Overview: Ultrasonic Detection
By Gary Mohr, UE Systems
Some of the most common plant applications are: leak
detection in pressure and vacuum systems (i.e., boilers, heat exchangers, condensers,
chillers, distillation columns, vacuum furnaces, specialty gas systems), bearing
inspection, steam trap inspection, valve blow-by, pump cavitation, detection of corona in
switch gear, compressor valve analysis, and the integrity of seals and gaskets in tanks,
pipe systems and large walk-in boxes.
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, and a short wave signal tends to be fairly
directional. It is therefore easy to isolate these signals from background noises
and detect their exact location. In addition, as subtle changes begin to occur in
mechanical equipment, the nature of 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: qualitatively, due to
the ability to "hear" ultrasounds through a noise isolating headphone, and
quantitatively, via incremental readings on a meter. This is accomplished in most
ultrasonic translators by an electronic process called "heterodyning", which
accurately converts the ultrasounds sensed by the instrument into the audible range where
users can hear and recognize them through headphones.
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 various equipment. That is precisely what makes these instruments so useful; they
allow inspectors to confirm a diagnosis on the spot by being able to discriminate among
various equipment sounds.
The reason users can accurately pinpoint the location of a
particular ultrasonic signal in a machine or from a leak is due to its high frequency
short wave. Most of the sounds sensed by humans range between 20 Hz and 20 kHz (20
cycles per second to 20,000 cycles per second). They tend to be relatively gross
when compared with the sound waves sensed by ultrasonic translators. Low frequency
sounds in the audible range are approximately 1.9 cm. to 17 meters in length, whereas
ultrasounds sensed by ultrasonic translators are only 0.3 - 1.6 cm long. Since
ultrasound wavelengths are magnitudes smaller, the ultrasonic environment is much more
conducive to locating and isolating the source of problems in loud plant environments.
Airborne ultrasound translators
are relatively simple to use. They consist of a basic hand held unit with
headphones, a meter, a sensitivity adjustment, and (most often) interchangeable modules
that are used in either a scanning mode or a contact mode (Figure 1). Some
instruments have the ability to adjust the frequency response from between 20 to 100 kHz.
An ultrasonic transmitter called a tone generator is often included.
Many of these features are useful in helping a user adapt to
a specific test situation. For example, if an ultrasound source is too difficult to locate
due to an intense signal, a user can focus on the exact site by adjusting the sensitivity
downward. In another instance, if a low-level leak occurs in a water valve, the frequency
tuning can be adjusted to help a user hear the trickle of the water leak.
The interchangeable modules allow users to adjust for
different types of inspection problems. The scanning mode is used to detect
ultrasounds that travel in the atmosphere such as a pressure leak or a corona discharge,
while the contact mode is used to detect ultrasounds generated within a casing such as in
a bearing, pump, valve or steam trap housing.
Leak Detection -
This category covers a wide area
of plant operations. It can be viewed as a way of keeping a system running more
efficiently. Some plants include it as part of an energy conservation program, while
others refer to it as fugitive emissions. Regardless, leaks cost money, affect product
quality and can reek havoc with the environment. Ultrasonic detection can often
locate the problem, whether the leakage occurred in a liquid or a gas system.
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 through the leak site to the low-pressure side, where it expands
rapidly and produces a turbulent flow (Figure 2a). This turbulence has strong
ultrasonic components. The intensity of the ultrasonic signal falls off rapidly from
the source, allowing the exact spot of a leak to be located.
Generalized gas leak detection is also very easy. An area
should be scanned while listening for a distinct rushing sound. With continued sensitivity
adjustments, 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. This is
done 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.
Vacuum leaks may be located in the same manner; the only
difference being that the turbulence will occur within the vacuum chamber (Figure 2b). For
this reason, the intensity of the sound will be less than that of a pressurized
leak. Though it is most effective with low-mid to gross leaks, the ease of
ultrasound detection makes it useful for most vacuum leak problems.
Liquid leaks are usually determined through valves and steam
traps, although some successes have been reported in locating water leaks from pressurized
pipes buried underground. A product can be checked for leakage if it produces some
turbulence as it leaks.
Valves are usually checked for leakage with the contact probe
on the downstream side (Figure 3). This is accomplished by first touching the upstream
side and adjusting the sensitivity to read about 50% of scale. The downstream side is then
touched and the sound intensity is compared. If the signal is lower than upstream,
the valve is considered closed; if it is louder than upstream and is accompanied by a
typical rushing sound, it is considered to be leaking.
Steam traps are also inspected easily with ultrasonic
translators. During examination, the steam trap is touched with the contact
probe. By listening to the trap operation and observing the meter, trap condition
can be interpreted. The speed and simplicity of this type of test allow every trap
in a plant to be routinely inspected.
Leaking tubes in heat exchangers and condensers as well as
boiler casing leaks are detectable with ultrasonic translators. In most power
plants, the problem of condenser in-leakage is a major concern. Condenser fittings are
often routinely inspected utilizing the leak detection method previously described.
If a leak is suspected in a condenser tube bundle, it is possible to locate the leak by
putting a condenser at partial load and opening up a water box of a suspected tube bundle.
After the tube sheet is cleared of debris, the tube sheet is scanned.
How to Locate Leaks
Select the Log setting on the meter selection dial. Use "fixed
band" position on the Frequency selection dial. If too much background noise is
present, try some of the shielding methods. Start off with the sensitivity selection a 10
(maximum). Begin to scan by pointing the module towards the test area. The
procedure is to go from the "gross" to the "fine"... with more subtle
adjustments made as the leak is approached.
If there is too much ultrasound in the area, reduce the
sensitivity setting and continue to scan. If it is difficult to isolate the leak due to
competing ultrasound, place the rubber-focusing probe over the scanning module and scan
the test area. Listen for a rushing sound while observing the meter. Follow
the sound to the loudest point. The meter will show a higher reading as the leak is
approached. In order to focus in on the leak, keep reducing the sensitivity setting and
move the instrument closer to the suspected leak site until you are able to confirm a
To confirm a leak, position the rubber focusing probe (if it
is on the scanning module) close to the suspect leak site and move it slightly back and
forth in all directions. If the leak is at this location, the sound will increase
and decrease in intensity as you sweep over it. In some instances, it is useful to
position the rubber focusing probe directly over the suspect leak site and push down to
seal it from surrounding sounds. If it is the leak, the rushing sound will
continue. If it is not the leak site, the sound will drop off.
Overcoming Competing Ultrasounds
If competing ultrasounds make it
difficult to isolate a leak, there are two options:
manipulate the environment, i.e., when possible, turn off the
equipment that is producing the competing ultrasound or isolate the area by closing a door
manipulate the instrument and use shielding techniques.
If environmental manipulation is not possible, try to get as
close to the test site as possible, and manipulate the instrument so that it is pointing
away from the competing ultrasound, and isolate the leak area by reducing the sensitivity
of the unit and by pushing the tip of the focusing probe up to the test area, checking a
small section at a time.
In some extreme instances, when the leak check is difficult
in the fixed band mode of the frequency selection dial, try to tune in to the leak sound
by tuning out the problem sound. In this instance, adjust the frequency selection
dial until the background sound is minimized and then proceed to listen for the leak.
Since ultrasound is a high frequency, short wave signal, it
usually be blocked or shielded. Note: when using any method, be sure to follow your
plant or company safety guidelines. Some common techniques are:
1. Place your body between the test area and the
competing sounds to act as a barrier.
2. Position a clipboard close to the leak area
and angle it so that it acts as a barrier between the test area and the competing sounds.
3. Using a gloved hand, wrap the hand around the
rubber focusing probe tip so that the index finger and the thumb are close to the very end
and place the rest of the hand on the test site so that there is a complete barrier of the
hand between the test area and the background noise. Move the hand and instrument
together over the various test zones.
4. In addition to a glove, use a wipe rag to wrap
around the rubber focusing probe tip (be sure not to block the open end of the tip).
This is usually the most effective method since it uses three barriers; the rubber
focusing probe, the gloved hand, and the rag.
5. When covering a large area, it is sometimes
helpful to use some reflective material, such as a welders curtain or a drop cloth, to act
as a barrier. Place the material so that it acts as a wall between the test area
and the competing sounds. Sometimes the barrier is draped from ceiling to floor; at
other times it is hung over the railings.
In ultrasonic inspection of leakage, the amplitude of sound often depends
upon the amount of turbulence generated at the leak site. The greater the
turbulence, the louder the signal; the less the turbulence, the lower the intensity of the
signal. When a leak rate is so low that it produces little, if any, turbulence that
is detectable, it is considered "below threshold". If a leak appears to be
of this nature build up the pressure (if possible) to create greater turbulence, or
utilize a liquid leak amplifier.
The Liquid Leak Amplifier is a specially formulated
liquid that produces a thin film through which the escaping gas will pass (Figure
4). When it comes in contact with a low flow of gas, it quickly forms a large number
of small soda-Re bubbles that burst as soon as they form. This bursting effect
produces an ultrasonic shock wave that is heard as a crackling sound in the headphones. In
many instances the bubbles will not be seen, but they will be heard. This method is
capable of obtaining successful leak checks in systems with leaks as low as 106 standard
If there are situations where a signal may be difficult to
isolate, it may be helpful to utilize the Frequency Tuning Dial. Point the system
toward the test area and gradually adjust the frequency tune dial until the weak signal
appears to be clearer and then follow the basic detection methods previously outlined.
Heat exchangers may be tested in a similar fashion. The
header is removed and the shell side is either placed under vacuum or is
pressurized. Mere will be some instances where it is difficult or too time consuming
to inspect under pressure or vacuum. In this case, a test unique to ultrasound is
This method uses an ultrasonic transmitter called a
"tone generator". The tone generators are placed in the various access
ports or fittings to produce an intense, uniform ultrasound within the shell side.
Since the sound waves are high frequency, they will tend to deflect off the surface of
solid, intact tubes, but will penetrate through the leak site of a tube. By scanning the
tube sheet, an operator listens for a distinct high frequency signal indicating the
leaking tube (Figure 5).
The preferred method is pressure or vacuum, but the tone
generator method is a good backup for difficult situations.
There are three basic electrical
problems that can be detected:
1. Arcing: An arc occurs when electricity flows
through space. Lightning is a good example.
2. Corona: When voltage on an electrical
conductor, such as an antenna or high voltage transmission line exceeds threshold value,
the air around it begins to ionize to form a blue or purple glow.
3. Tracking: Often referred to as
"baby arcing", follows the path of damaged insulation.
Although theoretically ultrasonic detection can be used in
low, medium, and high voltage systems, most of the applications tend to be in medium and
high voltage systems.
When electricity escapes in high voltage lines or when it
jumps across a gap in an electrical connection, it disturbs the air molecules around it
and generates ultrasound. Often this sound will be perceived as a crackling or
frying sound; in other situations it will be heard as a buzzing sound. Applications
include: insulators, cable, switchgear, buss bars, relays, contactors, junction boxes. In
substations, components such as insulators, transformers and bushings may be tested.
Ultrasonic testing is often used for evaluation at voltages
exceeding 2,000 volts, especially in enclosed switchgear. This is especially useful
in identifying tracking problems. In enclosed switchgear, the frequency of tracking
greatly exceeds the frequency of serious faults identified by infrared. It is
recommended that both tests be used with enclosed switchgear. Note: When testing
electric equipment, follow all your plant or company safety procedures. When in
doubt, ask your supervisor. Never touch live electrical apparatus with the system.
The method for detecting electric arc and corona
leakage is similar to the procedure outlined in leak detection. Instead of
listening for a rushing sound, a user will listen for a crackling or buzzing sound.
In some instances, as in trying to locate the source of radio/TV interference or in
substations, the general area of disturbance may be located with a gross detector such as
a transistor radio or a wide-band interference locator. Once the general area has
been located, the scanning module is utilized with a general scan of the area. The
sensitivity is reduced if the signal is too strong to follow on the meter until the
loudest point is located.
Determining whether a problem exists or not is relatively
simple. By comparing sound quality and sound levels among similar equipment, the
problem sound will tend to be quite different.
On lower voltage systems, a quick scan of bus bars often will
pick up a loose connection. Checking junction boxes can reveal arcing. As with
leak detection, the closer one gets to the leak site, the louder the signal. If
power lines are to be inspected and the signal does not appear to be intense enough to be
detectable from the ground, you can use an ultrasonic waveform concentrator (a parabolic
reflector), which will double the detection distance of the system and provide pinpoint
Ultrasonic inspection and
monitoring of bearings ia a reliable method for detecting incipient bearing failure.
The ultrasonic warning appears prior to a rise in temperature or an increase in driving
torque. Ultrasonic inspection of bearings is useful in recognizing the beginning of
fatigue failure, brinelling of bearing surfaces, flooding of or lack or lubricant.
In ball bearings; as the metal in the raceway, roller, or
bearing balls begins to fatigue, a subtle deformation begins to occur. This
deforming of the metal will produce an increase in the emission of ultrasonic sound
waves. When testing, changes in amplitude of from 12 to 50 times the original
reading is indication of incipient bearing failure. When a reading exceeds any previous
reading by 12 dB, it can be assumed that the bearing has entered the beginning of the
This information was originally discovered through
experimentation performed by NASA on ball bearings. In tests performed while monitoring
bearings at frequencies ranging from 24 through 50 kHz, the changes in amplitude indicated
the onset of, or incipient, bearing failure before other indicators; including heat and
vibration changes. (An ultrasonic system based on detection and analysis of modulations of
bearing resonance frequencies can provide subtle detection capability, whereas
conventional methods have difficulty detecting very slight faults.) As a ball passes
over a pit or fault in the race surface, it produces an impact. A structural resonance of
one of the bearing components vibrates or rings by this repetitive impact. The sound
produced is observed as an increase in amplitude in the monitored ultrasonic frequencies
of the bearing.
Brinelling of bearing surfaces will produce a similar
increase in amplitude due to the flattening process as the balls get out of round.
These flat spots also produce a repetitive ringing that is detected as an increase in
amplitude of monitored frequencies.
The ultrasonic frequencies detected by the system are
reproduced as audible sounds. This signal can greatly assist a user in determining
bearing problems. When listening, it is recommended that a user become familiar with
the sounds of a good bearing; often heard as a rushing or hissing noise. Crackling
or rough sounds indicate a bearing in the failure stage. In certain cases a damaged
ball can be heard as a clicking sound, whereas a high intensity, uniform rough sound may
indicate a damaged race or uniform ball damage. Loud rushing sounds similar to the
rushing sound of a good bearing only slightly rougher can indicate lack or lubrication.
There are two basic procedures of testing for bearing
problems: comparative and historical. The comparative method involves testing two or more
similar bearings and comparing potential differences. Historical testing requires
monitoring a specific bearing over a period of time to establish its history. By analyzing
bearing history, wear patterns at particular ultrasonic frequencies become obvious which
allows for early detection and correction of bearing problems.
- Use the
contact (stethoscope) module. Select LIN on the meter selection dial. Select the
desired frequency on the frequency selection dial. Select a test spot on the bearing
housing and mark it for future reference. Touch that spot with the contact
module. In ultrasonic sensing, the more mediums or materials ultrasound has to
travel through, the less accurate the reading will be. Therefore, be sure the
contact probe is actually touching the bearing housing. If this is difficult, touch
a grease fitting or touch as close to the bearing as possible. For consistency,
always approach the test spot at the same angle. Reduce sensitivity until the meter reads
20. Listen to the bearing sound through headphones to hear the quality of the signal
for proper interpretation. Select same type bearings under similar load conditions
and same rotational speed. Approach the bearings at the same angle, touching
approximately the same area on the bearing housing. Compare differences of meter
reading and sound quality.
Historical Bearing Test
- There are
two methods to historically trend a bearing. The first is a very common, field
proven method called the "simple" method. The other provides greater
flexibility in terms of decibel selection and trending analysis, and is referred to as the
"attenuator transfer curve" method. The attenuator transfer curve method
is used in the Bearing Trac software which provides trending, graphs, and historical
analysis. Before starting with either of the two historical methods for monitoring
bearings, the comparative method must be used to determine a baseline.
- Use the basic
procedure as outlined above in the comparative test. Note frequency, meter reading, and
sensitivity selection on your bearing history chart. Compare this reading with
previous or future readings. On all future readings, adjust frequency and
sensitivity level to the original level recorded in the bearing history chart. If
the meter reading has moved form the original 20 mark up to or past 100, there has been a
12 db increase. (Increments of 20 on the meter in the linear mode is about 3
decibels; e.g., 20-40=3db, 40-60=3db, etc.) Note: Increase of 12 db or greater
indicates the bearing has entered a failure mode. Lack of lubrication is usually
indicated by an 8 db increase over baseline. It is usually heard as a loud rushing
sound. If lack of lubrication is suspected, after lubricating, re-test. If
readings do not go back to original levels and remain high, consider that the bearing is
on the way to the failure mode and recheck frequently.
If a vibration program already exists for bearing analysis,
an ultrasonic bearing monitoring program can be of assistance. Ultrasound translators can
be used to aid a diagnosis. The high frequency, short wave characteristic of ultrasound
allows the signal to be isolated, so that a user can determine if a bearing has been
correctly diagnosed as failing.
At times there can be false signals generated by
equipment connected to a particular bearing. By adjusting the sensitivity, the
frequency, and listening to the sound, it can be determined whether it is the bearing, a
rotor or something else that is the root of the problem. The ability to hear what is
going on can prove very important. 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.
Sometimes there are so many bearings in a plant that not
every piece of equipment can be checked routinely by a limited staff of trained
technicians. Since ultrasound detectors require little training, a technician or the
machine operator can determine potential bearing problems and notify the vibration
technician for follow-up.
Lack of Lubrication
- To avoid lack
of lubrication, note the following: as the lubricant film reduces, the sound level
will increase. A rise of about 8 db over baseline accompanied by a uniform rushing
sound will indicate lack of lubrication. When lubricating, add just enough to return
the reading to baseline. Use caution. Some lubricants will need time to run to
uniformly cover the bearing surfaces. Lubricate a little at a time. Do not
Over-lubrication - One of the
largest causes of bearing failure is over-lubrication. The excess stress of lubricant
often breaks bearing seals or causes a buildup of heat which can create stress and
To avoid over-lubrication don't lubricate if the baseline
reading and baseline sound quality is maintained. When lubricating, use just enough
lubricant to bring the ultrasonic reading to baseline. As mentioned above, use
caution. Some lubricants will need time to uniformly cover the bearing surfaces.
Slow Speed Bearings -
Monitoring slow speed bearings is
possible. Due to the sensitivity range and the frequency tuning, it is quite
possible to listen to the acoustic quality of bearings. In extremely slow bearings
(less than 25 rpm), it is often necessary to disregard the meter and listen to the sound
of the bearing. In these extreme situations, the bearings are usually large (1-2
inches and up) and greased with high viscosity lubricant.
Most often, no sound will be heard as the grease will absorb
most of the acoustic energy. If a sound is heard, usually a crackling sound, there
is some indication of deformity occurring. On most other slow speed bearings, it
possible to set a baseline and monitor. It is suggested that the attenuator transfer
curve method be used, since the sensitivity will usually have to be higher than normal.
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. In some instances, manufacturers of ultrasonic translators supply
video cassette training tapes that show exactly how each type of trap can be inspected.
The method is quite simple. A steam trap is touched
with the contact probe. By listening to the trap operation and by observing the
meter, trap condition can be interpreted. The speed and simplicity of this type of
test allow every trap in a plant to be routinely inspected.
It is advisable to have
instruments that are sensitive enough to detect the type of problems you will encounter in
the plant. A wide dynamic range in an instrument will enable you to look for small
leaks on one end and locate gross mechanical problems on the other.
Since sound quality is an important consideration, make sure
the instrument heterodynes the ultrasonic signal. This will insure users that they
are getting an accurate reproduction of the ultrasonic signal, for signal clarity and
interpretation of the headphone sound. Noise attenuating headphones with good sound
quality are essential. If the sound quality is not clear, it will be difficult to
understand what is being sensed. It is advisable to get over-the-ear headphones that
will block out ambient plant sounds during inspections. Without proper RF shielding, stray
electronic signals will interfere with test results. In some instances, radio
programs have been heard, which totally confused operators.
Since every plant is different, there might be special
accessories needed to assist in some situations. For example, compressor valve
analysis might be easier with a magnetically mounted probe and an oscilloscope
interface. If you are going to inspect a variety of equipment or have fluids of
different viscosities, it would be useful to have the ability to change frequencies
(Figure 7). For leak detection of potentially explosive or flammable gases, it is
advisable to use equipment rated intrinsically safe.