Compressed air: leak reduction

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Leakage is usually the largest source of energy wastage associated with compressed air usage. Leaks typically waste around 20 to 30 percent of a compressor’s output, but some systems can lose 50 percent or more of their air through leaks. Not only do leaks increase energy costs; they also contribute to operating losses and reduced productivity because the pressure drops they create affect the performance of air-using equipment.

Leaks typically occur in pipe fittings and joints, hoses, valves, flanges, couplings and point-of-use equipment. Systems that operate at a higher pressure than is actually required can promote leaks and can also increase the risk of damage to the equipment.

A well-maintained system – which means having a leakage detection and repair program in place – should have a leakage rate of less than 10 percent.

In terms of energy savings, a general rule says that an effective air-leakage reduction program can reduce energy wastage by up to 80 percent.

Technical explanation

Leakage rates are a function of the supply pressure in an uncontrolled system and they tend to increase with higher system pressures. Leakage rates (expressed as flow rate) are also proportional to the square of the orifice diameter. An example of air losses for different equivalent orifice sizes is shown below.

Leakage rates
Figure 1: Leakage rates for different supply pressures and approximately equivalent orifice sizes
 

The leakage rate, expressed as percentage of compressor capacity lost, can be determined by measuring the compressor performance during non-production periods.

  • Run the compressed-air system during a non-production period (i.e. when there is no immediate demand for compressed air).
  • Make sure that no equipment is operating at the time (Note: You can isolate selected parts of the system to determine the main sources of leakage).
  • Measure the load and unload cycles and calculate the average power consumed. The compressor will load and unload because the air leaks will cause it to cycle on and off as the pressure drops due to air escaping through the leaks.
  • Plot power usage on the compressor performance graph (below), which represents the air leak rate of the system.
Compressor performance graph
 
 

Read the corresponding air leak rate in Free Air Delivery (FAD) or use the following equation:

Leakage (%) = [(T x 100)/(T + t)]

 Where:

T = on-load time (minutes)

 t = off-load time (minutes) 

The best method for identifying leaks is to use an ultrasonic acoustic detector. Contact your supplier to arrange a leak survey to detect and tag leaks. Fixing leaks can be as simple as tightening a connection or as complex as replacing faulty equipment; therefore it is convenient to prioritise repairs so that the largest leaks are fixed first (as this will be where most of your energy savings are concentrated).

Wherever feasible (i.e. without affecting or disrupting the operation of air-using equipment), try to reduce the pressure of the system so as to minimise the leakage rate.

Other methods of reducing air leaks

  • Install air isolation valves to individual production areas with shorter operating hours and close these valves when these areas are not being used.
  • Install solenoid valves on equipment so that the air supply to that equipment gets switched off whenever the power is turned off.
  • Implement a leak prevention program that incorporates identification (including tagging), tracking, repair, verification and employee involvement.

Evaluating quotes

Estimate the leakage rate using the method described, and once the survey has been conducted and leaks detected, compare quotes by looking at the cost-benefit of each repair option, i.e. cost of repair versus improvement in leakage rate (energy savings).

Further information

The Energy Efficiency Exchange (eex.gov.au) - ‘Opportunities – Compressed Air’

References

Queensland Government, n.d. Eco-Efficiency for Queensland Manufacturers, Stage 3 Opportunities for Improvement, Foundry Factsheets. [Online]

US DOE; EERE, 2004. Compressed Air Tip Sheet #3: Minimize Compressed Air Leaks. [Online]


This content was produced with assistance from Energetics
www.energetics.com.au

 

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