter removes the particles created by the plant compressed air distribution system. Filters local to the machine also often filter the air to a finer level than the overall plant filters.
Air prep also provides a means to regulate pressure to a known level, and to monitor it to ensure proper machine operation. The air prep also provides a manual and electrical shutoff for safety and maintenance requirements.
Compressed air, like electrical and hydraulic energy sources, can be hazardous, causing severe or fatal injuries, so it must be properly installed and used, with proper training for personnel. Typical hazards include flying debris and chips causing eye injuries, or a broken hose whipping around causing the same or worse injuries. At less than 15 psi, compressed air can even dislodge an eye. At high pressures it can be injected into the body and bloodstream and rupture eardrums and organs, or cause an air embolism and possibly a heart attack. Even the loud noise created by escaping air can damage hearing.
Compressed air supplied to a machine is normally between 80 and 120 psi. Most pneumatic cylinders, actuators and hand tools such as air guns require such pressures to operate effectively. However, OSHA limits compressed air’s use for cleaning purposes by regulating the air pressure to 30 psi or less, and also requires chip guarding. Users should also strictly limit the use of compressed air for cleaning a work area, and never use it to dust off clothing.
Improper installation, such as using PVC pipes for compressed air distribution, is also a hazard. PVC can shatter due to pressure, impacts or exposure to UV light.
Air hoses and pipes can also cause injuries, for example, if someone tries to grab a whipping hose or trips over an air line. Air can also remain in a pipe as stored energy, which can cause unexpected machine operation, so it is necessary to bleed or dump air from all air lines before maintenance is performed. For machines, a means of disconnecting the air and depressurizing the air lines by isolating the main air supply must be provided.
When it comes to pneumatic power presses or any cylinder that can produce tens, hundreds or thousands of pounds of force—there are many safety requirements beyond the use of air preparation. Two-hand control, redundant circuits, lockout/tagout of power sources and other requirements are of utmost importance in these types of applications but are beyond the scope of this article.
Air preparation components
Operator, maintenance and machine safety is an important part of an air prep system design. The following devices in the list below should be included in the order listed from upstream to downstream air flow.
Air perpetration devices
- Manual shutoff relief valve with lock-out
- Pressure switch
- Electrically operated soft start valve and air dump
- Lubricator, if needed; best practice is to install it as close as practical to components that need oil
Air preparation devices can be purchased separately or as an assembly. Total air prep units are also available, providing all the items in the Table connected and ready for integration onto the machine which can save time, money and provide a cleaner look for modern machinery.
Air preparation starts with a manual shutoff relief valve with lock-out. This safety-related lockout and tagout device is used to depressurize air lines before beginning service or maintenance. All machines should have one.
Moving downstream, the next device required is a filter or a combined filter-regulator. Multiple configurations are available, but some key features include filtration, drains and traps. A filtration level of 20 microns works for most machines, with 5 microns providing better protection. However, finer filtration will require more filter maintenance. The moisture drains and traps are a must and should include a manual or automatic drain to remove the liquid.
The regulator should include a local pressure gauge. A best practice is to use a digital pressure switch installed downstream of the regulator to provide a local, visual pressure reading — and also a discrete “pressure okay” input to the automation system.
Most machines have an emergency stop function, so an electrically-operated soft start and air dump valve activated by this function should be installed. An emergency stop activation will then dump pneumatic energy. This valve can also slowly increase supplied air pressure, providing a soft start when energy is reapplied.
A final component in an air preparation unit (that is not often used) is a lubricator. It provides an oil mist to pneumatic devices that may need it such as an air motor. Most cylinders and actuators should be supplied with compressed air upstream of the lubricator, with the lubricator mounted as close to the device needing oil as practical.
Ensuring sufficient air flow
While the availability of a compressed air source in a large facility is rarely a concern, the air pressure at a machine can sag without adequate air flow. This is especially true if supply pipes or hoses are not the proper diameters, and during times of high demand.
While pressurizing a single 2-in. diameter cylinder doesn’t require high flow, pressurizing five of the same size cylinders at the same time might. Other high demand times may be intermittent such as when blowing off chips or debris from a machine. If improper and unsafe blow-off pressures are used, and they often are, compressed air demand will rise dramatically.
For these and other reasons, oversizing the air supply system, including supply lines and air prep, is a good idea. Sizing the compressor for peak demand or future growth is also a good investment. A small storage tank, mounted at the machine, is another option to help mitigate compressed air pressure sags.
When it comes to air preparation, calculating machine compressed air usage and ensuring the assembled air prep unit exceeds these requirements is a simple design exercise. The calculations can be done manually or with one of the many online air-consumption calculators.
Air consumption is a function of the volume of a cylinder, the cycle time and the inlet air pressure. It is typically expressed in standard cubic feet per minute (scfm) of free air, where “standard” means at a temperature of 70 °F and at sea level (standard atmosphere). Depending on the size of the cylinders and their cycle times, air consumption can vary widely.
Calculate the air consumption of each cylinder as follows:
SCFM = 2(Area x Stroke) x Cycles
For example, to determine the air consumption of a 2-in. bore cylinder with a 4-in. stroke operating at 30 complete cycles (extend and retract) per minute at 80 psi inlet pressure:
1. Find the cross-sectional area of the piston based on bore diameter (A=πr^2)
3.1416 x (2-in. bore/2)² = 3.14 in.
2. Determine air consumption per single stroke
3.14 in.² x 4-in. stroke = 12.56 in.³
3. Determine air consumption per complete cycle, disregarding displacement of piston rod
12.56 in.³ x 2 = 25.12 in.³ per cycle
4. Determine the volume of compressed air consumed per minute
25.12 in.³ x 30 cycles/min = 753.6 in.³/min (of 80 psi air)
5. Convert cubic inches to cubic feet (1 ft³ = 1728 in.³)
6. Determine ratio of compressed air at 80 psi to “free,” uncompressed air at standard atmospheric pressure (CfRatio). It may be necessary to adjust the 14.7 psi constant if not operating at sea level or 70° F.
7. Apply ratio to determine cubic feet of free air used per minute
0.436 ft³/min x 6.44 compression ratio = 2.81 cu. ft. of free air used per minute (scfm)
The air consumption of the tubing between the valve and cylinder should also be considered, especially with longer tubing lengths. The same equation used for a cylinder’s air consumption applies, with the substitution of tubing length in inches for stroke, and assuming the tubes are connected to a double-acting cylinder with the same length tubing on both the extension and retraction ports.
While there is some consumption of air when tubes are pressurized due to compression, the major impact of long tubing runs is pressure loss due to friction. This loss causes the effective pressure at the actuators to be lower than the system pressure. Design consideration must balance the lower pressure loss when using larger tubing with the extra air consumption and slightly slower operation due to the lag caused during pressure buildup in the tubing.
Air preparation should be used to protect machines, assembled in the proper order and with sufficient flow, and oversizing the air preparation system can be a smart design choice. Care taken in the design, installation and maintenance of the air prep system will result in years of reliable and safe service.