P-F curve showing Ultrasound as early detection tool.
INTRODUCTION TO ULTRASOUND.
Airborne ultrasound can be considered an ideal technology to improve equipment reliability and reduce energy costs. These instruments can be used to detect a variety of potential problems or they can be used 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 rubbing. A brief overview of the technology, its applications, and suggested inspection techniques are explained.
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:
1. They are directional and problems can be easily located
2. They provide early warning of impending mechanical failure
3. Instruments can be used in loud, noisy environments
4. They support and enhance other PDM technologies or can stand on their own in a condition monitoring or energy conservation program.
Generically, applications for ultrasonic translators fall under three basic categories: mechanical inspection, leak detection and electrical inspection.
Mechanical equipment produces a "normal" sound signature while operating effectively. As components begin to fail a change in the original sonic signature occurs. This change can be noted on a meter/display panel or can be recorded if the detector has the ability to do so. The sound quality will be heard through headphones.
The key to mechanical inspection relies on a consistency factor. Variables should be kept to a minimum. To accomplish this, whether trouble shooting or trending equipment, a test point should be established. This test point can then be used for comparison with other test points on similar equipment or compared with itself over time.
As an example, for bearing inspection, in order to determine whether a bearing is in a good or failed mode, touch the bearing housing using the contact probe or a magnetic transducer, at one point, usually near the grease fitting, and adjust the sensitivity to get a specific meter reading (digital instruments will display the dB level). Compare this reading at the same sensitivity setting on a similar reference point on a bearing operating under the same conditions. The meter (dB) reading and the sound quality should be similar. This same reading can then be used to trend each bearing over time to determine lack of lubrication or failure mode.
Ultrasound detectors work well on slow speed bearings. In some extreme cases, just being able to hear some movement of a bearing through a well-greased casing could provide information about potential failure. The sound might not have enough energy to stimulate classic vibration accelerometers, but will be heard via ultrasonic translators, especially those with frequency tuning.
Electrical problems are also detected with ultrasonic translators. When arcing, tracking or corona discharges occur, they ionize air molecules producing ultrasound. Loose connections can be identified. Buss bars, switchgear, junction boxes, etc., can be listened to for the high frequency sounds of an electrical emission. This will usually be heard as a buzzing or frying sound in the headphones.
Another area of inspection for ultrasonic detectors is switchgear and overhead high voltage lines for location of corona or tracking problems. Although infrared has often been used to locate electrical problems, it has been found that these instruments are often "blind" to corona and tracking in high voltage systems (13 kV and up). Ultrasonic detectors "hear" the sound of corona and enable users to locate them quite quickly. For this reason, many inspectors now use ultrasonic translators to support their infrared electrical monitoring programs.
In fact, those inspectors that use both technologies often relate that they prefer to screen enclosed equipment such as switchgear with ultrasound instruments to detect the possibility of corona, arcing or tracking by scanning around door seals and air vents, before the cabinets are opened to perform infrared inspection. By following this procedure, the potential for an inspector being involved in an arc flash incident is dramatically reduced.
Leakage can occur in liquid or gas systems. The greatest advantage of ultrasonic detection is that it can be used in a variety of leak situations since it is sound sensitive and not "gas” specific.
The reason ultrasound is so versatile is that it detects the sound of a leak. When a fluid (liquid or gas) leaks, it moves from the high-pressure side of a leak through the leak site to the low-pressure side where it expands rapidly and produces a turbulent flow. This turbulence has strong ultrasonic components. The intensity of the ultrasonic signal falls off rapidly from the source. For this reason the exact spot of a leak can be located.
The method of generalized gas leak detection is quite easy. All one does is scan an area, listening for a distinct rushing sound. With continued adjustments of the sensitivity, the leak area is scanned until the loudest point is heard.
Some instruments include a rubber focusing probe which narrows the area of reception so that a small emission can be pinpointed. The rubber focusing probe is also an excellent tool for confirming the location of a leak by pressing it against the surface of the suspected area to determine if the sound of the leak remains consistent. If it decreases in volume, the leak is elsewhere.
Using digital ultrasonic detectors, leaks are measured in dB’s, then downloaded into available software that allows the user to assign a cost value to the leak as well as the potential reduction in greenhouse gas emissions when the leak is repaired. All of this information can be displayed in simple, custom designed, excel spreadsheet based reports.
Vacuum leaks may be located in the same manner. The only difference is that the turbulence will occur within the vacuum chamber. For this reason, the intensity of the sound will be less than that of a pressurized leak.
Valves are usually checked for leakage with the contact probe. The downstream side is used to determine leakage. This is accomplished by a method of touching two points on the upstream side and noting the intensity of both readings. The next step is to touch two points on the downstream side. If downstream levels are lower than the upstream levels, the valve is considered closed. If they are higher than upstream and are accompanied by a typical rushing sound, the valve is most likely leaking. If the second downstream point is louder than the first, this may indicate that the sound source may not be coming from the valve. The sound may be generated from a source further downstream, not associated with the valve.
Steam traps are also inspected easily with ultrasonic translators. It is important to determine exactly how a particular trap is supposed to operate. This can be accomplished by consulting with steam trap suppliers and using other resources such as the internet. When the operation of the trap is known, it is tested by touching near the discharge orifice of the trap and both listening and noting the intensity level. If a sound is heard that is consistent with the correct operation of the trap, the trap is working correctly, if a load hissing or rushing noise is continuously heard, the trap is likely failed.
Airborne ultrasound instruments are an important part of any reliability based condition monitoring programs as well as an essential tool for energy conservation programs. Their versatility, ease of use and portability enable inspectors to effectively plan and implement inspection procedures. By locating leaks, detecting high voltage electrical emissions and sensing early warning of mechanical failure, these instruments contribute to cost reduction, improved system efficiencies and reduced downtime, increasing a plants overall reliability.