IR for steam leaks

[micjk]

yesterday I was walking by one of our flash tanks, 450# cond to 90#steam, and heard a hissing sound. I couldn't really tell if it was a leak at the sight glass, which is common at our facility, or coming from somewhere else. I scanned the sight glass with our e-4 camera and saw nothing. Instinct told me there was a leak there somewhere so I took a piece of plastic laminate sheet and lo and behold found the leak . I then scanned the laminate and could see it with the camera. My question is; why could I not see the steam leak with the camera? Is it possibly due to the leak being minute or ambient conditions around the leak or both.

Roy Gariepy
Maintenance and Reliability Dept.
Dorlastan Fibers LLC.
Goose Creek, South Carolina USA

 

[electricpete]

I don't know the full answer but it's an interesting question. I have some rambling thoughts.

As you know, often you can't visually see a high-pressure steam leak until it goes far enough that portions of the steam vapor condense and form water droplets. I suspect it may be the same with infrared, even though seeing water droplets visually (based on their reflection of light) is much different than seeing with infrared (based on thermal radiation).

One thing that makes me think this way is looking at the blue sky - it is off-scale low (-40F) as if we are looking into outer space. But we are looking through air which of course is much higher temperature let's say 70F. For whatever reason, the air molecules don't seem to emit their radiation. But they do transmit (and to a lesser extent reflect, particularl if humid). I suspect maybe steam is the same way.

So now I have to ponder the question: what is the emissivity of air? Or the emissivity of steam? Emissivity is a surface property but air and steam don't really have a surface. Hmmm.

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[electricpete]

Maybe the relatively low density of steam and air (compared to solids and liquids) has something to do with it.

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[IRstuff]

Emissivity, transmission and reflectivity must all sum to 1. Therefore, if you have anything that is transmissive, it MUST be low emissivity.

In the 8-12 um band, transmission over short distances exceeds 90%, particularly in otherwise low humidity environments. Therefore, under those conditions, emissivity is less than 0.1

TTFN

 

[micjk]

Thanks for the replies,
So what I gather from the statements above, due to the highly transmissive properties of air, IR would not be a good resource for locating steam leaks, at the source, but maybe farther out as the steam begins to condense. Somewhat makes sense to me. I'll have to do a lttle experimenting.

Roy Gariepy
Maintenance and Reliability Dept.
Dorlastan Fibers LLC.
Goose Creek, South Carolina USA

 

[JKEngineer]

The other way to find leaks with IR is to look at the surfaces around the leak. If they are impinged by the leak, they will have their temperature affected by the leak. Therefore a hot steam leak will show a local hot spot at the leak point. Similarly an air in-leak at a turbine condenser will show a local cool spot. This is not totally reliable, since surfaces are not always impinged. For example a pinhole in a pipe wall, as opposed to a flange leak, will shoot straight out and barely impinge the surface, if at all.
Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[electricpete]

I have a proposed explanation. Right or wrong I'm not sure. This is more of a question than a statement.

EM radiation we are familiar with is caused by an accelerating charge. Electron dropping energy levels causes an X-ray. Or an ac current in a conductor (antenna) causes waves (radio waves if the current is at radio frequency).

Now try to fit that model onto thermal radiation. I picture that acceleration of molecules (together with their component atoms and charges) causes the em radiation.

If you have a solid all the molecules are bound into the structure and not exactly free to move. Sort of like they are bound into a crstalline structure by tine springs. Their kinetic energy must be in the form of oscilation like a mass spring system. Oscillation is acceleration and em radiation is emittted.

Now look at a gas. For the most part the molecules travel long distance at constant velocity, zero acceleration. They change velocity only when they hit another molecule. Then it is an abrupt change in velocity, not an oscillating acceleration. So no em radiation emitted.

Liquid would fall somewhere between the two. We know from our experience that water does show it's temperature on an IR camera... so for whatever reason, maybe liquids acts more like the solid model above.

Makes sense or way off base?

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[eyec]

sorry to be so basic, but maybe the temp difference between the steam and the attendant flash tank (near source) were not significantly different and what you may have seen was a cooler image area further away from the tank as the steam condensed, if you looked.

the film retained the thermal image in a static condition on the film, the steam was in a transient state.

 

[IRstuff]

"...heard a hissing sound." implies a continuous leak

TTFN

 

[cameraman1]

I think the problem is that a camera can ONLY see REFLECTED light, NOT light passing throug a transparent medium. Without the presence of a solid or liquid particle there is NO reflectance therefore there is no image to "see". Maybe thats why you can't see vampires in a mirror? Hmmm.
cameraman1

 

[IRstuff]

Sorry, not true. Otherwise, you wouldn't see anything through the air (a transparent medium). Note that "transparent," by definition, means that you're seeing something coming through the medium.

Moreover, for objects with emissivity greater than 0, you ARE seeing emission from the object, otherwise, the thermography industry is screwed.

TTFN

 

[cameraman1]

Hope I'm replying correctly to "irstuff":

http://www.eng-tips.com/userinfo.cfm?member=IRstuff

I am not sure of your correction. You said: "Otherwise, you wouldn't see anything through the air (a transparent medium). Note that "transparent," by definition, means that you're seeing something coming through the medium."

Your "transparent" definition is perfectly correct. However, you've missed my point. Micjk does not want to "see" the scene BEYOND the "steam" he wants to "see" the steam so he can detect its source. The image he wishes to "photograph" must be a real image, and it must be within the wavelength where he is attempting to snap a "picture". Neither the eye (nor a camera) can "see" light traveling through a transparrent (transparent to that wavelength) medium. If one "sees" light in front of an automobile at night it is ONLY because the light strikes particles in suspension such as fog or dust. If the air is clear you will only illuminate the subject located where the light beams are directed, namely the ground in front of the car. Mr. micjk cannot see his "steam" because the water is still in the gaseous state and it is therefore transparent to most wavelengths of light. The IR camera cannot "see" an image of warm gases because there is no object or particles to reflect light from (I assume you know enough thermodynamics to recognize the three very different states of water and their behaviors.) I would bet that if micjk were to sprinkle some type of powder over the region of interest then when the hot, transparent steam heated those particles then he would be able to "see" the location of the leak with his IR camera. ( Another method of finding the leak. One could also spray a fine mist of water over this area. This would likely condense so much liquid water from steam that an IR camera would not be necessary to find the leak.)
Am I not correct about the in-ability to "see" light traveling through a transparent medium?
By the way, what is the wavelength response range of micjk'S camera?
cameraman1

 

[IRstuff]

Back to EP's assertions. You're mixing classical and quantum phyics. While gases do behave per classical physics, classical physics cannot correctly predict thermal radiation. Thermal radiation is a quantum mechanical effect, executed by the vibrational energies within the molecular bonds.

You can clearly see, in the infrared, blowtorches, the sun, natural and other gases burning and rocket plumes. In all cases, the density, thickness and temperature are all contributing to the detectatbility of the infrared emission.

The steam leak, by the fact that it's not readily obvious in the visible band, has too low a density get a long-wave IR sensor excited. The continued impingement of the leaking steam on a piece of plastic is essentially a time integration of the energy from the leak. Collect enough of the energy to heat up the plastic and it becomes visible in the IR camera.

TTFN

 

[electricpete]

irstuff - I agree.

Except:
1 - It was a question or proposed explanation, not an assertion (read the first paragraph).
2 - Your comment "The steam leak, by the fact that it's not readily obvious in the visible band, has too low a density get a long-wave IR sensor excited"

From the original description of the system, I believe the steam would be too cool to see regardless of density.

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[electricpete]

A little more on #2 - I interpretted you are making a judgement on the low density of the steam based on the fact that it's infrared radiation is not visible as a glow.

Maybe I'm looking at it wrong - maybe you're just saying it's low density because you can't see it. Once again that wouldn't make sense. We can see a steam leak when it mixes with the air, cooling and condensing and producing water vapro. We can't see it when it is high pressure high density.

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[JKEngineer]

I would like to change the question a little.

What has been people's experience in seeing or not seeing high pressure steam leaks with IR? Short or long wave (please specify)
Thanks,
Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[eyec]

the only time this was near applicable was on a steam trap survey and this indicated thermal difference not active steam leaks.

have never heard of IR being used in the manner described, other than this thread.

thermal transfer of steam in the atmosphere may be an option but for other reason previously listed i do not think it an effective means for this purpose.

 

[PhilsTj]

We recently had a large steam leak on a manway that required cordoning off the whole hall way. At 25' you could visibly see large quantities of steam blasting away. With out FLIR long wave camera I could see very little disturbance in the image. For the most part the plume was transperent. Only the very hottest part (the center of the blast) showed some disturbance in the background image.

 

[IRstuff]

Oh yeah, one significant difference between LWIR and MWIR is that LWIR transmission takes a big hit in high humidity.

So, while the steam is hot and radiating, it's also absorbing in the LWIR band. Something like this would be very visible in the MWIR band.

TTFN