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Compressed Air

Compressed Air System Audit

Introduction

An audit should help you determine the operating costs of your system and identify any areas for improvement. When you audit a compressed-air system, it's very important to ask “why”, “who”, and “how”.

Compressed Air Energy Efficiency Reference Guide

Why?

Often the audit comes about as a request from one area in a facility to overcome a specific problem. Sometimes the audit is required because the facility wants to reduce its production costs by improving the energy efficiency of the plant.

Who?

Should someone in-house audit your system? Or should you hire a consultant to do it for you? Both choices have advantages and disadvantages.

Your plant's staff probably knows the system. and the audit will give you an opportunity to know more about what is happening in the facility.

Unfortunately, there are some skills and knowledge required to conduct a proper audit. And equipment can also be expensive, with high-end ultrasound guns costing up to $15,000 to $20,000. (See Purchasing Leak Detection Equipment). Contracting the audit to a professional engineering or audit firm guarantees results. The contracted firm will provide the necessary workers and testing equipment. Still, someone from your plant should be a part of the auditing team.

For a list of companies providing consulting services for compressed air, see the Natural Resources Canada web site Search for a Contractor.

How?

Before You Start the Audit

At the start, it is very important to establish the compressed air benchmarks for the system. The benchmark is how much compressed air you are using, and how much it is costing you. This “pre-improvement” information will let you calculate how much you have saved by the improvements.

Before starting the audit of the plant define these items:

  1. Set no more than three main goals to accomplish during the audit. Some typical goals are:

    • Eliminate the need for one or more air compressors.
    • Determine the air pressure requirements of the system.
    • Benchmark the system's components.

  2. Set no more than four minor goals to accomplish during the audit. For example, document the performance of components in the compressed air system.

  3. Define and document the loaded production cost of compressed air for the facility for the last four years.

    Power cost is probably the easiest and most common way that plants express the cost of the compressed air. But power cost gives you an incomplete picture. There are other areas that need to be addressed, including initial price, operating cost, depreciation of equipment, and maintenance. These costs can have a significant effect on the overall cost of the compressed air. We call this the “loaded cost” – the true cost, of compressed air.

    Table 1 shows the difference between a loaded and an unloaded cost based on a 3000 hp compressor running for 4200 hours per year with electricity costing 7 cents/kWh. The unloaded cost – the electricity alone – is 20 cents/cfm (cubic feet per minute). But the loaded (true) cost is 40 cents/cfm.

    Table 1
    Loaded and Unloaded Cost of Compressed Air

    Item Unloaded Cost Loaded Cost
    Electrical costs $59,138 $59,138
    Initial costs   $250,000 (amortized over
    10 years) = $25,000/year
    Water costs   $15,000
    Dessicant   $3,000
    Maintenance   $10,000
    Filters   $5,000
    Projects   $5,000
    Total Annual Cost $59,138 $122,138
    Cost per 1000 cfm $0.20 $0.40

  4. Calculate the highest compressed air pressure need the plant has.

  5. Determine the largest consumers of compressed air in the facility.

  6. Determine how many air compressors are in operation.

Performing the Audit

The audit should examine the compressed air system as a whole, as well as the individual components. Areas to study appear in the section Operation and Maintenance Tips.

Purchasing Leak Detection Equipment

There are two types of leak detection equipment:

  • ultrasound detectors
  • liquid leak detectors

Ultrasound Detectors

Ultrasonic detection is versatile and relatively easy to use. With it you can quickly scan a fully-operating compressor system to pinpoint the leaks.

These detectors “listen” in the ultrasonic frequency range, typically 20 to 100 kHz – sounds the human ear cannot hear. (The human ear hears up to 15 kHz.) Most of these detectors filter out sounds that you normally hear, while amplifying and converting the ultrasonic noise into frequencies that you can hear on headsets. Ultrasonic frequencies are easier to locate than lower frequencies since the ultrasonic frequencies echo less than lower frequencies.

You can also use ultrasonic detectors to check for leaks in shell and tube exchangers, valves and steam traps and worn bearings. These detectors can also check for electrical arcing, tracking and corona and many other things. Expect to pay from $500 to $10,000 each. Prices vary depending on sensitivity, range, flexibility and other features. Check if the unit has an intrinsically safe rating – especially if you plan to use it in potentially hazardous environments.

Another way to save money is to find reliable used equipment.

Some major manufacturers of leak detection equipment are:

Liquid Leak Detectors

These are basically soapy water that bubbles around leaks. Bubble size can help determine the size of the leak. One benefit is that you can buy a low-temperature version to use outside in cold weather.

Liquid detectors are inexpensive and easy to use. But they can be more time-consuming than ultrasonic detectors. And they can be messy. As well, you must be careful in clean and sanitary environments. If the leak is very large, the liquid detector may not bubble, but blow right off of the leak.

Figure 1. Leaks detected by a liquid leak detector

Leaks detected by a liquid leak detector

Large bubbles on the threaded connections indicate larger leaks.



A 236 ml bottle costs from $5 to $7 depending on its temperature rating. Most companies also supply detectors in 3.8 l (one Imperial gallon) containers.

For information on liquid leak detectors, consult:

Documentation

Sample System Profile

ABC Company

Peak requirements:

  • 110 psi maximum pressure,
  • 2000 cfm peak demand.

ABC Company asks a compressed-air system supplier to make up a package meeting these needs, ABC may get one compressor supplying 2000 cfm with pressure set to 110 psi as set out above.

When, however, ABC profiles its system by breaking down the requirements, the actual usage is:

Area A

  • 110 psi maximum pressure
  • 500 cfm peak demand, 400 cfm minimum demand

Area B

  • 80 psi maximum pressure
  • 1000 cfm peak demand, 800 cfm minimum demand

Area C

  • 80 psi maximum pressure
  • 500 cfm peak demand, 200 cfm minimum demand

Compressor Strategy

With this more specific profile, one strategy would be to install a single compressor running at 110 psi with an output of 500 cfm for Area A. Then, supply areas “B” & “C” with two compressors. The first would cover the minimum or average demand of 1000 cfm; the second would cover the additional peak demand-another 500 cfm.

We still have 2000 cfm capacity, but only 500 cfm is being produced at 110 psi and 1500 cfm is being produced at 80 psi.

You can also add storage tanks and other items to improve the system.

Bottom line: when your requirement profile is detailed, you can make better decisions to ensure that your compressed air supply system is more energy efficient.

Tagging the Leaks

Tags will let you accurately identify leak points and help the repair person fix the system correctly.

This is a sample of a leak tag:

Leak tag

For reporting, include this information:

  1. Date
  2. Location in plant
  3. System pressure
  4. Size of leak (Small, medium, large, and immediate attention)
  5. System fluid (Compressed air, instrument air, nitrogen, etc.)
  6. Location of leak (e.g. at the gasket, on the run of a tee, etc.)
  7. Type of leak (Thread, Valve, Gasket, Tube, Hose, etc.)
  8. Size (¼ inch, ½ inch etc.)
  9. Manufacturer – this will allow you to benchmark components in your system
  10. Annual cost of the leak
  11. Notes

It is useful to put the annual cost of the leak on the tag. This will encourage you to fix the larger leaks fast.

It is important to track the leak points so you can determine the performance of components in the system. If you simply replace a failed component with the same product without doing a root cause analysis on the reason for the failure, then you will likely have another failure at this point soon.

This table is an example of how to track the leaks.

Air Survey Data For Sample Co. Canada Survey performed by: Survey Date:
Tag Number
=
Area
Code +
Number
System
Pressure
(PSI)
Test
Result
Location
Description
Media Connection
Style
Mfg. Size Comments
1 100 Priority ESD valve Air Valve ABC ½ inch Very large leak on valve actuator
2 100 Large By door 2 Air Filter ABC N/A Leak out of bleed off valve
3 100 Small By tank 301 Air Hose ABC ¼ inch Pin hole leak about half way along the hose
4 100 Small By whse Air Thread ABC 1 inch Teflon tape not used on connection

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