One of the most common problems experienced in a compressed air system is low pressure or perceived low pressure. Symptoms can include machinery faults, inability of air cylinders to apply necessary force, or inadequate torque on an air tool.
Too often one of the following methods is chosen as a solution:
- Increase compressor pressure
- Add compressor capacity
Often, these actions may solve only some symptoms of an underlying problem. In order to effectively solve a low pressure problem, some analysis is required.
Attempt to observe a low pressure event from the point affected as well as from the compressor itself.
Say a machine is shutting down on low air pressure at 80 PSIG, and pressure at the machine is drawing below this level. However, the compressor is observed to be lightly loaded and maintaining 100 to 110 PSIG. In this case, we can determine that compressor capacity is certainly not the issue, and that there is significant pressure loss between the compressor and end use. While turning up the compressor pressure may prevent the symptom (machine shutdowns), it fails to solve the root of the problem (pressure drop), and causes additional issues (overloading of compressors, excessive artificial demand). The best long term solution is to solve the pressure drop problem.
If pressure is dropping at the compressor, then excessive demand or inadequate capacity may be the cause.
In the above example, if the compressor was noted to drop below 85 PSIG as well as the production machinery, then the system is drawing more air than the compressor can produce. This could be due to compressor malfunction (compressor not fully loading), excessive demand (such as leak rate increasing), or inadequate compressor capacity due to growth or changes in production machinery.
Compressor operation can be reasonably verified through several methods, including:
- Measurement of current (AMP draw). If a compressor is drawing near nameplate current (or even a bit above) at rated pressure, then it likely is producing air.
- A pump-up test can be performed, where a known volume of storage (preferably a “wet” air receiver) is depressurized, then isolated from the plant. By feeding only this volume from the compressor and measuring the time to pump it up to a given pressure, the air delivery can be estimated.
- Compressed air survey using flow meters and/or other means to evaluate system performance.
A compressed air survey (sometimes called an audit or assessment) can also give a wealth of insight into what is happening in the system, including evaluation of leak rate and analysis of peak demand events. This should be strongly considered before making major changes to an air system.
If excess demand is the problem, and leak rate appears under control, then the only viable solutions may be to move some production to off shifts or to add compressor capacity.
In the event that the compressors simply cannot supply the required pressure to a given machine, some other solution may be necessary.
For the examples above, let’s suppose that the production machine requires 100 PSIG of treated air, the compressor is only capable of 100 PSIG, and that there is a dryer, filter, and piping in the system totaling 10 PSI of pressure drop. In this case, the compressor can only supply 100 PSIG max, which drops to 90 PSIG at the point of use.
In some cases, the production machinery may be modified to allow operation at lower pressure. Often the cost of machinery is reduced by reducing the size of air cylinders and tubing, thus requiring increased pressure to apply the same force. In some cases, these may be retrofitted to allow larger bore cylinders to operate on lower plant pressure.
If the flow demand is low, but pressure demand is high, a pneumatic amplifier may be an good solution to boost pressure at the point of use while keeping plant pressure low.
A final alternative is to dedicate a high pressure compressor to the machinery. Many screw compressors are substantially quieter and smaller in footprint than traditional systems, allowing them to be placed directly on the plant floor. This should be considered if the machine demand is too large for an amplifier, but the demand is still only a small percentage of the plant-wide air demand.
Poor Air Quality
Some common causes of poor air quality include:
- Service bypass valves left open.
- Drain valves inoperable or drain lines stuck. This is often overlooked when a refrigerated dryer appears to be operating properly and is hitting the desired temperature. However, if the moisture is not adequately drained, it easily becomes re-entrained in the compressed air exiting the dryer.
- Undersized treatment. This commonly occurs when compressor discharge temperature is elevated if this was not accounted for in the treatment sizing, or if demand has increased over time.
- Lack of maintenance. Filters, in particular, often suffer from inadequate maintenance. Often maintenance personnel rely on differential pressure gauges supplied with filters. These differential gauges will only show high differential if they are observed at high demand times. If the elevated differential period is not observed, the filter media may even rupture, contaminating the system and eliminating the differential so that maintenance personnel never recognize the need to change the filter cartridge.
Frequent Down Time
Compressed air supply is critical to most manufacturing facilities as water, gas, and electricity, but it is one of the few utilities that are generated on site. As such, unplanned down time must be kept to a minimum.
Since air compressors often operate continuously, ingest high quantities of dirt and contamination, and generate loads of heat, regular preventative maintenance and occasional failures are events that must be accounted for. The most reliable way to prevent compressed air system downtime is to have backup compressor capacity available at all times. At minimum, it is recommended that there is sufficient backup so that any one compressor may be shut down for maintenance or repair. Depending upon the relative cost of downtime and/or the degree of specialization of the air system, additional backup may make sense.
Sources of backup may include standby compressors that are rotated by operators, sequencers, or master control systems or may be older compressors that are used only for backup purposes. In the case of dedicated backup machines, it is important to operate them on a regular basis (at least a couple of hours weekly). This ensures that they remain lubricated and are in good operating condition when needed.
Another important factor for eliminating downtime is proper preventative and predictive maintenance. In continuous operation, a compressor may operate 8,760 hours per year. To put that in perspective, car driving only 30 miles per hour would travel over 260,000 miles in that time. Regular maintenance will help to keep things operating properly and efficiently as well as noticing potential problems before they cause unplanned downtime.
Poor Efficiency / High Energy Cost
Virtually any change to the compressed air system can significantly affect energy efficiency. Here are some “low hanging fruit” that can help reduced costs.
- Operate the compressors at as low a pressure as possible. Do recall, however, that operating below the rated pressure of air treatment may de-rate the air treatment. Excess pressure requires additional compressor power as well as intensifying leaks and causing legitimate users to use excess air.
- Eliminate wasteful timer operated drains and manual bleed valves in favor of demand operated drain traps. Each timer operated drain wastes hundreds of dollars annually blowing off costly compressed air when little or no moisture is present.
- If multiple compressors are used, ensure that they are using a sequencer or master control system to coordinate them with minimal pressure fluctuation, and shutting off when not required.
- If inlet modulation controlled compressors are being operated without receiver tanks, consider adding receiver volume and using alternate control methods.
- Create an ongoing leak reduction and education program. End users and machine operators treat air leaks differently from water, oil, natural gas, and chemical leaks. The air is odorless, creates no mess or slip hazard, and is believed by many to have very little (if any) cost. Educating the users can help them to recognize that leaks are a costly expense. While a single round of leak repair never hurts, leaks are continuously being created in the air system, necessitating a continuous management program.