Follow these 5 air audit best practices before you call for help

Feb. 5, 2008
These five fixes will let you get the most from an air audit.

A growing number of groups, including manufacturer’s reps, independent auditing companies and energy services companies, offer compressed air auditing services. These services provide valuable benefits to operators of compressed air systems. Generally, you might consider purchasing audit services for one of two reasons.

First, there might be a specific problem in your compressed air system that needs attention. It might be low pressure in a specific part of the plant, moisture in the lines or inadequate overall capacity. The second reason is a desire to reduce compressed air system operating costs. In either case, you should take a number of easy, low-cost actions before the audit begins. If you have a specific issue that needs fixing, these actions might mitigate the problem and preclude the need for costly outside help.

If your desire is to reduce compressed air costs in general, performing these simple tasks will allow auditors to perform a higher-level system analysis. As air auditors, we find that all too often, we spend our time and our client’s money solving problems that only get the air system to a minimum level of operational efficiency. We can’t work on really getting a system working at optimum efficiency until these issues are addressed.

What follows is advice from auditors. Attention to these five points can significantly improve the quality of our auditing or let you avoid calling us in altogether.

1. Focus on filters

These are a critical but often overlooked component of a compressed air system. Frequently, they’re not replaced at appropriate intervals. When filters are clogged, the pressure drop across them literally skyrockets as the filter element reaches the end of its service life. Filters generally operate with a pressure drop between 1 psid and 5 psid. However, as they reach the end of their service life, the pressure drop can exceed 15 psid. This plays havoc with your system and severely limits the air pressure available for air users. Don’t let the pressure problem you’re having on the plant floor come from dirty filters in the compressor room.

Change filters once a year at a minimum. In many facilities, filters must be changed more frequently. To ensure you have the opportunity to change filters before the pressure drop across them skyrockets, install differential pressure gauges on the filters if you don’t already have them. These gauges are available as add-ons to nearly every filter.

Frequently, plant personnel deal with low-pressure problems by increasing the compressor’s output pressure. Remember, for every 2 psig increase in output pressure, energy consumption increases by 1%. The pressure problem might be “solved,” but the underlying issues remain.

2. Locate leaks

A simple fact is that every compressed air system has leaks. Often, a prime reason for calling in an auditor is to perform a leak survey. However, even if a leak survey is part of the audit, make an effort to repair known leaks before the audit starts because auditors don’t get an accurate picture of how the system operates if it has large leaks.

These affect the pressure and flow in an entire compressed air system. During a recent audit, facility engineers complained of pressure problems at a large group of critical air users. Upon further investigation, we found a 1/4-inch leak on a filter/regulator/lubricator (FRL) on the air line servicing the group of equipment. Simply fixing one hole solved the pressure problems for the equipment and increased the pressure available to the other air users.

Pay special attention to quick-disconnects and other hose fittings, the most common place for leaks. Fortunately, they’re also the easiest leaks to repair, often requiring no more than tightening a hose clamp or replacing a quick-disconnect. Inspect and repair connections and quick-disconnects before any air audit.

3. Fix the FRLs

Filter/regulator/lubricators (FRLs) are a critical part of every compressed air system. They protect and lubricate end-use equipment while minimizing the air used. However, FRLs often are in poor operating condition and should be repaired or replaced. First, check the bowls on the filters. If any are cracked or leaking, replace them immediately. If any are full of condensate, drain them and put a system in place so they can drain as needed in the future. If the manual drains on any bowls are cracked open or wide open, close them and inform operators that they shouldn’t be left open. Also, ensure that connections on the FRLs are sound and not leaky.

Second, make sure the regulator gauge is readable and in good working order. As air auditors, we see too many regulator gauges with the housings broken off, gauges filled with condensate, gauges reading zero pressure, gauges that don’t respond when the regulator position is changed, and gauges that have corroded. Replace any regulator that doesn’t display downstream pressure accurately.

Once your regulators are in good working order, make sure they’re set to deliver an air pressure that matches the end-use specifications. Frequently, regulators are set significantly higher than they need to be. They might have been set too high initially or, more likely, machine operators decided that they weren’t getting enough air pressure and have cranked up the regulator.

Common machine operator thinking is, “If I can make 20 widgets a minute with 30 psig, I can make 40 widgets with 60 psig, right?” Alternatively, operators might have had low pressure one day and cranked the regulator pressure up without remembering to reset it. One lesson we’ve learned as air auditors is that after being set initially, regulator pressures go up, but never come back down.

How do you check that your regulators are working properly? Whenever we perform an air audit, we connect a simple digital pressure gauge equipped with a quick-disconnect fitting to verify air pressure on nearly every piece of equipment. Delivering more pressure than end users need increases air consumption.

An air cylinder that was specified to use 30 psig will use 70% more air at 60 psig, 230% more air at 90 psig and 300% more air at 120 psig. Virtually all air-powered equipment works this way – the more pressure delivered, the greater the air used. Auditors call this phenomenon artificial demand.

Furthermore, delivering more pressure than needed can be a safety hazard, especially for hand tools. If you find that machine operators are constantly changing regulator settings, consider using tamper-resistant regulators. These are widely available and will discourage unauthorized changes in regulator pressures.

4. Unauthorized modifications

Plant personnel often regard compressed air as a free and infinitely available utility. This can lead to some very ingenious uses for air. Our personal favorite unauthorized use is as a personal cooling device. Tape down the trigger of a blowgun and hang it so that it points toward your station and you’ll be cool all day. Maybe drill a couple of holes in a pipe and mount it so it blows on you. In a metal casting facility, we found a copper pipe with seven 1/8-in. holes drilled in it being used to cool a workstation. This cost the plant more than $10,000 a year in energy costs and reduced system pressure to the point that every air user on the system was adversely affected. Another of our favorite unauthorized air uses is for cleaning floors. Sweeping is much easier with an air hose and 100-psig air than it is with a broom.

These unauthorized air uses affect a system just like a large leak, reducing pressure and flow available to other air users. Are there any enterprising machine operators on your plant floor?

5. Train the drains

Moisture in compressed air can quickly destroy compressed air system components. What should you do if you find moisture in your air? Simple – before you call an auditor, even before you call in your dryer service company – make sure that the drains in your system are working properly. Although this sounds obvious, wherever we find people complaining about condensate, we find inoperable drains.

In one case, a client spent in excess of $10,000 on an air audit with the express target of determining and eliminating the source of moisture in compressed air lines. After discussions about replacing two air dryers and conducting a comprehensive system audit, we traced the cause of the excess moisture to a failed automatic drain attached to the refrigerated dryer. Had this drain been tested and repaired, the entire audit could have been avoided.

Electronic drains are easily checked by pressing the ubiquitous “Test” button found on virtually every electric drain, whether they are zero-loss drains or timer drains. Other mechanical drains typically have a manual bypass. If you open the bypass and a significant amount of condensate comes out, the drain needs to be repaired or recalibrated. Remember, manufacturers recommend that mechanical-type drains be taken apart and cleaned monthly to prevent clogging.

Some facilities consider the maintenance that electronic or mechanical drains need to be excessive and, therefore, elect to install manual drain valves, fully intending to open these drains periodically. Then, they fail to do so. It’s not uncommon for auditors to open a manual drain valve on the bottom of a coalescing filter or air receiver only to have gallons of nasty-looking condensate and oil drain from the equipment.

If you have condensate drains at any of these locations, ensure they function properly by checking the following:

  • Air/lubricant separators
  • Air receiver
  • Air dryer
  • Coalescing filters
  • Distribution low points

So, you still want an air audit?

Getting one is pretty simple, but make sure to complete the above checklist of five air system maintenance items before you call anyone. Chances are good that your system will perform significantly better. You might even eliminate the immediate need to pay for an outside air audit. If you still want to improve performance and reduce the air system operating cost, go ahead and make the call. As independent auditors, we seek to develop cost-saving recommendations that reduce air system operating cost by 25% to 50% while improving system performance. So clearly, air audits are a sound investment. Just complete the checklist before we arrive and make us really work for our fee.

Andrew Sheaffer is a research engineer for the Energy Resources Center at the University of Illinois in Chicago. Contact him at [email protected] and (312) 413-3615.