Ultrasonic Noise Condition Monitoring

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Contents

Sound

The human ear can respond to minute pressure variations in the air if they are in the audible frequency range, roughly 20 Hz - 20 kHz. This is call airborne sound or noise. Sound can travel through gas, liquids or solids. When sound travels through a solid it is also a high frequency vibration. The speed that sound moves through material is constant for any particular material.

What is Ultrasonic Noise? - Sound can be produced by vibratory or shock events. Ultrasonics is the science of sound waves above the limits of human audibility.

The frequency of a sound wave determines its tone or pitch. Low frequencies produce low or bass tones. High frequencies produce high or treble tones.

Ultrasound is a sound with a pitch so high that the human ear cannot hear it. Frequencies above 20 Kilohertz are considered to be ultrasonic.

Low-frequency sounds in the audible range have wavelengths of ~1.9 cm up to 17 metres. Those detected by ultrasonic instruments have a wavelength of only 0.3–1.6 cm and can travel through small openings.

Amplitude or loudness of an ultrasonic noise falls off exponentially from the source. The noise is localized and can easily be isolated for detection and analysis.

The frequencies used for ultrasonic condition monitoring instruments are usually from 20kHz to 40kHz.

What is Ultrasonic Noise? - The diagram below uses the coils of a spring to represent individual molecules of a sound-conducting medium. The molecules are influenced by adjacent molecules in the medium in much the same way that the coils of the spring influence one another. The compression generated by the sound source as it moves, propagates down the length of the spring as each adjacent coil of the spring pushes against its neighbour. The individual coils remain in their same relative positions, being displaced first one way and then the other as the sound wave passes. As a result, each coil is first part of a compression as it is pushed toward the next coil and then part of a rarefaction as it recedes from the adjacent coil.

Figure 53 – The Nature of Sound Waves 

In much the same way, any point in a sound-conducting medium is alternately subjected to compression and then rarefaction. At a point in the area of compression, the pressure in the medium is positive. At a point in the area of a rarefaction, the pressure in the medium is negative.

Frequency and Amplitude of Sound Waves – Figure 54 demonstrates frequency and amplitude using the spring model introduced earlier. In the diagram, if A is the example base sound wave, B with less displacement of the media (less intense compression) as the wave front passes, represents a sound wave of less amplitude or "loudness". C represents a sound wave of higher frequency indicated by more wave fronts passing a given point within a given period of time.


Ultrasonic noise measurement instruments

Ultrasonic Transducers - Piezoelectric transducers convert mechanical energy to alternating electrical energy directly through use of piezoelectric crystals.

Piezoelectric crystals produce an electrical charge when force is applied to it. Contact ultrasonic sensors are technically identical to accelerometer vibration transducers but are not as sophisticated as they only need to measure a limited frequency range.

Non-contact ultrasonic sensors use the same technology but are equivalent to a microphone.

Ultrasonic Noise Instruments – Most ultrasonic noise instruments allow the measurement of the level of ultrasonic noise and also have electronics that convert ultrasonic noise into the audible range. This allows the human ear to be able to analyse ultrasonic noise patterns.

Sources of Ultrasonic Noise

The typical sources of ultrasonic noise are:-

  • Turbulent Fluid
  • Friction and Impacts
  • Electrical Noise

Turbulent Fluid - High turbulence from fluid flow is usually created by a pressure differential. Examples of high turbulence are:

  • Pressurised air leak from a pipe
  • Leakage of hydraulic oil across a worn hydraulic valve
  • An operating pressure relief valve
  • Vacuum leak into an evacuated tank

Friction and impacts - Examples of friction or impact could be:

  • Rubbing a piece of metal on the table
  • High speed rolling element bearing operating normally
  • A gate valve closing under pressure
  • Material falling into a chute

Electrical noise - Some examples could be:

  • Fluorescent light starter
  • Electrical arc welding
  • Current tracking on an insulator

What are Ultrasonic Noise Instruments used for?

  • Air leakage surveys (also for other gases such as CO2)
    • Air system leakage gives high electricity & upgrade costs
  • Condition Monitoring of rolling element bearings
  • Fast & cheaper condition monitoring method for less critical bearing systems
  • Condition Monitoring for slow speed bearings
    • Has a number of advantages for slow speed bearing monitoring
  • Steam trap and steam leakage surveys
    • High potential savings in energy costs & component life
  • General equipment defect identification
  • Finding internal valve or other leaks in hydraulic or fluid pipework systems
  • Electrical equipment surveys or defect identification
    • Find arcing or electrical corona defects

Other Ultrasonic noise instruments

Ultrasonic noise is used for many applications other than Condition Monitoring. Examples are:

  • Ultrasonic flow measurement

(See Figure 65)

  • Medical ultrasounds
  • NDT thickness testing
  • NDT crack testing
  • Ultrasonic cleaning

Non-contact Ultrasonic Noise Techniques

Non-contact ultrasonics relies on the sound from a defect of interest being transmitted through the air. It is the best technique to use to detect external gas leaks from pressurised systems or to find vacuum systems leaks. The sound source is in the high velocity area of the leak and can transmit its ultrasonic noise freely into the air.


Figure 66 – Turbulence Caused By Pressure & Vacuum Leaks

The technique is also used where the source of the noise is dangerous, such as with electrical systems or is inaccessible eg. where the source is guarded. Non-contact measurement is used where speed of inspection is very important ie. where the number of components is large eg. conveyor trough & return rollers. Non-contact analysis is limited by the intensity of the sound transmitted to air and the level of background noise.

Using an Ultrasonic Noise Generator - If the pressure driving the potential leak is small then the intensity of the measured ultrasonic noise will be low and will require the ultrasonic instruments microphone to be very close to detect the leak.

Figure 67 – Leak Detection Without Pressurisation In some situations it may be impractical to measure the leakage while the system is pressurised. Battery operated ultrasonic noise generators are available as an accessory. These units can be turned on inside a compartment or vessel and leakage paths to the external environment will be easily detected with a non-contact ultrasonic meter. A standard applications is for pressure vessels.

Contact Ultrasonic Measurement

For all non-external leakage ultrasonic noise applications, contact measurement is the preferred technique unless safety, access or speed of monitoring makes it unacceptable. This is because ultrasonic noise is typically being generated inside equipment components. Ultrasonic noise can be transmitted from the surface of the component into the air but contact measurement will be far more sensitive to detect, monitor and diagnose the defect. Two standard applications of contact measurement of ultrasonic noise are monitoring of bearing friction and excessive steam trap leakage.

Appling Ultrasonic Measurement Techniques - There are three general monitoring approaches used:-

Trending of ultrasonic noise levels - A defect is identified by:-

  • A significant step increase in measured noise level
  • A slower trend up in measured noise level
  • Measured noise level reaches a predefined alarm level from history or dB increase)

Comparative analysis with an identical or very similar component - This approach relies strongly on the analysis of the sound patterns coming from the component in comparison to the reference component (the reference pattern could be from memory). The measured noise level is also usually compared.

Defect identification Approach – This is used where multiple noise levels and patterns have to be analysed from the component and from surrounding systems to understand the problem. Other observations and measurements are usually made and all are analysed together to determine if there is a defect and what the cause may be. This approach is often used after a defect is found by one of the above two approaches.

Monitoring Bearings and Machine Friction

Ultrasonic monitoring of rolling element bearings is a simple form of high frequency vibration monitoring. Bearing damage and bearing lubrication defects generate high frequency noise and give early warning of problems.

Good vibration monitoring practice for bearings measure both high and low frequencies. There are some very sophisticated & successful techniques to verify that a bearing is defective using spectrum analysis. Ultrasonic monitoring of bearings relies on trending of the high frequency noise levels and a comparative analysis of the sound patterns. It gives early warning of defects. It does not give a good indication of when a bearing may fail.

Monitoring of higher speed bearings (>600rpm) relies more on trending, while lower speed bearings rely more on analysis of sound patterns, but they are always used together. Ultrasonic noise is generally not suitable for monitoring oil lubricated journal bearings.

Higher Speed Bearings Sounds Patterns – Listen to ‘Normal Bearing” sound pattern and ‘Bad Bearing’ sound pattern from the included CD. The sound pattern difference is not great but is obvious if compared to a reference. This is because higher speed bearings always produce some friction noise. On under loaded and vertical shaft bearings, roller skidding can also occur producing noise.

There are machine components other then bearings that could be subject to friction problems. These can be monitored in exactly the same way as bearings. The sound patterns will be different depending on the component. Examples could be shaft-sealing areas, sliding components, jacking screws etc.

Ultrasonic vibrations are directional and travel in straight lines. This makes it important to understand the direction of the load that the shaft applies to the bearing. Figure 72 shows the typical loaded area for a C2 (normal clearance) bearing.

It also assists to understanding the machine housing design and where the bearings are located within, to find the most direct transmission path (Example in Figure 73).

Machines such as fans and pumps have speeds above 600rpm and a significant component of the force on the bearing is balance. In this situation there should not be a large radial variation in reading around the housings. If there is a large local high reading on these machines, it may indicate a defect such as misalignment.

Finding the Best Monitoring SPOT - Ultrasonic vibration will reduce (attenuate) over distance from the source. This attenuation is especially severe at machine casing joints, so it is very important to try to monitor the bearing by measurements on the same casing that hold the bearing.

Select the monitoring Spot by determining the loaded direction, estimating the likely transmission paths and trial measurements over the selected area to determine largest reading (Figure 74 & 75). Check effect of probe angle & select the final spot taking into account measurement practicality and reading level.

For difficult applications where the bearing casing is not accessible, try grease nipples, bolted connections into casing or the closest casing connecting to the bearing. Outboard motor bearings are often difficult. See Figure 76.

For dead-shaft pulley bearings, measure from the end of the shaft on the outer diameter on the loaded side of the pulley. Ultrasonic noise will often be transmitted better along the surface of a solid object. On a dead shaft pulley, noise generated in the load zone is transmitted along the outer diameter of the shaft to the end of the shaft.


Ultrasonic Vibration Severity for Bearings

  • 6 dB variations around average value (+ or - 3 to 4 dB) may be normal variation. A normal sound pattern is muffled smooth whirring, rushing or hissing noise.
  • 8 dB gain over baseline indicates a very early bearing defect or lack of lubrication, overloading or excessive speed. The sound pattern is a louder (rougher growl) and is more strident & high-pitched.
  • 12 dB increase establishes the very beginning of the bearing defect or hard contaminant particles in lubricant. Sound pattern is a more crackling noise.
  • 16 dB gain indicates advanced failure condition
  • 35-50 dB gain warns of a very severe issue

High frequency noise is an early warning parameter and can assist with finding the Root Cause of a defect. Before serious repair action is decided upon, correlation with other condition monitoring parameter should be used, to be able to make a better prognosis of failure.

Inaccessible locations - Where a bearing is not directly accessible for contact with the probe, then probe extensions to the standard probes can be used. These may be supplied with some instruments or they can be made to suit the requirement. Probes up to 3 meters long can be made up. In some situations non-contact microphone detection can be used if the friction noise is high.

Bearing Lubrication

In normal use, rolling element bearings experience little or no wear and are very reliable. This is due to the lubricant film being maintained between components. The diagram below shows what happens to the microscopic film thickness if lubrication is poor or if loads are high. Due to the surface roughness there are microscopic contact between the rolling elements. The surface peaks are crushed together and weld, then break apart producing microscopic impact noises. These impacts result in increased noise at ultrasonic frequencies and generate a sound pattern that can identify poor lubrication or overloading.

Monitoring Bearing Lubrication - Some ultrasonic detectors offer a lube adaptor that attaches an ultrasonic contact sensor to any standard grease gun. This permits ultrasonic monitoring and lubrication simultaneously. Another approach is to use a magnetically attached ultrasonic transducer directly onto the bearing housing. Monitoring lubrication has many advantages:-

  • Confirms grease is getting into a bearing
  • Identifies bearings that are damaged, under lubricated, overloaded, underloaded or running at excessive speed
  • Ensuring that greasing quantity and frequency are adequate, to ensure service life is achieved
  • Can help optimises lubrication intervals
  • Can help determine quantity of grease required
  • Can help guard against over-lubrication (when to stop pumping)

Ultrasonic greasing is not a method to completely replace normal greasing practices. It is best used to supplement and optimise your current greasing practices.

Some effects of over-greasing are:-

  • Over pressuring bearing cavities and damaging seals (Grease guns can produce 200 psi pressure)
  • Getting grease into motor windings (Figure 81)
  • Causing grease churning creating excess bearing

temperature

Ultrasonic Bearing Lubrication Procedure (with measurements) – Set up the ultrasonic instrument for the normal bearing monitoring procedure.

  • Attach the grease gun to the machine grease nipple.
  • Listen to the sound pattern from the headphones, begin pumping the grease gun, give only a half pump at a time. By using half pumps with the grease gun we are trying to avoid over-lubrication and pressurisation of the bearing housing. Too much grease can upset bearing dynamics and causes grease churning. This is important for critical motors and other higher rpm bearings but is not necessary for lower speed bearing where excess grease is not a problem. (lower speed is where shaft dia. (mm) X rpm < 20,000)
  • The sound pattern should change as the grease reaches the bearing. In most cases the ultrasonic level will decrease. (it may be after a few, half pumps). Continue greasing until the specified quantity is supplied or there is an increase in ultrasonic noise. This increase in ultrasonic noise indicates a significant quantity of grease has been forced into the rolling elements. If this increase quickly dissipates continue greasing.
  • If after only the initial half pump of grease you noticed an increase in the sound level, stop lubricating and wait 10-15 seconds for the ultrasonic noise level to stabilize. If it does not stabilize, stop lubrication of the bearing as the bearing likely has enough grease already. If the sound level does not vary at all, review the possible reasons for this.
  • The lower value of ultrasonic noise and the dB reduction should be recorded, if a full monitoring service is required. If a greasing services is being supplied, only record information if greasing variation is unusual (> 8dB change). Also record if the measured lowest level is more than 8dB over the recorded baseline level or the sound pattern indicates a possible bearing defect.
  • If a bearing defect is suspected then carry out your normal bearing defect identification. If an early bearing defect is present then the ultrasonic noise reduction from greasing may only last a short period (1 to 30 minutes).

Monitoring Bearing Lubrication - If a bearings ultrasonic noise does not decrease during greasing, investigate whether there is a problem. Good judgment is needed. Reasons for ultrasonic noise not decreasing during lubrication:-

  • The grease is not getting into the bearing rolling elements
  • The grease is not getting to the bearing cavity (blockage or leakage)
  • The bearing was well lubricated already
  • Background noise is too high to hear the noise change
  • Poor casing transmission of ultrasonic noise to the grease nipple
  • Speed is too low to generate lubrication noise
  • The bearing load is very low relative to design capacity
  • Not enough grease yet

If it is considered that ultrasonic noise is not detecting lubrication defects then controlling grease quantity by ultrasonics should be discontinued and standard greasing practice carried out.

If a bearing has a remote greasing point then obviously a grease gun attached ultrasonic instrument will not be effective. A standalone ultrasonic meter could be used in this circumstance (probe extensions may be required).

Grease Quantity - If ultrasonic noise increase is being use to limit the quantity of grease being applied, it is recommended to use some Caution. Its application is best for higher speed bearings (>600rpm) and some medium speed application where external contamination is not a major issue.

Grease may need time to flow to the right area of the bearing and may indicate higher level of ultrasonic noise before adequate quantity is added. Lubricate a little at a time to prevent under or over lubrication.

Consider whether grease enters trough the centre of the bearing or through the housing cavity at the side of the bearing. Centre entry should give instant grease to the bearing. Side entry may require more pumps before grease gets into the rolling elements.

Understand the grease relief mechanism and required grease flow path through the bearing. Consider what damage could be done by over-pressure or over-lubrication. Understand the quantity of grease that is being added compared with the volume of the bearing housing. Understand seal type & watch for signs of purging from seals. If it is not coming out, where is it going?

Slow Speed bearings - Monitoring of slow speed bearings with vibration analysis can be difficult and time consuming but is a successful technique. A faster and lower cost approach is to monitor with ultrasonic sound patterns. The Japanese Steel Industry successfully uses assisted listening techniques to monitor many slow speed bearing applications and using ultrasonic listening significantly improves on this. If it is integrated with greasing it becomes cost effective but as with vibration monitoring it may not be guaranteed to catch every failure.

The problem with monitoring of slow speed bearing is that the friction noise is very low. Ultrasonic instruments have a wide sensitivity range. Defects are detected and monitored by listening to the sound patterns from these bearings more than monitoring of noise levels. It is important that the levels of background ultrasonic noise be low to monitor successfully.

On slow speed bearings it is possible to set a baseline and monitor as for normal bearing ultrasonic monitoring.

Be patient when monitoring slow speed bearings. Set your ultrasonic meter to Log (if applicable) and listen for at least 8-10 second. This is because defect noises may not occur at every rotation. The slower the bearing the longer you should listen.

In extremely slow bearings (less 25 RPM), it is often necessary to disregard the meter display and only monitor by the sound pattern from the bearing. With large bearings and high viscosity grease, no friction sound may be heard as the grease may absorb most of the acoustic energy. If a sound is heard it usually is a crackling or clicking sound and indicates a possible defect. Grease analysis is the best way to confirm a low speed bearing defect. Observation of grease colour changes and any signs of magnetic debris in ejected grease is important. There are a number of simple ways to get a used grease sample (chapter 4).

Diagnostics for Bearings and Lubrication defects –

  • Determine the source of the noise by finding the location of maximum sound pattern and noise level (seal or bearing). The maximum level around the bearing housing may indicate defect location or load direction.
  • Check for and eliminate possible sources of ultrasonic interference external to the bearing.
  • Check for noise with listening rod. Check velocity vibration with meter or hand. Check bearing temperature and if practical shaft temperature with a non-contact meter.
  • Determine the defect type, its severity and any repair recommendations if defect is severe.
  • Severity needs to be assessed from the prognosis of the defect from levels and trends (how quickly will it deteriorate) and the possible effects of the problem (Safety, Environmental and economic consequences).
  • If possible determine root cause of the defect and how to eliminate any future reoccurrence.

Critical bearing are typically routinely monitored so defects are often found by small changes indicated from trend graphs or by comparing to historical baseline levels. Sound patterns are often more important in recognising bearing defects than in finding leaks. As defects such as poor lubrication only produce small sound pattern changes, so using reference component comparisons is very important to verify a defect.

Ultrasonic noise is not the best indicator for bearing condition severity. Verifying by correlation with other condition monitoring techniques may be necessary before a failure prognosis is given or a component change-out is recommended. Some correlation techniques are:

  • Vibration analysis using a Vibration Meter
  • Oil analysis or grease analysis
  • Various 5 Senses monitoring techniques

Bearings Trending - Tending ultrasonic noise gives a history of the bearings condition & lubrication.

  • It can help pick out small changes that might otherwise be regarded as random variations.
  • It gives an instant visualisation of the variability of a measurement point.
  • It gives an instant visualisation of the significance of changes

Other Mechanical Defects - There are a range of equipment defects that ultrasonic noise can assist with. The example below is cavitation. Another is piston compressor valve monitoring.

Cavitation - In a non-elastic fluids such as water, sound and pressure fluctuations are transmitted normally as long as the amplitude is relatively low.

As amplitude is increased, however, the magnitude of the negative pressure in the areas of rarefaction eventually becomes sufficient to cause the liquid to create a partial vacuum bubble, due to the negative pressure. This causes a phenomenon known as cavitation. Cavitation can be a significant problem in the negative pressure zones around pump impellors or vanes.

As a sound waves or pressure fluctuations pass, the cavitation "bubbles" grow and eventually get to an unstable size. Finally cavitation "bubbles" collapse violently resulting in implosions, which cause shock waves to be radiated. The human ear can easily hear significant levels of cavitation indicating the collapse of many large cavitation “bubbles”. As cavitation produced ultrasonic noise, even very low levels of cavitation can be detected with an ultrasonic instrument. Detecting the initiating location of cavitation can assist in understanding its cause and in finding ways to eliminate the defect.

Contact Ultrasonic Noise Measurement – Errors

  • Measurement of components that are not operational
    • Produce wasted effort and confusing results.
  • Measurement of the wrong component or the wrong location on the component
    • May cause incorrect diagnosis or may just cause confusion and waste time.
  • Other sources of ultrasonic noise interfering with or overpowering component noise
    • Potential to miss some defects or create a false diagnosis.
  • Incorrect probe connecting force, direction of the probe to the surface or loose probe connections
    • Causes unnecessary variation in measurements, potentially missed defects and confusion.
  • Not selecting the correct sensitivity range
    • This could result in inadequate audible headphone levels and incorrect level measurement, potentially missing defects and wasting effort
  • Not recognising a specific defect sound pattern
    • Can miss defects that are not identified by magnitude but by sound pattern eg. Very low speed bearing.
  • Incorrect instrument settings (Filter, Mode etc. instrument specific)
    • Gives inadequate instrument performance and missed defects.
  • Not identifying the exact maximum level location on the component
    • Potentially resulting in the wrong diagnosis.
  • Not identifying the correct defect type, defect severity or maintenance action. Need to understand the failure modes for the component.

Identify & Correct Errors or Reject Bad Data

Irregular variation in amplitude

  • Check if caused by small movements of the probe or probe looseness. Try to stabilise & hold steady. More difficult at high sensitivity settings.
  • Check for presence, level and possible source of the background vibrations that could be affecting ultrasonic noise amplitudes. Measure at two places on each transmission path. Large vibrations from nearby equipment can be a cause eg. Gearbox or materials chute. Collect levels when the background interference is lowest and note the issue.

Measurement lower than expected

  • Check to ensure the right component and spot location has been measured and that the machine is operational.
  • Check ultrasonic instrument adjustments.
  • It is unusual for equipment ultrasonic noise to reduce significantly without a specific cause eg greasing, realignment, force change etc. Check for evidence of maintenance.

Other issues that can effect ultrasonic noise are noise from nearby components, speed changes or nearby fluid flow variations. All the above issues are also relevant for below.

Measurement higher than expected

  • Check for possible causes of higher ultrasonic noise by 5 senses inspections
  • More detailed 5 senses inspections and diagnostics to be performed if the increase is more than 200%, a trend (or other pattern) is recognised or if a defect has already been identified. Ensure to checks for external sources of ultrasonic noise.

Monitoring external leakage (Pressure or Vacuum)

Leakage in pressurised air or gas systems are a significant source of energy losses in many industrial plants. A 0.4mm diameter hole, or equivalent, leaking at 690kPa gives 18 m3 per day or 6750 m3 per year increased flow requirements and is a significant extra usage of electricity. Air leaks are a significant contributor to the background environmental noise with OH&S consequences.

Ultrasound is a high frequency, short air pressure wave. The intensity of the ultrasound produced by a leak drops off rapidly as the sound moves away from its source. For this reason, the leak sound will be loudest at the leak site. Ultrasound is very "directional" and therefore, locating the source of the leak is quite simple with a directional sensor.

Because the intensity of the signal falls off rapidly from the source, you can locate the exact spot of a leak, whether it’s pressure from compressed air or other gas, a vacuum leak, a condenser heat exchanger leak, or a leak that’s under ground or behind a wall

Understand the System Being Investigated

The more familiar you are the system being checked the easier and quicker a survey will be. Understand the number of compressors, operating pressures, where pipework goes and where maintenance and new installation work has been carried out. New does not mean without leaks. Sections of pipework may be isolated and this needs to be understood.

Diagrams – Obtain drawings of the system and check if they are up to date. Use these drawings to map out a route that will be followed each time a survey is done. Use copies of the drawings for marking leaks if the site is complex.

Monitoring External Gas Leakage

General guideline for finding leaks - First, set to Ultrasonic instrument to fixed band Frequency and the mode switch to Log (if relevant). Then set the volume level sensitivity so that the normal background noise of the plant is near the bottom of the hearing threshold.

Pan the instrument along the pipes of the air/gas system or around plant area to be checked. When the typical leak sounds are heard, slowly move the instrument up, down, right, and left in the shape of a cross to determine which orientation sounds the loudest.

With the instrument pointing in the loudest direction, begin moving toward the sound until the leaking component is found. If it is hard to determine the direction of the noise, reduce the sensitivity until direction can be established.

In order to confirm the leak site, move the scanner back and forth over the suspect area. The sound level should increase as you pass over the leak. In some loud site environments, frequency tuning may be advantageous (if relevant). Mark leak with black felt tip pen and or tag. Record the location of the leak, the type of leak & severity.

Take a close-up photo of the leak location and a photo from a greater distance to identify the specific plant location (See Figure 89). In some situations a photos will not uniquely identify the location so a tag must be attached to the leak location. An alternative to the tag is use of a can of paint spray, if it is allowed.

Detection of Small Leaks - For small leaks a flexible focusing attachment is added to the microphone. The smaller opening on the end of the attachment limits noise from other sources giving precise location. Liquid Leak Amplification (LLA) is a method used to detect very small leaks. This is very important if leak detection is required for expensive gasses. A special liquid (or water) is poured over a leak test area. The LLA liquid will begin to create minute bubbles that collapse as soon as a trace of escaping gas hits it. The collapse will create an ultrasonic shock wave, which will be registered by an ultrasonic sensor.

Leak Severity

Very rough CFM (ft3/minute) flow rate estimates for sensitivity settings for the Ultraprobe 2000

Very rough CFM flow rate estimates for dB outputs

Plant Leak Improvement Strategy - In many plants 40 - 50% of compressed air capacity is used to supply leaks. Many of these leakage defects are difficult to detect without ultrasonics and require an integrated longer-term strategy to eliminate. The following approach is recommend.

  • Use a plant wide leak audit or pilot area audit to determine savings from leak reduction.
  • Determine frequency of a full leak surveys (Typically 1/yr). Carry out survey & repair leaks.
  • Repairs should be tested with soapy water to confirm success. Set a standard for repairers to check leakage after installation and repair of fittings & connectors.
  • Review the pattern and type of leakage defects detected. Determine if changes are required to standard for hoses, fittings & connectors used or to maintenance procedures.
  • Monthly recording and trending of compressor total loaded hour operation to monitor for any significant unexpected increase in air usage.

• A second stage of ultrasonic leakage testing is also recommended to detect internal leakage defects from cylinder pistons, isolation valves, DCV’s, check valves and relief valves.

Leak Survey Reports – The ideal way the manage air leakage defects is to find the leaks and fix them immediately. In many situations this is not practical due to isolation requirement or the size of the job. In this situation the leak location has to be identified so that it can be found at a later stage. One ideal way to do this is with defect tags.

Another way to document a leak is with digital photos, downloaded in MS Excel or similar software. An example is given below. This format uses two photos, a close-up on the defect and one of the general equipment area. The equipment area photo makes it easy to identify the location and an arrow is drawn between the two photos to identify the exact defect location.

Monitoring internal leakage or fluid flow restriction in pipework or pressurised systems

A gas or liquid passing through an orifice generates ultrasonic turbulence that is picked up with the contact probe. Often valves start to leak internally & it is very difficult to determine which valve is leaking without an ultrasonic instrument.

Place the probe upstream and downstream of the valve, steam trap or restriction and compare ultrasonic readings. Louder levels downstream indicate the steam trap or valve is open. Lower or similar level indicate its closed. A fully open larger valve may produce little or not ultrasonic noise. A partially open (cracked) or leaking closed valve will have a pressure differential across the valve and generate turbulence & ultrasonic noise.

Ultrasonic noise is generated when turbulent flow occurs, such as the circular currents that form when liquid or gas flows through a partially open valve. The intensity of the noise is directly proportional to the flow rate. If a valve is completely open and its configuration is such that it does not significantly interfere with the flow through the pipe or the flow rate is slow enough, then the flow will stay laminar and the downstream flow will generate little if any ultrasonic noise.

Steam Trap Monitoring - Steam traps remove condensate from steam systems without letting steam escape. Leaking steam traps are a major source of waisted energy.

Ultrasonic noise inspection of steam traps is one of the most reliable monitoring methods available. Blow-by, oversized traps or line blockage are all easily detected. Frequency tuning can enhance discrimination between condensate and steam flow (if relevant).

Inspection methods vary depending on the type of valve or steam trap. You need to know the details of your system and recognise the different types of steam traps.

In order to determine leakage or blockage, touch upstream of the valve or trap and reduce the sensitivity of the instrument until the meter reads about 50. Next, touch downstream of the valve or trap and compare intensity levels. If the sound is louder down stream, the fluid is passing through. If the sound level is low, the valve or trap is closed. Always check that the noise is not transferred from another source by checking pipework and attachment points.

All steam systems no matter how well insulated will lose some steam to condensation. The condensation must be removed or else it will puddle in the low points of the system. When enough condensate collects it can be create a slug of water. This water slug will proceed to slam into a bend or connected components at a high velocity. This is called water hammer and is very destructive.

Steam traps are placed periodically through a steam system to remove the condensate from steam. There is no good quick method, other than ultrasonics analysis, for determining whether a steam trap is operating correctly unless it discharges to atmosphere. Condensate is usually collected for reuse.

When a steam trap is sized correctly and is operating correctly, it will discharge and close periodically. If the trap is not discharging at all, then the trap is either isolated or failed closed. Listening to the outlet line can monitor the trap discharge cycle. During discharge, the ultrasonic intensity level at the trap outlet should be higher than the inlet. However, even if the trap is heard to be discharging periodically, the outlet noise should be characterized to insure that it is indicative of turbulent flow noise, and not the rushing sound of steam. This may require some experience to assess.

Another obvious test is to measure the inlet and outlet temperatures and compare the results with some obvious conditions. For instance, if the temperature of the outlet line is substantially above 212° F (100° C), then there is a high probability that the trap is allowing steam to blow through and it may have failed open. If the inlet pipe has cooled to ambient temperature, then there is probably little if any flow through the trap. This may be caused by the trap failing in the closed state, or by some other problem, such as an upstream valve being closed when it shouldn't be.

There should be a temperature difference between the inlet and the outlet lines of the trap, since when operating correctly, a mixture of steam and condensate enters the trap and only condensate leaves the trap. Many energy conservation programs start with a steam trap survey because leaking traps can raise a company's overhead operating expenses by as much as a third. Energy audits and repairs often save companies hundreds of thousands of dollars.

Experts estimate that in a plant with no active steam trap testing and repair program, 50% of the traps are blowing steam. With regular inspection and prompt repair, this figure can be reduced to under 3%. Example: One trap with a 3/32" orifice operating at 100 psi will cause a loss of over $2,000 a year.

Generally speaking, there are two types of steam traps: intermittent and continuous flow. Intermittent traps normally operate in a cycle of open-close-open-close. Continuous-flow traps usually modulate according to condensate load and a failure most often occurs in the closed position. Continuous flow traps include float, float & thermostatic and thermostatic (bellows). A failure usually occurs in the open position causing a constant rushing sound. Each trap in this category has its own particular method of operation and pattern of open-close. Other intermittent traps include inverted bucket, bucket, thermodynamic (disk), bi-metallic and, at times, thermostatic. Change from the typical operating sound signifies trap failure.

Monitoring electrical defects

Arcing, Tracking, & Corona Discharge – These electrical defects produce ultrasonic noise at the site of emission. The signal is heard as a frying or buzzing sound in the headset. As with pressure or vacuum leak detection, the closer the instrument is to the discharge, the more intense the signal. Test: switchgear, transformers, circuit breakers, buss bars, relays, junction boxes, insulators, and other electrical gear. Before beginning any inspection of mid or high voltage equipment, review your plant safety procedures.

When it is not possible to get close to the equipment it may be desirable to use an Ultrasonic Noise Concentrator. This highly sensitive parabolic dish can double the detection distance of the instrument and provides pinpoint accuracy.

Traditional use of infrared cameras reveals "hot spots" that the naked eye otherwise misses. Corona, arcing, and tracking do not always generate significant increases in temperature. Ambient high temperatures can also mask them from the camera. They do however generate distinct noises in the ultrasonic range and are detectable using ultrasonic listening equipment. Ultrasonic detection can significantly assist Thermal Imaging in electrical equipment inspection, especially when some cabinet door can’t be opened.

As with generic leak detection, the area of inspection is scanned using a high sensitivity level. To help discriminate direction, reduce the sensitivity. If it is not possible to remove covers, or plates, scan around the seams and vent slots. Any potentially damaging discharges should be detected. Searching for an electrical defect is similar to searching for a gas leak, in that with the airborne sensor attached, pan the gun in the direction of electrical equipment and listen for electrical defect sounds like popping, buzzing, or crackling. Then, move toward where the sound is the loudest. A parabolic reflectors will help with location. Electrical equipment is normally silent, although some transformers may produce a constant 50 cycle hum or steady mechanical noises. These should not be confused with the erratic, sizzling, uneven, and popping sounds of an electrical discharge.

There are times when it may be difficult to access electrical equipment with a Thermal Imaging instrument, especially if the equipment is enclosed or TI camera is not available. There are situations such as for high-voltage where for safety reasons you can’t get too close. In such cases, you can use an ultrasonic detector with a parabolic microphone or parabolic attachment. These devices have a narrow field of view (10° or 5°) and can detect defects at more than double the distance of standard scanner microphones.

There are some situations where a contact ultrasonic probe can be used on electrical equipment. One application is checking of accessible insulated high voltage joints. This is routinely carried out in some underground mining applications.

Internal Leakage Diagnostics/ Defect identification

An example of Leakage Diagnostics is determining if a closed valve is leaking. Below is a suggested approach.

  • Understanding the process. What are the fluid characteristics such as pressure, flow direction, temperature, viscosity, compressibility, cost, hazard etc.
  • Understanding the equipment. Where on the component is the pressure drop occurring (across the valve plate for a gate valve) ? Where on the component would give the best ultrasonic transmission downstream from the pressure drop location (around the valve body in area closest to the valve plate)?
  • Measure ultrasonic noise level and listen to the sound pattern on the valve body. Compare this to the level away from the valve body. A higher level on the body indicates a leak. Find maximum noise location & confirm it is not stem leakage.
  • Test for external interference. Measure and listen on at least two points on each possible external transmission path. If outer points are greater than inner points, this indicates possible interference.
  • Try to eliminate airborne interference to the valve body.
  • If a leak is suspected, move the valve handle to see if leak changes.
  • Open valve slightly to confirm leakage noise pattern (if allowable).
  • If unsure look for ways to verify the leakage
  • Determine the defect type, its severity and any repair recommendations

Figure 97 – Valve Leakage Testing Procedure

Severity needs to be assessed from the prognosis of the defect (how quickly will it deteriorate) and the possible effects of the problem (Safety, Environmental and economic consequences). If possible determine root cause of leakage and how to eliminate any future reoccurrence

Airborne Measurement

Error Sources and their Effects - (for leakage also read electrical noise).

  • Measuring items that are unpressurised (eg isolated pipeline) wastes effort.
  • Other sources of ultrasonic noise interfering with or overpowering leakage noise. Potentially missing some leaks
  • Not getting close enough to the noise source to detect the defect. Missing leaks
  • Not being rigorous enough on component or area leak scanning. Missing leaks
  • Not selecting the correct sensitivity range. This could result in inadequate audible headphone levels and missed leaks
  • Not recognising a specific defect sound pattern. Can miss defects that are not identified by magnitude but by pattern eg. Electrical Corona.
  • Incorrect instrument settings (Filter, Mode etc. - instrument specific). Gives inadequate instrument performance and missed leaks.
  • Not identifying the exact leak location on the component. Potentially resulting in the wrong repair task being carried out and waisted effort.
  • Not identifying the leak location well enough to be found by repairer. Resulting in waisted effort and missed leak reduction opportunity
  • Not recognising a potential leak source plant location. Gives missed opportunity
  • Not identifying the correct defect type, defect severity or maintenance action.

Identify and Correct Errors or Reject Bad Data

  • Measuring items which are unpressurised (or switched off electrically)
  • Where possible verify directly eg pressure gauges, opening valve to atmosphere etc.
  • Where not possible get operator input, check valve positioning etc.
  • Other sources of ultrasonic noise
  • Identifying sources of other ultrasonic noise in the area or at least their direction
  • Identify the noise level of these sources and determine how much interference they will be compared to the expected level of the leakage source.
  • If practical, switch off the other noise source during monitoring
  • Identify the angle the instrument is held that minimises interference and use this angle where practical for the measurement location
  • Shield the Scanning Module with a rubber attachment (Ultraprobe). Two types available. One for close work identifying exact locations & one for distance work.
  • Shield the Scanning Module with a part of your body.
  • Shield the Scanning Module with some sort of mechanical barrier eg. a welding screen
  • If other techniques are not adequate try tuning the instrument filter to try to minimise the interference

Not getting close enough

The practical measurement distance from a possible leak source is determined by the likely pressure drop at the leak area, the fluid type (gasses transmit better), the size of the leak, the amount of background ultrasonic noise and the likelihood of obstructions (eg access to only one side of pipe or component). The closer the better as long as inspection efficiency or safety is not affected.

Not being rigorous enough

Pipework and pressurised systems can be complex so some care may be required to ensure all areas in a location are covered. If leakage pressure differential being measured is small, mobile platforms (or other) may have to be organised to get close enough to the areas of interest.

Not selecting the correct sensitivity range

Leak detection primarily uses the headphones to determine maximum amplitudes. Different instruments will have different adjustments. Sensitivity must be adjusted consistently during the inspection to ensure the right range is being used. Overall area scanning will require higher sensitivity than close-up leak position identification.

Not recognising a specific defect sound pattern

Specific equipment defects give specific sound patterns, which must be either remembered or identified by comparing to reference components. Review training videos included in this course to learn sound patterns.

Incorrect instrument settings

This is specific to each instrument, so you need to fully understand what each adjustment does and how they should be set. Before you start using an instrument you must check all settings If you make special adjustments during inspections (eg. Changing filter settings) you must reset them to standard immediately after. For general leak testing an Ultraprobe 2000 the filter should be set on ‘Fixed Band’ and the Mode Switch set on ‘Log’.

Not identifying the exact leak location on the component

You need to identify the location of a leak to determine the repair required & to ensure the right item is repaired. The closer potential leak locations are the more accuracy is required. Use a rubber focus probe or similar for smaller leaks. If you can’t get close to the item, try using a plastic pipe extension to measure directly from the leak area. If not practical or safe use a distance focusing attachment.

Not identifying the leak location well enough to be found

The repairer must be able to find the leak you have identified. Mark exact location with felt tip pen or marker paint. Tags can also be used. Take at least two digital photos, one close-up showing the exact defect location and one standing back to locate the defect to obvious land marks (see reporting). If the item is in a location where there are no obvious land marks (eg. Tunnel), mark location with a large painted arrow by spray can, if this is allowed.

Not recognising a potential leak source plant location

Enough information about the pressure systems being investigated is required to identify all areas that require a leakage survey. Schematic diagrams are the ideal.

Not identifying the correct defect type, severity or maintenance action

Leaks are generally easy to diagnose if the component can be viewed close-up. The severity can be judged by visual and audible sound observation and by the leakage rate charts given previously.

If the exact location of the leak is not known, specify the repairer to check with soapy water before the system is isolated for repair. Find out the difficulty in isolating the system to understand if online repairs should be considered. Where a possible on-line repair is simple and safe, do-it-yourself. If unsuccessful specify the off-line repair.

Ultrasonic Accessories

Contact Measurement - The Ultraprobe has standard probe extensions that can help reach difficult spots. Custom probe extensions can be made up to reach more inaccessible locations. Great care should be used with extension probes around moving equipment. A plastic tube can be placed around probes to limit external interference. Magnetic attachment ultrasonic transducers are also available (see below)

Non-Contact Measurement - Standard covers are available for ultrasonic microphones that focus down to a small area or increase range of detection (see below & in electrical section). Long extensions can be made of plastic or cardboard tubes to measure close-up from a distance eg. Overhead pipework.

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