Ultrasonic Measurement and Analysis

This article aims to present the basics of ultrasonic measurement and analysis, as well as the applicability of this technology for machine diagnostics, bearing condition monitoring, and leak detection tasks.

Before delving into ultrasonic diagnostics in more detail, we will review the fundamental concepts and relationships related to the technology. Sound waves are longitudinal waves that propagate by exciting the molecules of the medium. Each molecule - with some damping - transfers the received excitation (energy) to the next molecule. In our atmosphere, this naturally refers to the air molecules, and the transmission itself occurs at the speed of sound (343 m/s at 20 °C). Sound waves also propagate in other media, but with different speeds (denoted as v, unit m/s) and damping. Some examples of different propagation speeds include steel or aluminum at 5100 m/s, concrete at 3800 m/s, rubber at 40 m/s, and assuming a water medium at 1460 m/s. Generally, it can be stated that sound waves propagate faster in solid materials (with rubber as an exception) and liquids than in gases. In gases, sound propagation depends on pressure and temperature: the higher the pressure or temperature, the faster the wave travels. The propagation within a medium is linear, and its direction remains unchanged.

Figure: wave curve with main characteristics [source: PIM]

The properties of sound waves are expressed in terms of frequency (denoted as f, unit Hz) or wavelength (denoted as λ, unit m), as well as amplitude, and task-specifically, for example, sound pressure (Pa) or sound intensity (dB). Frequency indicates how many periods occur in one second (1 Hz = 1/s). Given a known frequency and propagation speed, the wavelength can be easily calculated, which is the ratio of propagation speed to frequency (λ = v/f). Therefore, for a given medium, specifying either the frequency or the wavelength is sufficient, as one can be derived from the other. According to the relationship, higher frequency sound waves have a shorter wavelength.

Sound waves behave according to several physical laws. From a measurement perspective, it is important to mention the laws of reflection, modulation (change in a characteristic due to an external influence), and interference (interaction of waves). Reflection on solid surfaces and liquids follows the well-known optical law: the reflection of the wave occurs at an angle equal to the angle of incidence. During this process, neither the frequency (wavelength) nor the amplitude changes, except for minimal losses. It is noteworthy that reflection occurs not only between air and solid or liquid surfaces but also at any interface between media with different sound propagation speeds.

Interference occurs when multiple sound waves (at least two) meet. If two sound waves have the same frequency, whether they reinforce or weaken each other, or even cancel each other out, depends on their phase difference (the relative shift in wave periods between 0-360°). A phase difference of less than 90° or greater than 270° results in reinforcement, while a difference between 90° and 270° leads to weakening, and a precise 180° phase difference results in complete cancellation according to vectorial summation. (This is also the basis for noise reduction using anti-noise.) When two sound waves of different frequencies meet, the resulting sound wave has a frequency equal to the sum of the source wave frequencies, but its amplitude can increase or decrease. (This depends on the phase angle difference and amplitude ratio.) For sound waves with significantly different frequencies, modulation occurs. Sound intensity is commonly expressed in dB (decibels). dB is essentially a logarithmic scale that expresses sound intensity as the base-10 logarithm of the sound energy P. Depending on inherited abilities, age, and health, the human ear can perceive sounds between 20 and 20,000 Hz. Since not all frequencies are heard with the same sensitivity (some frequencies are perceived more strongly, while others are significantly weaker), sound intensity measurement corresponding to human auditory perception is carried out with an A-weighting, using an A-weighted frequency characteristic filter. Sound intensity measured with consideration for human frequency sensitivity is typically indicated in dB(A). The quietest - still audible - sound is 0 dB(A), while the loudest (bearable by our ears) sound intensity is 120 dB(A). The human ear cannot perceive frequencies above 20 kHz (although dogs can), so ultrasonic sounds generated by various physical processes are inaudible. Therefore, we need devices that allow us to measure these sounds, display their intensity (e.g., in the form of a diagram or digital value), and make them audible by transforming and amplifying them into a lower frequency range. Ultrasonic measurement enables the examination of the condition and operation of numerous devices and equipment. For instance, bearings operate with high-frequency vibrations depending on their condition and lubrication, with the frequency range falling into the ultrasonic range in the case of incipient faults or inadequate lubrication. Thus, through ultrasonic measurement - whether through contact using a so-called stethoscope or through air using a microphone - early bearing faults or insufficient lubrication can be detected. If the applied ultrasonic measuring device is capable of transforming ultrasonic waves, the extent of bearing failure can even become audible. As a practical application, ultrasonic monitoring combined with bearing lubrication can be mentioned: lubricant should be applied to the bearing until its sound becomes appropriate.

With this method, excessive lubrication can be avoided, which can lead to undesirable consequences such as heating due to mechanical resistance exerted by the fat and resulting undesired decrease in bearing clearance (bearing seizure).

Figure: ultrasonic time signal and spectrum in good and bad bearing cases [source: CSI]

Ultrasonic Applications

Ultrasonic measurement is also suitable for checking bearings on conveyor belts (detecting faulty bearings). While with vibration measuring instruments, vibrations on each bearing would need to be measured and analyzed contactlessly, ultrasonic waves can be observed passing by the conveyor belt. Strong ultrasonic sources indicate damaged or extremely poorly lubricated bearings. (Of course, faulty bearings on conveyor belts can be more precisely and quickly detected from the detected heating with thermography, but the investment cost of a thermal camera required for thermography is orders of magnitude higher than the procurement cost of ultrasonic measuring instruments.)

Figure: ultrasonic time signal in case of insufficient lubrication [source: CSI]

The density can also be examined in any gas and steam system, as turbulence generated in the gas escaping through narrow gaps (holes) produces ultrasonic waves that can be localized and their strength and characteristics examined. This way, the proper operation of valves, steam traps, condensate separators, and pressure reducers can be verified. To further assist this, state-of-the-art instruments incorporate evaluation procedures that analyze data from multiple frequency bands, different parameters measured at multiple locations successively, including expert procedures, allowing for a clear determination of whether the inspected equipment is good or faulty.

Figure: CSI SonicScan: multi-frequency ultrasonic sensor and analysis [source: CSI]

It is important to mention that the intensity of emitted ultrasonic waves is not proportional to the size of the gap or hole. Smaller and narrower gaps produce stronger sound than larger and wider holes. Additionally, the pressure difference between the outflow and the external environment influences the detectability of leaks. The greater the pressure difference, the stronger the expected ultrasonic waves.

In the case of insulated or multi-layered gas and steam pipes, it should be considered that ultrasonic waves appear on the surface that can be inspected with the measurement, where they can "leak" through the upper layers. Often, these external gaps are more or less distant from the leakage point of the pressure-bearing pipe, connection, or tank, so the ultrasonic waves are not measured where the actual fault is located.

Figure: detection of leaks with ultrasonic measurement [source: CSI]

Among the diagnostic possibilities, the inspection of electrical equipment is noteworthy. It happens that high-frequency vibrations occur in transformers, distribution boards, or voltage transformers due to loosened elements, and voltage surges, as well as leakage currents, occur due to faulty connections or insulators. These phenomena (in low, medium, or high voltage networks) mostly occur accompanied by ultrasonic waves. It is also characteristic that with the shortening of the spark gap, the accompanying ultrasonic frequency increases.

Evaluation of Results Obtained

The most advanced ultrasonic measuring and analyzing devices make the measured ultrasonic waves audible through headphones and display them numerically (in dB), and are also capable of displaying various evaluations of digitally filtered frequency ranges - mean value, absolute peak value, continuous peak value, peak multiple amplitude value, etc. Furthermore, they offer possibilities for further signal processing (such as the CSi SonicScan 7000 device), as they can provide digitally generated envelope signals for machine analyzers or other signal processing devices, such as oscilloscopes. The identification of faults is often facilitated with funnels or parabolic reflectors. Various sensors can be used to better distinguish different phenomena: tactile spike or magnetically attached body sound sensors, ultrasonic microphones. For example, when inspecting tanks for leaks without pressurizing them, ultrasonic sources available with the instruments can be used. In this case, ultrasonic waves are excited inside the tank, and then it is measured where they exit the tank. Based on the above, it is evident that ultrasonic measurement excellently complements vibration diagnostics, thermography, and various material tests in the assessment of the condition of machinery and equipment. Our table summarizes the most common application areas.

• Mechanical inspections with trendable data
  • Condition and lubrication of bearings
  • Operation of pumps (presence of cavitation)
  • Operation of valves (closing-opening process)
• Detection of leaks
  • steam traps, condensate separators
  • steam system pipes, connections, seals
  • compressed air and gas system connections, seals
  • pneumatic systems (e.g. brake systems) fittings, valves, and seals
  • elements and seals of boilers, heat exchangers, and other pressurized equipment

Figure: pump with a damaged blade (ultrasonic signal and spectrum) [source: CSI]

Rahne Eric (PIM Ltd.) pim-kft.hu, gepszakerto.hu

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