Up Time Magazine- April/May 2008 - (Page 55) eliminate the background noise and aid the end-user in finding the leak quickly. Some ultrasonic receivers are even narrower in frequencies, allowing the end-user to hear 38 kHz plus or minus 2 kHz. Some may be tuned to 40 kHz plus or minus 10 kHz allowing background sound to sometimes interfere or compete, making locating the leak more difficult. Still, the fact remains that an ultrasonic receiver, whether it is tuned with a narrow or wide band, will aid you in finding the leaks. Figure 3 - Actuator and Valve control air line leak. away from the leak it was when identified. Years ago when I was selling airborne ultrasound equipment, I would often hear excuses such as the ones below: • We are running screw compressors, the air leaks are not costing us anything. • I don’t need ultrasound to hear my leaks, we can hear them without ultrasound. • We don’t have any large leaks, just a lot of small leaks. Recently, I went back and reread a newsletter from the Department of Energy (DOE). At one time the Department of Energy would publish the Compressed Air Tip Sheet in which they would discuss equipment, techniques, cost savings, etc… In the August 2004 Tip #3 newsletter, they gave an example of a chemical plant that had performed a leak-prevention program (leak audit) and found several leaks. Leaks found were listed as: 100 leaks of 1/32” opening or orifice 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 savings as 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. Formula used: Cost Savings = # of leaks x leakage rate (cfm) x kW/cfm x # of hours x $/kWh Using values of the leakage rates from Table 1 Start Your Leak Audit… • Start your leak audit by first having safety personnel come out and scan the plant for noisy areas using a decibel meter. • Record the locations and average decibel reading for each location. • Using an ultrasonic receiver, start in the compressor room and scan for leaks. • Follow the supply lines leaving the compressor room. • Prepare to tag and identify – Identify each leak – Tag each leak with a tag giving the location and size of the leak. – Create a record of each leak by taking a photo with a digital camera. Always have the tag that identifies the leak in the picture whenever possible. • Ultrasound decibel to CFM charts are misleading and are grossly exaggerated, so avoid using such charts. • Instead use Small / Medium / Large / XLarge to describe your leaks. • Assure that the tag identifies the instrument used and how many inches or feet and/or broken, and actuator and valve bodies leaking due to internal leaks or bypassing air. (Figure 3.). Another noisy leak was found on the relief valve of a compressed air supply holding tank. As I walked by the tank scanning with the SDT 170 Ultrasonic Receiver and 31” Flexible Wand, I heard a moderate leak coming from the relief valve. As bad as this plant was starving for compressed air, this relief valve should not have been relieving (see Figure 4.). Figure 4 - Compressed air holding tank relief valve leaking. Leakage rates1 (cfm) for different supply pressures and approximate equivalent orifice diameters Why Ultrasound? Well, if you could hear high frequency sounds above 20 kHz, and were enlisted in the US Navy, you would probably be a sonar technician on a naval vessel some where in the South Pacific or, possibly, the Mediterranean. But even if you could hear high frequencies above 20 kHz, you would still not be walking around the plant finding air leaks. Why not? Because of too many interfering frequencies, you still would not be able to determine the location very easily. Using an airborne ultrasound receiver that enables the end-user to listen within - say 38-40 kHz - which helps www.uptimemagazine.com Pressure (psig) 70 80 90 100 125 Orifice Diameter (inches) 1/64 0.29 0.32 0.36 0.40 0.48 1/32 1.16 1.26 1.46 1.55 1.94 1/16 4.66 5.24 5.72 6.31 7.66 1/8 18.62 20.76 23.1 25.22 30.65 1/4 74.4 83.1 92 100.9 122.2 3/8 167.8 187.2 206.6 227 275.5 Table 1 - 1For well rounded orifices, multiply the values by 0.97, and for a sharp-edged orifice, multiply the values by 0.61. Courtesy of U.S. Department of Energy, Compresses Air Tip Sheet Aug 2004 Tip #3 55 http://www.uptimemagazine.com
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