Ultrasonic Detector Locates Boiler-Casing Leaks

John Reason

Many undetected casing leaks plagued one of our large watertube power boilers, even after extensive work was done to repair it.  We tried smoke bombs to pinpoint the location of the leaks, but this proved frustrating and misleading because of the effect of insulation and lagging material.

We decided that, instead of looking for smoke leakage, we would try to listen for air leakage with our portable ultrasonic testing instrument. This instrument senses airborne ultrasonic vibrations generated by a pressure or vacuum leak.  It is sensitive only to ultrasonic vibration and, in most cases, is not affected by audible background noises from plant machinery.   It converts ultrasonic signals into the audio range, which can be heard through earphones, and has a meter for a visual indication. Also, our detector has a tuning dial capable of adjustment from 20 to 100 kHz.  We found this feature particularly useful.

To conduct the test, we started the forced-draft fan and gradually closed the boiler outlet damper. This pressurized the furnace casing to about 12" H₂0.   We then scanned the casing with the detector to locate leaks.  The first attempt proved confusing because of competing ultrasonic sound generated by another boiler close by, but by adjusting the tuning dial, we were able to reduce these confusing sounds dramatically.

We found leaks in cracked and unfinished welds, in the membrane wall, and in several refractory seals. It took us less than a day to scan the boiler casing completely and to locate all the leaks accurately. We estimate that, without the detector, it would have been necessary to remove both the inner and outer casing panels and insulation, which would have taken about one week.