Uptime Magazine - October/November 2008 - (Page 63)

Leak Detection Applications Industrial Compressed Air Leaks Cryogenic Gas Leaks Auto-Clave Leaks Expansion Joints Boiler Tube Leaks SF6-Gas Leaks Steam Leaks Dust Collectors Coal Ducts Underground Leaks (water/air/gas) Vacuum Leaks Composite Repair (vauum bags) Condenser Leaks (waterbox) Heat Exchanger Leaks (on/off-line) Boiler Casing Leaks Containment Wall Inspection (nuclear) Nitrogen Blanket Leaks (oil-filled transformers) Hydrogen Leaks (generator) Air-in-Leakage (csution lines) HVAC Condenser Leaks (high-vac side) Aviation Oxygen or Cryogenic Bleed Air System Anti-Ice Ducts Inflatable Gear Aircraft Fuel Systems (wet wings/bladders) Cockpit Pressurization # Water Guide Pressurization Pitot-Static Air Marine Water Tight Hatches # Cargo Bay Hatches # Automotive Wind/Water Leaks # # - Tone Method Figure 3 - Ultrasonic Leak Detection can be applied to a vast number of applications, several are listed here. that you are not able to see. Airborne ultrasound instruments can typically detect a leak of 1x10-3 std/sec. When used with an ultrasound detector, a wetting solution may allow the instrument to hear leaks at 1x10-4 to 1x106 std/sec. I have an air tank that I used in my two-day workshop recently at the Museum of Natural History. Two weeks earlier, this air tank was pressurized with 38 lbs. of compressed air. After 14 days my tank had 28 lbs. of air, so, of course, there is a leak (which is on the filler pipe fitting). When I placed just a drop of the liquid wetting solution on this leak, I got several thousand, very small bubbles. This is good because when I listened with the ultrasound detector, those leaks sounded like a bowl of Rice Krispies a thousand times over, and could be heard from several feet away. The liquid I used is a low-surface tension surfactant. Like other liquid soap solutions, this liquid is a water based solution, probably made with de-ionized or de-mineralized water. Some of you might be thinking - why bother with the ultrasound detector if you can see the bubbles? It’s a good question, and I have a good answer. The wetting solution should be used when you are looking for leaks that are below the range of the ultrasound detector. It is particularly useful for leaks that are hidden from line-of-sight. A nitrogen blanket leak on a transformer is a www.uptimemagazine.com good example. There is a gasket where the bushing slips into the top of an oil filled transformer. The nitrogen blanket pressure is typically only .5 psig of nitrogen. Without using mirrors you can’t see along the gasket. Spraying a wetting solution, such as light surface tension fluid or even a 10 weight oil, may create small bubbles. These can now be detected by the ultrasound receiver. The idea is that a low-level leak can still break the surface tension on a low-surface tension liquid, which creates bubbles. You must research your wetting solutions carefully. Dish washing liquid is quite common and might seem to be a good solution, but it has a heavy surface tension, so a low-level leak may not be able to break the tension to create any bubbles. Important Note - Don’t buy into the notion of being able to translate a compressed air leak that you found using airborne ultrasound to an actual “CFM” (cubic feet per minute). It doesn’t work! A widely circulated CFM chart was actually created by one user for his particular instrument. Even instruments of the same make and model could be as much as 30 decibels different from his at certain sensitivity settings. This brings to mind a story that illustrates this point There was a technician that was very proud to have found several leaks in his facility. He calculated the total as over 1300 cfm’s in leaks. So, he went to his energy manager and reported the leaks found and his calculations. His energy manager looked at him with a smile and said that is fantastic, but we only have an 1100 cfm compressor! Needless to say he was embarrassed, and I am reminded of it every time I see him. Those charts were only meant to be an estimate. Motor Diagnostics Leak detection is fun and, I think, quite rewarding. But what about motor diagnostics or motor bearing analysis? Well, it may surprise some of you to know that ultrasound can also be effective for motor and bearing diagnostics and gearbox inspection. The best way to use the technology is to listen to bearing, establish a baseline decibel level and watch for a rise in decibels over a period of time (see Figure 4). I have been around longer than I care to remember, going back to ‘old school’ ultrasound. The ultrasound instruments of the late 80’s and early 90’s were great for helping you determine when a bearing was bad. Back then, the manufacturers were not afraid to tell you the set range of decibels that, when reached, indicated the bearings were bad. I don’t mean the instruments could actually tell you a bearing’s condition, only that the end-user could listen to the bearing and retrieve a baseline number or dB. Then he/she could easily set-up the instrument with the same previous setting when they returned to the motor or bearing and retake the reading. If the new decibel reading was high enough within a predetermined condition range (incipient failure, failure or catastrophic failure), the bearing would be scheduled for removal. NASA even published this information in one of its Tech Briefs, “bearings monitored in the 24-50 kHz changes in amplitude indicated the beginning of or incipient bearing failure be- Electrical Leaks - Leak detection methods and applications have grown over the years. For many years, electrical leakage was not considered a leak detection method by anyone, let alone an application for airborne ultrasound. Besides, who had ever heard of corona discharge? But today, electrical leakage is now considered a leak detection application. Corona, tracking and arcing is a disruption in the air, or turbulence. Turbulence in the air creates friction, which is white noise that the ultrasonic receiver can hear. I truly believe the use of airborne ultrasound for this application is saving lives. Structural Envelope Leaks - Detection of building envelope leaks is another ultrasonic leak application which is gaining ground, especially when used in conjunction with infrared thermography. During a building envelope leak inspection you would use the tone method, which requires the end-user to use an ultrasonic transmitter. This transmitter emits high frequency sound that can flood an area such as an office, a vault, or an automobile. Humans cannot hear the high-frequency sound, only the ultrasonic receiver can. An ultrasonic receiver is then used by the end-user to hear the leak. For instance, to detect a leak in a car or truck, we would place the tone generator inside, then scan around the windows and doors outside the vehicle to find where the sound is exiting the car body. This sound could then be interpreted as the leak path for wind/water, dust and dirt intrusion. 63 http://www.uptimemagazine.com

Table of Contents Feed for the Digital Edition of Uptime Magazine - October/November 2008

Uptime Magazine - October/November 2008 Contents Upfront Upclose: An Asset Management Oasis in the Desert Information Technology: Streamlining Through Simplicity Lubrication: Phases for Framing a Solid Foundation Infrared: Do Your Homework Before You Buy Maintenance Management: Get It Together by Getting Together Motor Testing: Key Elements in Motor Decision Making Precision Maintenance: Shedding Some Light on Tolerances Reliability: RCM & the Mortgage Meltdown Ultrasound: A Multidimensional Tool Vibration: The Whirling Pump Mystery, Solved with ODS Upgrade: Are You Looking in the Right Place for Your Wear Debris?

Uptime Magazine - October/November 2008

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