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Wednesday, February 2, 2011

Minimize Compressed Air Leaks

Leaks are a significant source of wasted energy in a compressed air system, often wasting as much as 20%-30% of the compressor’s output. Compressed air leaks can also contribute to problems with system operations, including:
  • Fluctuating system pressure, which can cause air tools and other air-operated equipment to function less efficiently, possibly affecting production
  • Excess compressor capacity, resulting in higher than necessary costs
  • Decreased service life and increased maintenance of supply equipment (includ-ing the compressor package) due to unnecessary cycling and increased run time.
Although leaks can occur in any part of the system, the most common problem areas are couplings, hoses, tubes, fittings, pipe joints, quick disconnects, FRLs (filter, regulator, and lubricator), condensate traps, valves, flanges, packings, thread seal-ants, and point-of-use devices. Leakage rates are a function of the supply pressure in an uncontrolled system and increase with higher system pressures. Leakage rates identified in cubic feet per minute (cfm) are also proportional to the square of the orifice diameter.

Leak Detection
The best way to detect leaks is to use an ultrasonic acoustic detector, which can recognize high frequency hissing sounds associated with air leaks. These portable units are very easy to use. Costs and sensitivities vary, so test before you buy. A simpler method is to apply soapy water with a paintbrush to suspect areas. Although reliable, this method can be time consuming and messy.

Example
A chemical plant undertook a leak-prevention program following a compressed air audit at their facility. Leaks, approximately equivalent to different orifice sizes, werefound as follows: 100 leaks of 1/32” at 90 pounds per square inch gauge (psig), 50 leaks of 1/16” at 90 psig, and 10 leaks of 1/4” at 100 psig. Calculate the annual cost savings if these leaks were eliminated. Assume 7,000 annual operating hours, an aggregate electric rate of $0.05 kilowatt-hour (kWh), and compressed air generation requirement of approximately 18 kilowatts (kW)/100 cfm.

Cost savings = # of leaks x leakage rate (cfm) x kW/cfm x # of hours x $/kWh

Using values of the leakage rates from the above table and assuming sharp-edged orifices:

Cost savings from 1/32” leaks = 100 x 1.46 x 0.61 x 0.18 x 7,000 x 0.05 = $5,611
Cost savings from 1/16” leaks = 50 x 5.72 x 0.61 x 0.18 x 7,000 x 0.05 = $10,991
Cost savings from 1/4” leaks = 10 x 100.9 x 0.61 x 0.18 x 7,000 x 0.05 = $38,776
Total cost savings from eliminating these leaks = $57,069

Note that the savings from the elimination of just 10 leaks of 1/4” account for almost 70% of the overall savings. As leaks are identified, it is important to prioritize them and fix the largest ones first.


Suggested Actions
  • Fixing leaks once is not enough. Incorporate a leak prevention program into operations at your facility. It should include identi-fication and tagging, tracking, repair, verification, and employee involvement. Set a reasonable target for cost-effective leak reduction—5%-10% of total system flow is typical for industrial facilities.
  • Once leaks are repaired, reevaluate your compressed air system supply. Work with a compressed air systems specialist to adjust compressor controls. To maximize energy savings, compressor run time must be reduced to match the reduced demand.
Source: http://www1.eere.energy.gov/industry/bestpractices/pdfs/compressed_air3.pdf

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