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AMAZON multi-meters discounts AMAZON oscilloscope discounts This predictive maintenance technique uses principles similar to vibration analysis. Both monitor the noise generated by plant machinery or systems to determine their actual operating condition. Unlike vibration monitoring, however, ultrasonics monitors the higher frequencies (i.e., ultrasound) produced by unique dynamics in process systems or machines. The normal monitoring range for vibration analysis is from less than 1Hz to 30,000Hz. Ultrasonics techniques monitor the frequency range between 20,000Hz and 100 kHz. ULTRASONIC APPLICATIONS As part of a predictive maintenance program, ultrasonic instruments are used for three primary applications: airborne noise analysis, leak detection, or material testing. Airborne Noise Analysis All plants are required by Occupational Safety and Health Administration (OSHA) regulations to meet ambient noise levels throughout their facilities. These mandates have forced these plants to routinely monitor the noise levels within each area of the plant and to provide hearing protection in those areas where the ambient noise level is above acceptable levels. Ultrasonic meters are the primary tool used to monitor the ambient noise levels and to ensure compliance with OSHA regulations. In addition, some plants use simple ultrasonic meters to survey noncritical plant equipment and systems for unusual noise emissions. This latter application is limited to a simple "go/no-go" measurement and has practically no ability to diagnose the root-cause of the abnormal noise. Leak Detection The principal application for ultrasonic monitoring is in leak detection. The turbulent flow of liquids and gases through a restricted orifice (i.e., leak) will produce a high frequency signature that can easily be identified using ultrasonic techniques. There fore, this technique is ideal for detecting leaks in valves, steam traps, piping, and other process systems. Materials Testing Ultrasonics has been, and continues to be, a primary test methodology for materials testing. Typical test frequencies start at 250 kiloHertz (kHz), or 250,000 cycles per second (cps), up to 25 Mega-hertz (MHz), or 25 million cps. Testing materials generally consist of introducing an energy source into the material to be tested and recording the response characteristics using ultrasonic instruments. These tests may be as simple as striking the material with a hammer and recording the results with an accelerometer and ultrasonic meter. Ultrasonic testing relies on the measurement of time and amplitude or strength of a signal between emission and reception. Because of a mismatch of acoustic properties between materials, the sound will partly reflect at interfaces. The quality of reflected energy depends on the acoustic impedance ratio between two materials. For example, sound transmitted through steel reaching a steel/air boundary will cause 99.9 percent internal reflection, whereas a steel/water boundary would reflect only 88 percent within the material and transmit 12 percent into the water. If impedance ratios are widely different, such as an open crack with a steel/air interface, then adequate reflection will occur and permit detection of the flaw. Conversely, a small crack in a compressive stress field that does not have oxidized faces will yield a steel/steel boundary and cannot be detected using this method. TYPES OF ULTRASONIC SYSTEMS Two types of ultrasonic systems are available that can be used for predictive maintenance: structural and airborne. Both provide fast, accurate diagnosis of abnormal operation and leaks. Airborne ultrasonic detectors can be used in either a scanning or contact mode. As scanners, they are most often used to detect gas pressure leaks. Because these instruments are sensitive only to ultrasound, they are not limited to specific gases as are most other gas leak detectors. In addition, they are often used to locate various forms of vacuum leaks. In the contact mode, a metal rod acts as a waveguide. When it touches a surface, it’s stimulated by the high frequencies, ultrasound, on the opposite side of the surface. This technique is used to locate turbulent flow and/or flow restriction in process piping. Some of the ultrasonic systems include ultrasonic transmitters that can be placed inside plant piping or vessels. In this mode, ultrasonic monitors can be used to detect areas 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 don’t 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 positive cost benefit for including ultrasonic techniques in a total-plant predictive maintenance program. 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. |