INTRODUCTION TO ULTRASOUND

Airborne ultrasound is considered the ideal technology to improve equipment reliability and reduce energy costs. These instruments can be used to detect a variety of potential problems and can be complimented with other PdM technologies such as vibration and infrared. Usually portable, these instruments detect leaks in both pressurized gas and compressed air systems, vacuum systems, valves and steam traps. Additional applications include inspection of high voltage apparatus for corona, arcing and tracking. They are also used to trend bearings and detect conditions such as lack of lubrication and impacting.

In companies worldwide, there has been a compelling need to improve equipment and plant reliability, and reduce costs. An integral part of this process has been to implement programs using condition monitoring technologies. Use of these condition monitoring technologies has led to improvements in production, reduced maintenance costs, reduced energy consumption, improved efficient use of personnel and increased profitability.

One reason for the improvements in equipment and plant reliability has been the development and advancements in many of the condition monitoring and PdM technologies. Airborne/Structure Borne Ultrasound. Instruments based on this technology can monitor a wide range of plant operations and yet are simple enough to be used with minimal training for basic, effective inspection routines. Ultrasound has become an integral part of an overall reliability program in companies around the globe.

Lightweight and portable, ultrasound instruments may be used to inspect potential problems in practically every type of equipment and system in a plant.

Some typical applications include:

Leak detection in pressure and vacuum systems, compressed air audits, leak detection on specialty gas systems, bearing inspection, testing gears/gearboxes, pumps, motors, steam trap inspection, valve testing, detection of cavitation in pumps, and testing for arcing, tracking, and corona in electrical apparatus.

What makes airborne/structure borne ultrasound so effective? All operating equipment and most leakage problems produce a broad range of sound. The high frequency ultrasonic components of these sounds are extremely short wave in nature. A short wave signal tends to be fairly directional. It is therefore easy to isolate these signals from background plant and operating equipment noises and to detect their exact location. In addition, as subtle changes begin to occur in mechanical equipment, ultrasound allows these potential warning signals to be detected early, before actual failure.

Airborne ultrasound instruments, often referred to as "ultrasonic translators", provide information two ways: qualitative through the ability to "hear" ultrasounds through a noise isolating headphone and quantitative via incremental readings on a meter or display panel.

Although the ability to gauge intensity and view sonic patterns is important, it is equally important to be able to "hear" the ultrasounds produced by operating equipment. That is precisely what makes these instruments so effective. Ultrasonic instruments allow inspectors to confirm a diagnosis on the spot by being able to clearly discriminate among various equipment sounds. This is accomplished in most ultrasonic translators by an electronic process called "heterodyning" that accurately converts the ultrasounds sensed by the instrument into the audible range where users can hear and recognize them through headphones, and in some instruments, record the sounds for further analysis and documentation.

The high frequency, short wave characteristic of ultrasound and the ability of these instruments to sense that sound, enables users to accurately pinpoint the location of a leak or of a particular mechanical anomaly.

Most of the sounds sensed by humans range between 20 Hertz and 20 kilohertz (20 cycles per second to 20,000 cycles per second). The average human threshold is approximately 16.5 kHz. Low frequency sounds in the human audible range are approximately 1.9 cm (3/4") up to 17 m (56') in length. The high frequency sounds sensed by ultrasonic translators are only 0.3 cm (1/8") up to 1.6 cm (5/8") long. Ultrasound wavelengths are therefore magnitudes smaller than those of low frequency, which makes the "ultrasonic environment” much more conducive to locating and isolating the source of problems in loud plant environments.

The basic advantages of ultrasound and ultrasonic instruments are:

They are directional and problems can be easily located
They provide early warning of impending mechanical failure
Instruments can be used in loud, noisy environments
They support and enhance other PdM technologies or can stand on their own in a condition monitoring or energy conservation program.