ULTRASOUND APPLICATIONS

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In general, applications for ultrasonic translators fall under three basic categories: mechanical inspection, leak detection and electrical inspection.

 

MECHANICAL 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.

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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.

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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.

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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 INSPECTION

Electrical problems are also detected with ultrasonic translators.  When arcing, tracking or corona discharges occur, they ionise 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. 

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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 (13kV 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.  

 

LEAK DETECTION

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.

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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.

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The method of generalised 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.

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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 decibels (dB) 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

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

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. 

 

Wrapping up

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.