Using Ultrasonic Instrumentation To Enhance Plant Maintenance
Ultrasonic technology’s primary function — to detect air and gas leaks — plays a significant role in reducing plant energy costs and is an important part of any plant maintenance program.
The human ear cannot always hear trouble coming, but ultrasonic detectors can. They sense compressed air, vacuums, gas leaks, and other sounds that cannot be heard or seen (Fig. 1). These devices reliably detect leaks in underground tanks and storage containers that might otherwise go unnoticed. They are also used to identify the high-frequency sound created by the metal-to-metal contact of worn bearings. Their versatility and relatively low cost make them a popular instrument in maintenance programs.
How ultrasonic technology works
Ultrasonic detectors (Fig. 2) are similar to electronic stethoscopes, actually hearing the ultrasonic sounds that result from various types of physical, mechanical, and electrical phenomena. More sensitive than the human ear, ultrasonic devices detect and measure high-frequency sounds over 20 kHz.
For example, when gas or air flows from high to low pressure through a small hole, turbulence is generated. This turbulence creates a high-frequency sound in the ultrasonic region where background noise from machinery is usually small. Undisturbed by background noise, ultrasonic sensing devices focus on the sound generated by the leaking air or gas.
Ultrasonic detection systems typically consist of a handheld instrument, high-impedance headphones, localization sound probe, ultrasound transmitter, and precision contact probe. An electronic circuit within the detector converts ultrasonic vibrations into an audible frequency that can be heard through the headphones or a built-in speaker.
Some ultrasonic devices include a digital readout for quantifying results. In the past, readings were recorded and charted manually. This cumbersome process is being replaced by interface devices that connect the detectors to computers for systematic data collection and trending analyses.
Some early ultrasonic detection systems were sensitive to background noise, which interfered with the reception of a clear signal. Modern detectors do not react to extraneous, audible noise and can detect sounds in environments with noise in excess of 80 dB.
Detecting leaks with ultrasound
In general, the higher the decibel reading sensed, the larger the leak. The relationship between leak size and ultrasonic intensity, however, is nonlinear because detector response depends on several elements. Primary factors include distance from the leak, volume of the leak, velocity, and direction. Although the dependence on these factors is nonlinear, a simple calibration for typical measurement conditions lets the user estimate the size of a leak.
Figure 3 shows how ultrasonic response increases with pressure, but it also depends strongly on distance. For example, the measured intensity falls as 1/d(super n) with n between 1 and 2. More simply stated, the noise gets louder, the closer one moves to the object being measured. Ultrasonic intensity may also be smaller when the detector is directly in line with the leak than when it is 30 to 60-deg off-axis. In fact, the variation can be more than 50%.
Figure 4 illustrates the nonlinear relationship between decibel readings and leak rate. This concept is particularly important to understanding the results of ultrasonic measurement. As leak rate grows, the ultrasonic intensity also increases, but in a nonlinear fashion.
A decibel is a logarithmic unit, and sound intensity is nonlinear with leak rate. An increase in a decibel reading of 10 means an increase in sound intensity from the leak by a factor of 10 and an increase in the leak rate by a factor of 6.
Ultrasonics for other applications
Although many users initially use ultrasonic devices for detecting compressed air leaks, they soon find the instruments have other applications. These include detecting wear on bearings, gears, and gaskets throughout the plant. Ultrasonics can be used to identify faulty steam traps, leaking valve packing, switchgear malfunctions, gear noise, and other harmful friction in plant machinery.
For lubrication checks
In some facilities, the use of ultrasonics in the performance of lubrication checks on ball and roller bearings has become routine. Observing the noise level of grease-lubricated ball and roller bearings helps maintenance determine the point at which the grease reaches and quiets the bearing, but is not overgreased. It is also possible to determine when additional lubrication is needed and whether the proper lubrication viscosity was used.
For condition monitoring
Rolling element bearing conditions can also be monitored using ultrasonic detectors. In the early stages of failure, rolling element bearings emit high-frequency signals. The level of these signals can be monitored by graphing the results by hand over time or by connecting the detection instrument to a tape recorder, data collector, and computer. Defective or worn gears produce similar ultrasonic sounds.
For electrical problems
Ultrasound technology has also effectively detected corona and electrical discharges. Electrical corona or arcing disturbs the air surrounding the arc, causing it to vibrate and generate a sound readily detected by ultrasonic devices. Some plants now include corona checks as a regular part of their maintenance surveys.
Other types of ultrasonic devices
New and different applications for ultrasonic technology are being discovered regularly. Different types of transmitters and sensors applicable to more conditions are increasingly available. For example, a bisonic transmitter can switch between two ultrasonic frequencies up to nine times a second to produce a more distinct and measurable tone. This transmitter measures ultrasounds over wide areas, making it possible to canvas a plant quickly and with fewer labor costs.
Multidirectional devices provide similar benefits. These units contain up to 13 individual transmitters that can be adjusted in nine different directions. Each transmitter can be operated in a single frequency to pinpoint a leak precisely in a specific spot or in the bisonic mode to cover large areas.
Mobile sensors enhance the ability of ultrasonic detectors to transmit or receive long-range signals or signals from difficult to reach spots. As a result, locations previously inaccessible can now be inspected and far more leaks identified and corrected.
Mobile sensors can be placed in and above liquid in single-wall underground hydrocarbon storage tanks to test their integrity. Such tests have proven reliable in both empty and filled tanks. The EPA certifies devices for this use.
Maintaining ultrasonic detectors
Ultrasonic detection equipment requires little maintenance. Most units are designed for industrial applications and are durable and able to withstand the rigors of plant use. General maintenance involves keeping the units, transmitters, and sensors clean and recharging the battery after each use. Most units contain an LED battery test indicator.
The use of ultrasonic technology for multipurpose condition detection is expected to expand as plant maintenance personnel become familiar with its varied applications. It is a cost-effective, simple method for verifying machine conditions detected by measurement devices that use other technologies (pressure and vibration measurement units, for example). Having several ways to measure the same physical phenomena leads to greater confidence in determining and interpreting abnormal equipment behavior.
— Edited by Jeanine Katzel, Senior Editor, 847-390-2701, firstname.lastname@example.org
Ultrasonic detectors reliably sense the high-frequency sounds generated by leaks, worn bearings, or abnormal equipment operation, making them an important part of plant maintenance programs.
Interface devices connect detection systems to computers for systematic data collection and trending analyses.
Most ultrasonic detection equipment is durable and able to withstand the rigors of plant use, and requires little maintenance.
The authors are available to answer technical questions about this article. Contact Howard Malm at 604-936-1030 and Fernando Halpern at 707-577-8053. The SDT USA website is located at www.sdt-usa.com.