Page 267 - Intro Predictive Maintenance

258 An Introduction to Predictive Maintenance

of sonic penetration along the container’s surface. This ultrasonic transmission method
is useful in quick checks of tank seams, hatches, seals, caulking, gaskets, or building
wall joints.

Most of the ultrasonic monitoring systems are strictly scanners that do not provide
any long-term trending or data storage. They are in effect a point-of-use instrument
that provides an indication of the overall amplitude of noise within the bandwidth of
the instrument. Therefore, the cost for this type of instrument is relatively low. The
normal cost of ultrasonic instruments will range from less than $1,000 to about $8,000.
When used strictly for leak detection, little training is required to employ ultrasonic
techniques. The combination of low capital cost, minimum training required to use
the technique, and the potential impact of leaks on plant availability provide a posi-
tive cost benefit for including ultrasonic techniques in a total-plant predictive main-
tenance program.

11.3 LIMITATIONS
Care should be exercised in applying this technique in your program. Many ultrasonic
systems are sold as a bearing condition monitor. Even though the natural frequencies
of rolling-element bearings will fall within the bandwidth of ultrasonic instruments,
this is not a valid technique for determining the condition of rolling-element bearings.
In a typical machine, many other machine dynamics will also generate frequencies
within the bandwidth covered by an ultrasonic instrument. Gear-meshing frequencies,
blade-pass, and other machine components will also create energy or noise that cannot
be separated from the bearing frequencies monitored by this type of instrument. The
only reliable method of determining the condition of specific machine components,
including bearings, is vibration analysis. The use of ultrasonics to monitor bearing
condition is not recommended.