Message Board Thread - "Thermography in Fired Heaters"

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Thermography in Fired Heaters Andyred 2/4/2004
When gathering data to calculate emissivity for tubes in fired heaters should we try to gain the flame temperature as this is likely to be the highest reflected temperature seen on the heater tubes. This is going to be significantly higher than the surrounding refractory temperature.
 
Re:Thermography in Fired Heaters ron lucier 2/4/2004
Measuring the flame temperature is very difficult and may not be meaningful. The convective atmosphere in heaters is extensive and ever changing. We tried - unsuccessfully - to characterize both in fossil fired boilers many years ago.

The heat absorbed by the heaters is the sum of the radiant energy (flame, refractory, structure) plus the convective (combusting gas, products of combustion). In many instances, particularly with heaters long into their run, the tube emissivity is between 0.8 and 0.9. This is obviously good news. However, in horizontal heaters, when looking in from the ends, the reflected energy from the opposite end of the heater is a huge contributor and that is typically refractory.

Add the above to the fact that the apparent emissivity of most metals increase with angle (rather than decrease) and the net result is we're not in control of the measurement mission.

The heater inspection procedure, soon to be posted on our website, reflects these realities. I try to bound the tube temperature by varying emissivity +/- 0.05 and Treflected by +/- 100 F. Tube temperature are reported to be 875 F +/- 22 F for example. Materials Engineers would like exact temperatures and that is the point of single thermocouples. However, it is impractical to instrument the entire surfaces of the tubes with thermocouples so where IR comes in is to get a handle on the bigger picture. If everything is done correctly, and the reference thermocouple is in calibration and the IR measurement is close to it, the errors from the IR readings in other areas have to be close. Yes, there will always be uncertainty and yes, this is an inexact science but within the bounds of the stated imager accuracies, I think IR does an outstanding job.

EVERYONE WHO READS THIS! - If you are actively involved with heater or boiler tube thermography, jump in and comment. Better, send in an abstract for our Inframation conference in October and present your results. FLIR & ITC's prupose is to get you great cameras and superb training. You are the ones with the day to day field experience that makes us jealous so stand up and tell the world what you are up to!
 
Re:Thermography in Fired Heaters HeatSpy 2/4/2004
A) The article is fairly general and somewhat accurate. I don't see how anyone can get the impression that "apparent emissivity of metals increase with angle". Based on the book edited by D.P. DeWitt and Gene D. Nutter "Theory & Practice of Radiation Thermography" It ain't so!!!
B) Flame temperature will only be used for the "atmospheric" part of the equation (vs Ambient). This will give you the correct "path to the object" correction.
C) In reference to the comparisson with local thermocouples, there is much more to it than looking at one thermocouple and assuming the others are correct by thermography without taking into account the external fouling of the tubes in each area.
D) Is this "Heater Inspection Procedure" mentioned here available in it's present and unfinished state, available for revue?
 
Re:Thermography in Fired Heaters Doctir bob 2/4/2004
Most thermographers use a 3.9 micrometer narrow bandpass filter in their IR camera, as a clean flame has almost 100% transmittance at this wavelength. They measure the reflected apparent temperature using standard techniques. The challenges are to know the reflectance of the region of interest, and where the reflected energy is coming from. Measurement of reflected apparent temperature for different regions of a fired heater will give different values.

The directional reflectance of the pipe can help with evaulating what to use for reflected apparent temperature. Surface Optics Corporation can make these measurements at their facility, or they sell an instrument that does it at the customer's facility. Knowledge of the directional reflectance of various samples of fired heater tubes of different types and ages would definitely move the technology forward. To learn more about Surface Optics, here is their web link:

http://www.surfaceoptics.com/index.htm

Directional reflectance and emissivity are challenging theoretically and experimentally. I was surprised by one response that the emissivity of metals decreases with viewing angle. On page 113, Figure 2-7 in Theory and Practice of Radiation Thermometry, the plot clearly shows the emissivity of metals increases out to 80 to 85 degrees from normal then drops rapidly at grazing angles.
 
Re:"Thermography in Fired Heaters" and Ron's invitation -PIOTR PIOTR 2/8/2004
Following to Ron Lucier's invitation to “jump in and comment” I respond with pleasure - but my comment, here in any way, can’t be as clear and full as this particular problem requires…In general, I have to agree with Andyred’s remarks about significant differences between observed reality and “habits” of corrections. In my opinion the main weakness, subjective element in these measurements by means of IR thermography seems be to be impossible to be fully solved. Fortunately, due to applying news of technology a new and more reliable method can be applied. The idea is simple and omits very sophisticated but too often unpractical models, yet requires tools more specialized. For part of practical tasks, it's worthy to be considered because obtained results showed to be significantly more accurate.
So, it is my pleasure to inform that the most important elements of this project executed together with team of experienced thermographers from Polish Petrochemical Concern PKN ORLEN, illustrated with instructive examples both in the form of real-time movies, and static data, will be presented during SPIE Thermosense XXVI Conf. [April 13-16,Orlando, www.thermosense.org . The paper is entitled: “Advanced multispectral dynamic thermography as a new tool for inspection of gas- fired furnaces”]. As the discussed problem is very complex, additional paper, pointing quantitative problems and entitled “A new experimental techniques for operating furnaces by use of IR radiometers” will be presented during 7th QIRT Conference [July 5-8, Brussels, http://www.vki.ac.be/qirt2004 ]. Directly after both meetings, texts of these papers will be to added to web page: http://www.pirs.pl/varia ;
Yours, Piotr Pregowski /PIRS/
 
Re:Thermography in Fired Heaters Kim H.A. 2/9/2004
I would like to comment on Mr. Lucier's suggestion of using thermocouples as reference for IR measurements. I fully agree that it is a way to "solve" the problem. And possibly also the best available at the moment. But detailed simulations of installed thermocouples and comparison of measurements with pyrometry shows a significant difference between the thermocouple reading and the "true" temperature. In some cases it has been found to be more than 15 degC which must be considered significant. Again this will obviousley depend on the purpose of the measurements.

With regards to Andyred's emissivity calculations, have you ever used laser measurements of the emissivity? I have some experience with it and it seems to give very reliable results.
 
Re:Thermography in Fired Heaters Michael 7/25/2005
If thermocouples are welded to tubeskins, There is typically an area that has been grinded smooth for weld attachment. This area will usually appear significantly darker on tubeskin due to lack of scaling. Check temperatures of your thermocouples (if you trust them) and then set E=value accrodingly. This is how I get started. It gives me a reference point.
If there considerable scale or residue on tube surfaces, get a coarse file, heat it, bend it 90*, and weld it to a rod. Stick in the heater and knock some of the scale off to get a better reference of actual tube temp. DO NOT use this to calibrate or shoot other tubes. Other tubes are scaled up and will not have the same E.
You can purchase a calibration tool made of a 2" Chrome pipe(or whatever material) that has been cut in half, and has two Type K thermocouple welded to ID and OD sides. This assembly is welded to a stainless tubing handle. This can be placed in the heater through a porthole, left to heat up, and then shot from across the heater. This is another reference and will start you on the right path to set variables based on internal heater conditions. There is no fluid cooling the tube, so this number is a reference as well.
On large can-style heaters, just adjusting E and background temp, from my experience will not do the trick. Adjustemnts to transmiss, and possibly reference value will be required. When shooting over extended distances in that violent of an environment, all bets are off. Keep shooting and keep making adjustments. No two tubes are the same, no two flames are the same, and no two views are the same. Every porthole is a new world that you have to figure out. You can get generally close with a little effort, but if you want to get real close, you gotta burn the backside of your right hand. For me, it just takes standing there and working it out. Taking the things I "know" and applying that to the unknown to come up with a best guess.

I've seen coker heater that were not inspectable. They were in-direct fired and refractory dust covered every tube in the box. Anyone figures that one out, let me know.

This is only a small niche of the IR world, however, it seems to be one of, if not the most, complex. I find it frustrating that there are many very experienced people with mounds of knowledge, yet it is not shared. Furthermore, it is treated like Top Secret material. I am in no way an expert but if I do pick something up along the way, I have no problem sharing it. In the end, we all benefit from sharing this info. I seriously doubt that if some technique was shared and use began by others that any of us heater rats would be out of work. It would only further our reputation and increase the clients faith in our ablities.
 
Re:Thermography in Fired Heaters Andyred 9/5/2005
Thank you for some very interesting replies.I am currently re-writing our procedures and inspection intervals and will post some of them on this forum if there is some interest.

Andyred
 
Re:Thermography in Fired Heaters paulpartin 9/8/2005
Yes, please post your procedure when you can. I am very interested in finding out how other people are doing it. I am one of the "heater rats" that need some good info on this subject. Thanks in advance for your input. paul
 
Re:Thermography in Fired Heaters Safety 11/8/2006
Just joined in. I am interested in the safety of personnel who has to be exposed to the heat when ports are opened to shoot the tubes. Are there any wriiten procedures or special personal protective equipment that can be used. Appreciate the help.
 


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