Simple Solutions for Shop Compressed Air System Improvement

Technicians rely upon their shop’s compressed air system to keep fleets up and running. Minor leaks in the air lines can put a major drain on the system and cause energy costs to skyrocket.

There are some simple steps you can take to identify leaks and improve the efficiency of your system to save the shop time and money.

You can examine the system yourself or hire an auditor to measure the volume of air consumed by leaks. An air system auditor can leverage advanced data acquisition equipment and system analytics to provide the most thorough assessment of your compressed air system.

However, this level of service requires a significant monetary investment and may not be justifiable for smaller systems. If your compressed air system is under 30 hp, a few simple do-it-yourself-type tests can deliver significant results. The first step is to get control of compressed air leaks.

In an industrial system, 10 to 30 percent of compressed air demand is typically associated with leaks. Statistically, most leaks are less than 3 standard cubic feet per minute (scfm) and the average leak is around 2.1 scfm.

It is no surprise that we find a limited number of leaks larger than 3 scfm. Leaks above this volume are easier for maintenance managers to hear in an industrial environment. Along with the squeaky wheel getting the oil, the audible leak gets repaired.

If your compressor is rated for 20 to 100 scfm, a small number of 2 scfm leaks can easily represent a significant percentage of your total capacity.

GET LEAKS UNDER CONTROL

If there are too many leaks in your system, you may unintentionally purchase or upgrade to a larger compressor than you need just to support air that is discharging into the atmosphere.

Air leaks may also cause the compressor to load more frequently, and the increased run time and heat can shorten the life of the compressor. This larger load can, in turn, increase your electrical costs.

There are a few easy tests to help you assess and quantify leaks in your system. It is best to test the system when air-consuming applications are turned off. If the source cannot be isolated, make note of the application and how it is operating so you can gather consumption data and subtract it from your total measurement.

It is surprising how often someone estimates leak load based on consumption measurements taken on a Sunday without walking the system to account for production equipment. The difference can be substantial.

TEST METHODS

The first test method is based on compressor flow and percentage of time the compressor is loaded. This test is applicable to rotary or reciprocating compressors that operate using a loaded/unloaded or stop/start type control based on pressure.

With power on to the compressor and compressor unloaded/off, start your stopwatch when the compressor automatically starts/loads. Record the time when the compressor stops/unloads automatically, but keep your stopwatch running and record the time when the compressor starts again.

This will give you the loaded time and the total cycle time. Dividing the loaded time by the total cycle time will give you the load percentage. Multiplying compressor capacity (cfm) by this percentage will give you your system demand.

Assuming nothing else was consuming compressed air, this will be your leak load.

The second test method is based on pressure decay. This test is useful for variable speed, and other compressor control systems.

For this test, turn off power to the compressor and record how long it takes for the pressure to drop 10 psi. Make sure the pressure is within your normal operating range when performing the test, since pressure can influence the results.

Using the graph in Figure 1, identify the colored curve based on the total volume of air receivers (tanks) in your system. From the bottom (x-axis), find the decay time in seconds from your test and mark where it intersects the appropriate curve.

Relative to this intersection point, draw a horizontal line to the left side (y-axis) to determine the leak load in scfm.

To increase accuracy, add the storage volume from your pipe based on the values listed in the graph.

Now that you know the volume of air consumed by leaks, a simple assumption is one leak for every 1 to 2 scfm of leak load.

Start by checking for audible leaks. Turn off everything that is making noise and slowly walk along the air piping. If there is a large leak present, you should be able to hear it.

An air leak can create a static charge, acting like a magnet for dust and other contaminants floating around your shop. Watch for visible signs of leaks, e.g., black grimy film or fuzziness around the leak area.

Another simple way to locate leaks is to apply soapy water around common leak locations - threaded pipe fittings, quick disconnects, filter/regulator gaskets and drains - and look for expanding bubbles.

ULTRASONIC LEAK DETECTION

The most comprehensive method of finding leaks uses an ultrasonic leak detector to filter out background noises and focus on the frequency of an air leak to identify small - yet significant - leaks which would otherwise go undetected.

It will cost a few thousand dollars to purchase an ultrasonic leak detector yourself. Consider hiring a service provider who has experience using the ultrasonic technology to find the leaks for you. Depending on the size of the area, they may be able to find and tag all of your leaks for only a few hundred dollars.

Check with your local utility company to see if they offer energy incentives to help cover the cost.

THE COST OF AIR LEAKS

Detecting air leaks can cost you time and money, so it makes sense to convert your leak volume into a dollar amount to justify the audit and repairs. Assuming electrical power costs $0.10/kWh, use the following formula to determine the cost of your system’s leaks:

Cost = 0.027 x (scfm from graph) x (hours per year compressor power is on)

For example, if your 60-gallon compressor has power on and is available to run six hours per day, five days a week and takes 30 seconds to drop 10 psi, you could be losing $631.80 on an annual basis.

0.027 x 15 scfm x (6 hours/day x 5 days/week x 52 weeks/year) = $631.80

This is based on a compressor operating stop/start. Depending on compressor design, control method and running hours, this could increase to more than $9,000 if you have a modulating-type rotary screw compressor running around the clock.

TAKE ACTION

It is always important to be proactive to keep air leaks under control. If you hear or feel a leak, take action to repair it. It is a simple fix that will help to preserve your system and improve your overall efficiency.

If you’re upgrading your system, invest a little more money up front for quality components and engineered piping systems that use connection methods other than threaded fittings to minimize the potential for leaks and save money over the life of the system.

Being cognizant of wasted compressed air is paramount to achieving a best-in-class air system.

Elisabeth Kjellberg and Carlos Estrella are air system audit engineers with Ingersoll Rand Industrial Technologies. Ingersoll Rand is a world leader in creating and sustaining safe, comfortable and efficient environments in commercial, residential and industrial markets. www.ingersollrand.com. Its Industrial Technologies sector provides products, services and solutions that enhance our customers' energy efficiency, productivity and operations.