In the photos that follow, you can see how the duct work appears to be in pretty good shape. You don't see any obvious openings or rusted holes. You do, however, see a number of flanges. With fans blowing the air at high rates of speed, leaking flanges can allow not only air, but moisture into the duct work. In most cases, a good RTV sealant and tightening loose bolts and screws can remedy this problem.

Fig 1

Fig 2

Since Ultrasound is typically capable of hearing a 5 psig leak from .005 opening at 50 ft., it is highly effective in locating these leaks from a distance (Figure 2).

A recent graduate of the Ultrasound Technologies Training Systems (USTTS) Level I Class wrote me regarding his plant's dust extraction system. Mr. Michael McGuire, a PdM Technician with Owens-Corning, Summit, IL, shared his experience regarding water intrusion somewhere in his duct system. I had mentioned in my class about how ultrasound can be highly effective in detecting duct leaks of forced air systems. Michael explained their situation and how they collect dust off the production line (i.e., fiberglass, slate granule dust). The air lock (rotary valve) expels the dust at the bottom of the collector. When moisture enters the system it binds up the dust, and clings to the walls. When this occurs they have to shut down the collector and open access doors to clean out the lower portion. Many hours of labor and downtime were required to perform this fix. After the repairs were made they had some nasty weather. Fortunately, there was no evidence of any more moisture entering the duct work, meaning their fix was a success.

Imagine if your duct work is old and the supporting structure of angle iron is weak. This is especially bad for outside duct work. Duct work out in the elements can collect snow and ice, with the weight of the snow eventually weakening the structure allowing it to bow inward. When this happens, snow, ice, and water sit on the duct. Over time this area rusts and becomes a perfect place for water intrusion.

Inspecting the duct work is as easy as taking an ultrasound instrument and scanning the duct work during operation and listening for air in-leakage. In several of the photos you can see the technicians at Owens-Corning locating their leaks using a UE Systems Ultraprobe 10000 ultrasonic instrument. The Owens-Corning Sumit Roofing Plant produces roofing shingles for residential and commercial applications. The plant's products include built-up roofing asphalt, coatings for asphalt shingles, and specialty industrial products.

The beauty of the Ultraprobe 10000 instrument is its data-logging capabilities. A route can be made on the fly while scanning the duct work and leak locations given a point ID for follow-up.

Fig 3

This unit can also be frequency tuned to focus in on the in-leakage. Like pressure leaks, the ultrasonic frequency used to scan for vacuum or in-leakage is 38-40 kHz. Figure 6 shows that on Michael's UE Ultraprobe 10000 back panel 38 kHz has been selected to scan for leaks.

If you are unsure of the frequency, simply hold the ultrasound below your nose. In other words, hold the unit with the scanning module (minus the rubber cone) aimed at your nose. With your hand on the Sensitivity Knob advance the kilohertz setting up or down until you can clearly hear and define the sound of air as you breathe in through your nose. You may be surprised to learn that the frequency selected is between 38- 40 kHz. This is also a good practice before walking into a noisy environment to perform an air leak audit using any ultrasound instrument with tunable frequency.

Fig 4

These leaks of course will sound a lot like a pressure leak from an air hose. The reverse of pressure is vacuum or in-leakage. Keep in mind, you are listening for air in-leakage which is going to have the turbulence inside the flange or duct. Therefore, you may want to be close to the duct work or use an accessory such as a long-range horn attachment if your instrument has one available.

Use a rag or glove on your hand when scanning the duct work for leaks around the flanged areas. Use a rubber cone over the receiving end of your unit and your gloved hand, cupping your hand around the front of the cone, moving the instrument slowly while listening for the difference in sound.

Some companies have made a certain module available for close-up work. One manufacturer, UE Systems, Inc., has a Close Focus Module. This attachment gives you a narrow focus of about 25-30 inches, with an increase in amplitude of about 12-14%.

Fig 5

Other companies such as SDT North America has a 31-inch Flexible Wand with an 8-foot pigtail universal cable that allows the enduser to access those hard-to-reach flanges. Great for us guys that barely make the 5'7" mark. No short people jokes allowed.

In some cases using the parabolic dish may be necessary, particularly with duct work over 30 feet away. Be sure when using any of the long-range accessories, these accessories need to be moved slowly and deliberately. Scan as if you are trying to scan every inch of the duct work.

Another option is using the UE Systems, Inc. Warble Tone Generator, or the SDT Bi-Sonic Tone on Tone device. If you have a chance to inspect the duct work during some downtime, you may want to place one of these devices inside the duct work and slowly scan outside, listening for the dedicated sound to penetrate small openings.

Fig 6

These high frequency tone generators emit high frequency sound around 35-40 kHz. Even the smallest of the units can fill approximately 3000 sq. ft. of space. Of course these units are also available in much larger instruments. Some can flood the inside of a Boeing 747 aircraft or the cargo bay of ocean-going vessels.

When using these instruments to flood an area, keep in mind that the sound is carried away from the transducer like a flashlight beam. Let's say you have a main duct that has multiple branches extending off of it. Just as light would only enter the branches a few inches, so would sound. Your sound will not flood those branches, so you would have to put a transmitter inside that branch to leak test it properly.

Another place for water intrusion to occur is around the duct work and flanges close to the ground. As water puddles near a flange that is leaking, water can be sucked from that nearby puddle into the duct. Check your duct for proper clearance between the ground and possible leak points.

Fan assemblies that have service doors or windows which do not shut tightly, have latches that are loose, or a broken or missing door gasket will also pull in air and moisture.

Fig 7

Pay careful attention to long runs of duct (Figure 7). You can see how long this horizontal run is, some 70-75 feet. If not structurally sound, these long horizontal runs can, over a period of time, sag, causing flanges to leak.

The primary raw materials from which steel is made are iron ore, limestone, and coke. Coke, a by-product of coal, fuels the blast furnace and allows it to reach temperatures ranging from 3400°F to 3600°F, the intense temperatures necessary to make steel. When the raw materials are combined and heated in the blast furnace, iron is "smelted" or separated from the ore, and slag forms from the limestone. The slag floats to the top of the molten iron, where it can be drained, leaving only the molten iron which will be transformed into molten steel. If you happen to be working with an explosive product or a product that is highly combustible such as coke, then you want to use an ultrasound instrument that is Intrinsically Rated (IS). UE Systems, Inc. still makes the Ultraprobe 2000, which is Factory Mutual Rated at Class I, Div I, Groups A, B, C, and D. This is the most comprehensive package of any ultrasound receiver out in the marketplace.

Another reason to scan this type of duct work is the inert gas blown into the system to lower the volatile nature of the coke. Usually, this is a nitrogen charge. Nitrogen can be very expensive. For the plants using nitrogen for this operation, take note of your high cost of nitrogen and think duct work leaks. Again, use an IS-rated instrument for this inspection.

Duct work carrying high temperature air for sterilization or for heating water is very susceptible to rust. I have inspected a number of duct works in plants where the duct work structure was only being held in place with wire or maybe a strap here and there welded to another piece of angle iron. Not exactly the model for efficient duct work.

I was in a food processing plant that transported hot furnace air to manufacture steam and for sterilization. While I was there teaching them how to find leaks in their duct work, I witnessed technicians cutting the fingers from their gloves and stuffing the fingers into holes after globbing RTV sealant onto the fingers before inserting them into the holes to seal them. Not exactly the type of repair I'd recommend.

Duct work carrying tobacco, flour, or the like can be listened to for clogging. I realize that most plants have shakers, or vibrators, that sense a clog and immediately initiate the motor to turn on and shake or vibrate the product loose. However, if your plant is not equipped with a shaker or vibrator, use the ultrasound instrument to take a baseline reading while the product is going through the duct smoothly. The product should sound the same as the baseline reading all the time unless there is a clog.

As you can see by some of the pictures, your duct work can appear to be in great shape. No rust or broken supports. But, air in-leakage can still occur. Find a good sealant approved for the product being blown through the duct and the material out of which the duct work is made, and have it on hand.

Assure your personnel are familiar with how to use the ultrasound receiver. No matter whose ultrasound instrument you use, most will detect an air leak of 5 psig from .005-.050 inch opening at 35-50 feet away in a non-noisy environment. A noisy environment may mean you need to be closer or have an accessory that brings that leak closer to you. You may want to check out what accessories the manufacturer of your instrument has to offer.

All photos courtesy of Owens-Corning Summit, IL Roofing Facility.

Jim Hall is the president of Ultra-Sound Technologies (UST), a vendor-neutral company providing on-site predictive maintenance consultation and training. UST provides an Associate Level, Level I & II Airborne Ultrasound Certification. Jim is also a regular provider of online presentations at Reliabilityweb.com and is a contributing editor for Uptime magazine. Jim has provided airborne ultrasound training for several Fortune 500 companies in electrical generation, pulp and paper, petrochemical and transportation (marine, automotive, aerospace). A 17-year civil service veteran, Jim served as an aerospace engineering technician for the Naval Aviation Engineering Service Unit (NAESU) and with the Naval Aviation Depot, Jacksonville Florida (NADEP). Jim is also president of All Leak Detection, LLC, a leak detection company providing air leak audits above and below ground. Jim can be reached at (770) 517-8747 or jim.hall@ ultra-soundtech.com

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