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newvibe

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Reply with quote  #1 
Hi guys,

I am looking for a way to measure solid build-up on the inside of exhaust ducts using non-destructive testing?

Recently, we are noticing many of our exhaust ducts have a thick layer of build-up on the inside (accumulate over the years) and it's affecting the flow of the system. The way we are doing to find out which part of the duct has a lot of build-up on it at the moment is to drill a hole, then go in with a borescope to inspect that section of the ductwork. However, we do not want to drill too many holes along the ductwork for this. That is why I am looking at an alternative way.

Please share your experience? Thanks in advance.
Curran919

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Reply with quote  #2 
Never worked with ducts, so don't know if there is an existing standard method, but if I am allowed to be creative:

You could use something like a bump test and ping the walls and either listen to the resulting tone or measure with a miniature accelerometer. As the dust builds up, the mass increases and therefore the tone(s) (natural frequencies) will decrease.

There are also ultrasonic flowmeters that are sensitive to buildup on the inside of a pipe. As part of our portable 'strap-on' flowmeter, we also have a probe that measures the pipe thickness. Maybe this or something similar could work on measuring the dust thickness.
OLi

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Differential pressure over the pipe? Just a U-tube barometer from pvc tubing depending on flow, length etc.
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Danny Harvey

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How about a camera mounted on a robot?

Or you could maybe find an industrial duty Roomba and clean as you go.
MarkL

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Reply with quote  #5 
Ultrasonic thickness gauges for test metal thickness are common we have a customer uses them to check pipe thickness in abrasive mineral transfer pipes.
Maybe something like that? https://www.caulfieldindustrial.com/p/atp-rs-232-ultrasonic-thickness-gauge/p-k10132
fburgos

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I think Ultrasound ndt could be what you looking for, we use it to measure thickness but don't know about buildup
newvibe

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Reply with quote  #7 
Thanks everyone. I was looking into ultrasound thickness gauge and it seems to be very good for measuring pipes thickness. However, as fburgos said, I am also unsure if it can measures build-up. Perhaps the best option is to purchase one and test it out.
fburgos

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Reply with quote  #8 
wouldt buildup restrict area, increasing velocity and generating turbulence, airbone ultrasound?
Walt Strong

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"I am also unsure if it can measures build-up. Perhaps the best option is to purchase one and test it out."

I would not purchase thickness gage with first testing one. I doubt that it would work. It would indicate thickness of duct steel wall, but it unlikely to give indication of fouling thickness; that has different acoustic properties. You could contact one or more venders such as Panametrics to see if it might work and to get a demonstration, loaner, or short term rental.

You did not provide duct dimensions, fouling material composition and thickness.

Walt
OLi

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Reply with quote  #10 
Just vibrate with a vibro motor like a storage pocket?
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Dave Reynolds

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Reply with quote  #11 
Is duct / piping insulated, how large is duct, what is material of construction, how thick is metal on duct/piping

Depending on temperature, you could use thermography. Clean an area of duct on the inside, paint black target in same clean area on the outside of duct and use it as a reference to compare to suspected dirty areas. Assumption is the dirt in duct would either show warm or cold, the bigger the temperature difference between outside air temp and product air flow temp gives you best chance of success.

Process monitoring using air flow or pressure to detect buildup is occurring inside of duct

PM to inspect every X days/weeks/months, use bore scope to look inside of duct

http://lixi.com/wp-content/uploads/2019/09/LIXI_ImageScope.pdf
http://lixi.com/profiler-indications/

Possible solution https://videray.com/


Dave

newvibe

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Reply with quote  #12 
Thanks everyone for your input.

The duct sizes range from 24in up to 48in, 1/2in up to 3/4in thick. The duct material is fibreglass and the build-up is mostly chemical residue (crystalized). The thickness of the build-up layer can be up to 3in.
Sinski

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Reply with quote  #13 
As Dave says thermography can work well. I had an old boss who used this effectively on pulp stock pipes. You could see the build up of dried up stock attached to a pipe wall as cold spots.
RustyCas

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Reply with quote  #14 
I vote for thermography as well.  It's ridiculously cheap, especially when you don't need accurate temperatures - $200-$400 for iPhone units, and $400-$500 for dedicated units from Flir.  It's crazy.
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Walt Strong

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Reply with quote  #15 
"I had an old boss who used this effectively on pulp stock pipes. You could see the build up of dried up stock attached to a pipe wall as cold spots."

I assume the pulp pipe was metal and the fouling pulp stock on the inside wall was a good thermal insulator compared to the metal pipe. I believe the OP situation is not the same, so passive thermography may not work well. The fiberglass duct wall and the crystalized chemical residue inside may not have a significantly different heat transfer (insulation resistance). If thermography does work, then it would be a good screening tool to locate suspect areas of the duct with fouling. I would not buy a camera for this method without first trying it.

I have done some fiberglass flaw detection testing using ultrasound flaw analyzer (not a thickness gage). It was difficult to detect internal flaws, because either the transmitted sound would blast through the material (performance car seat frame), or the reflective wave was too weak. I suspect it would be difficult to detect the primary sound wave from the fiberglass thickness and the secondary wave from the crystalized chemical residue. I would not buy a UT flaw analyzer without first trying it.

I would try the acoustic sounding method that is based on applying and impact and measuring the sound (microphone) or vibration (accelerometer) response. The presence fouling on the inner duct wall could be expected to change the mass and possibly the stiffness of the fiberglass duct wall. The test method may be done with system in service, if the background sound/vibrations are low enough. 

Walt
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