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by Steve Johnson
Computational Systems, Inc
Today's cost-sensitive maintenance environment
dictates an effective, simple-to-use, high payback technology where
materials cost and personnel training are concerned. Ultrasonic
monitoring is such a technology. Most plant systems and equipment
generate operational noise of some sort and during failure modes the
noise characteristics can change dramatically. Detecting this change
and fixing the problem before failure can result in higher quality
production output, reduced downtime, reduced maintenance overtime,
and greatly reduced costs.
In its simplest form, noise is a vibration of molecules through a
medium, like air, grease, or metal, which moves spherically outward
from its source. Vibrations can be broken down to energy levels at
discrete frequencies and when the human ear detects these levels and
frequencies, it translates them into intensities and tones which are
then transmitted to the brain. The human ear can detect frequencies
between 20 Hz and 20 kHz. This range is referred to as the audible,
or sonic, range. Frequencies above this range are referred to as ultrasonic.
Ultrasonic instruments measure the ultrasonic frequencies and,
through a process called heterodyning, translate the frequencies
down into the audible range. Furthermore, the instruments will
typically give a type of visual indication (a digital display or an
analog meter) of the noise intensity either as some percentage of
the instrument's output or an actual calibrated sound measurement in
dB.
As previously stated, what makes ultrasonic noise monitoring so
useful, is the large number of systems and equipment which emit
ultrasonic noise when problems develop or failures occur. The most
effective industrial applications are detecting leaks in pressurized
or vacuum systems, inspecting steam traps, mechanical analysis
(especially bearings), inspecting valves, and electrical corona or
discharge detection. Due to the nature of high-frequency waves,
ultrasonic noise tends to be very localized and highly directional.
These two aspects tend to make ultrasonic noise detection and
analysis simple enough that, with little training, maintenance
personnel can begin making big contributions to the bottom line. One
final note: Always follow plant safety procedures when around
operating equipment, pressure and steam lines, or electrical
equipment.
Leaks in pressurized systems
By far, the most common usage for
ultrasonic noise detection is to locate leaks in pressurized gas or
vacuum systems. When a gas, such as air, escapes from high pressure
to low pressure through an orifice, the flow becomes turbulent,
generating ultrasonic noise. The noise is characteristically
identified as a 'rushing' or 'roaring' sound with few discernible
frequencies. General guidelines for finding leaks are as follows:
first, set the volume level of the gun such that the normal
background noise of the plant is near the bottom of the hearing
threshold. Then pan the gun along the lines of the air/gas system to
be checked. When the typical leak sounds are heard, slowly move the
gun up, down, right, and left in the shape of a cross to determine
which orientation 'sounds' the loudest. With the gun pointing in the
loudest direction, begin moving toward the sound until the leaking
component is found.
A common question being asked is, "Can the material cost of a
leak be determined from the noise level of a leak?" There are
many factors involved in this type of analysis, such as pressure
difference, hole shape, hole size, whether the flow is choked or
not, humidity, temperature, and instrument calibration. While a
general relationship between noise and flow rate can be assumed
based on empirical data, these numbers are mere guidelines at best.
Further research is necessary to determine how the other factors
will influence the calculations.
Steam Trap Analysis
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 will begin to ripple
into waves due to the steam passing rapidly above it, much like a
lake ripples when the wind blows over it. As the rippling continues
the size of the 'waves' will grow until the steam will actually lift
the condensate, forming it into a type of plug, and push it through
the pipe. This water plug will proceed to slam into the next bend,
or worse, component (i.e. turbine, boiler, etc.) at a high velocity.
This is called 'water hammer' and is, obviously, something to be
avoided. Steam traps are placed periodically through the system to
remove the condensate from steam systems. Steam traps are, in
essence, 'smart' valves, whose purpose is to keep the steam in a
system, but let all the condensate, air, and other gasses escape to
return lines for reconditioning. Due to the complexities of steam
systems, there is no quick formula for determining whether a steam
trap is operating correctly or not. While there are some obvious
tests which may be performed, no single method will work in every
situation. Some of these tests are: listening for the trap discharge
cycle, checking the temperature of the inlet and outlet, and
comparing the inlet and outlet temperatures.
When a steam trap is sized correctly and operating correctly, it
will discharge periodically and close periodically. If the trap is
not discharging, then the trap is either failed open or failed
closed. The trap discharge cycle can be monitored by listening to
the outlet line. Generally speaking, 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 experience before reasonable accuracy can be
expected).
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 oF
(100 oC), 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.
Additionally, 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. While each type and size of trap
can be unique and should be monitored to determine its own
temperature characteristics, a small temperature difference between
the two locations is a good indicator that further checking should
be performed.
Mechanical Inspection
One of the main sources of ultrasonic
noise in equipment is friction between two objects moving relative
to each other. This is most commonly seen in bearings. Whether due
to improper lubrication conditions or mechanical defects,
metal-to-metal contact will generate significant amounts of noise.
Traditional vibration analysis has shown that most mechanical
defects (pits, cracks, spalls, etc.) generate noise in the sonic
range. However, ultrasonic guns which have frequency adjustment and
are sensitive enough can detect such defects. To effectively monitor
bearings, first establish a baseline dB level for the bearing when
it is running correctly. Use another technology, such as vibration
analysis, or compare this bearing with other bearings in similar
applications to make sure that the bearing is indeed in a good
state. Next, trend that value through subsequent readings and look
for changes. There are some general guidelines for evaluating
bearing conditions based on increases in the dB level, i.e. for a
6-9 dB increase check lubrication, a 10-12 dB may indicate the
beginnings of a bearing fault.
A common cause of bearing faults is over- or under-lubrication. An
under-lubricated bearing will allow metal-to-metal contact and wear,
while an over-lubricated bearing will heat up and possibly pop the
bearing seals. Under-lubricated bearings will sound like general
noise (little or no impacting) and will generate noise around 25
kHz, though heavy loads on bearings may tend to lower this somewhat.
To effectively lubricate a bearing, a baseline reading at 25kHz
should be established. Then as further checking indicates a dB rise
at 25kHz, monitor the bearing while lubricating. As lubrication is
added, the noise levels should decrease. When the dB levels return
close to the baseline, cease lubricating. Do not over-lubricate a
bearing. One word of caution: It may take some time before
lubrication spreads itself throughout the bearings inner surfaces,
so proceed slowly.
Valve Inspection
Valves are inspected similarly to
steam traps by checking sound intensity levels both upstream and
downstream of the valve. If the sound level downstream of the valve
is greater than upstream of the valve, then the valve is, depending
on the type, probably partially open or maybe completely open.
Sometimes if the sound level downstream of the valve is less than
upstream (or very small) it means that the valve may be closed.
However, there is not always true. Ultrasonic noise is generated
when turbulent flow occurs, such as the back currents which form
when liquid or gas flows through a partially open valve.
Furthermore, the intensity level 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. To put it succinctly, hearing nothing below
the valve does not necessarily mean that the valve is closed. To be
more sure, use a sonic detector downstream of the valve (or maybe on
the valve stem) because laminar flow will generate sonic noise. If
listening with a sonic sensor and an ultrasonic sensor yields no
sound downstream of a valve, then the valve is probably closed.
Electrical Inspections
One final application for ultrasonic
monitoring is the detection of arcing (electricity traveling through
the air), corona (ionization of air around electrical conductors),
or electrical insulation breakdown. Searching for electrical
problems is similar to searching for gas leaks, in that, with the
airborne sensor in, pan the gun around through electrical equipment
and listen for electrical fault sounds like popping, buzzing, or
crackling. Then move toward where the sound is the loudest. For
situations where the source of the noise is out of reach and/or the
exact location cannot be easily determined, most ultrasonic vendors
have parabolic reflectors as accessories. A parabolic reflector will
usually at least double the effective range of a standard airborne
sensor and will greatly enhance the directionality.
Conclusion
Ultrasonic instruments have a wide
range of effective applications and can serve very capably as a
first line 'defense' against breakdowns. While no single technology
can provide total solutions for all the possible maintenance issues
which could arise, coupling ultrasonic monitoring with other
technologies, such as vibration, oil, and thermographic analysis can
greatly reduce equipment failures, reduce personnel overtime, reduce
energy consumption, and improve system and product quality.
Steve Johnson is the Engineering Product Champion for Ultrasonics
and Infrared Thermography at Computational Systems, Incorporated. He
oversees research, current and new development, and customer
applications for both product lines. Previously, at CSI he worked
for four years on the development and support of MasterTrend. He
received a BS and MS in Mechanical Engineering from the University
of Tennessee, Knoxville.
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