Posted on behalf of Francois Brenkman
Natref Inspection

We do have an AGEMA Thermovision 470 infrared camera with a flame filter installed. We would like to do a infrared scan on the tubes of a Hydrogen Reformer to determine the actual skin temperatures. There are no thermocouples on the tubes to have a reference temperature for Emissivity adjustment and no firebox temperature (only outlet temperature on ducting).

1) How can I determine the firebox temperature ?
2) How do I calibrate for Emissivity on the tubes ?

Regards,
Francois

Reformer and Agema sound familiar to me. I remember we have to do the calibration using an oven. The details I'm not so sure because it's done by inspection and instrument guys. Rgds

Can you not attach a make-shift K-type thermocouple to the tubes and run it out and read it with a DMM? Also wrap a 6x6 piece of AL w/ 1/2 sandblasted and take on a piece of wrinkled up AL foil and maybe use a hand-held IR device of know calibration...grin. Never hurts to have too much information but can hurt not having enough. Some will also use a piece of glass in sight for reflective reference.

Hopefully someone will weigh in much more knowledgable than me.

This Message Was also Posted on the IRINFO Group System.

The following is the Reply I gave to the IRINFO Group. I have decided to post it here also for anyone else who needed the information.

Dear Francois,

1. Firebox Temperatures... I am assuming you are referring to the environment temperature within the reformer's firebox. Stick a high temperature Thermocouple inside the firebox at the different levels and distances. But try not to stick the thermocouple in the path of the burner flames. (Use a TC on a steel rod. Make sure the 2 leads of the TC are properly fused together in order to get environment temperatures). Also make sure and leave enough time for thermal equilibrium of the TC.

In the future, try to install permanent Thermowells.

2. Emissivity of Reformer Tubes... Now here is something that you will get many different answers from many different thermographers. This is because each thermography might have their own opinion on the best way to attain reference temperatures. Yup that's right, you must get Reference temperatures. What I tell people is to obtain their reference temperatures anyway that will satisfy them as to the accuracy of the temperature. The 3 most common ways are
a) via embedded thermocouples in direct line-of-sight at different elevations and distances
b) via reference coupons made from the same tube material as inside your furnace
c) via a Gold Cup Pyrometer

Each method has their disadvantages. So it's like what I said... Whatever satisfies you as to what is accurate for you.

When your dealing with reformer tubes, you need to be aware of Background, Emissivity, Distance, Spot Size, Viewing Angle, Focus, Range, Burner Location, Flame Path, fuel used, convectional winds at each elevation and keeping your equipment shielded from the high temperatures. All of these variables affect temperature in reformers.

Nothing beats a good visual inspection of your tubes. So before you look through the camera, look at your tubes with your own eyes first. Identify potential overheating areas, potential burner firing problems, potential tube wall anomalies, and then look through your camera to confirm the problem and perhaps the cause.

Temperature accuracy is very tricky. Rememebr that the higher you go in temperature, the larger the degree of error! So if your settings are off by a small amount, it will show a significant temperature difference. And with reformers, 50 deg C difference is concidered a big problem. Also rememebr that your settings at the top elevation may not be the same as the midlle or lower elevations or even at different distances. The more references you can get, the better.

Hope this helps in some way.

Best Regards,

Please go through the attachment. You will find answers for all your querries.

Jayapal

Modern_Techniques_for_Optimisation_of_Reformer_Operation.pdf (498 Kb, 25 downloads) Modern Trends in Reformer Optimisation

I remember reading this paper and the case studies some years ago. It is a good paper and had some very good points and resources.

However, when it comes to the actual way they took the temperature measurements with both the Gold Cup Pyrometer and Spot IR Radiometer there are a few considerations that have not been taken and mentioned.

I agree that using a gold cup pyrometer for references is one of the best ways to do so, but one of the biggest limitations is that you can only take temperatures of the closest tubes at each row (Maybe tubes 1 through 8). You cannot get to the tubes deeper in the reformer and closer to the middle.

When it comes to using a Spot IR Radiometer/Pyrometer there are even more problems that you encounter such as:

Spot Size: This is one of the biggest issues. These pyrometers have a targeting circle that is seen when you look through the eyepiece. This targeting circle is your Spot Size. When looking inside your reformer and looking at the tubes, you will notice that the first few tubes are not difficult to inspect and the target of the tube adequately fills the spot size. But when it comes to the tubes closer in the middle of the reformer, about tubes 14+ you start getting more than 1 tube into the spot size-targeting circle. So what is happening here is you are now measuring 2 tubes and getting the average in 1 temperature. This obviously is wrong data.

Hemispherical Emitters: Tubes are hemispherical emitters and therefore we know that the edges of the tube show up much cooler than the actual tube temperature. From my experience about 50 to 100 Deg C cooler. That is why it is never recommended to take temperatures with IR equipment on the edge of tubes. Now due to the Spot Size issue with these Spot Pyrometers, you cannot get away with getting the edge of the tube within the targeting circle thus leading to wrong temperatures.

Now there is no way that I know to compensate for these above errors by any calculations or software. This is a spot Size issue and is inherent to the spot pyrometer.

Another issue with these spot pyrometers is that not all are created equal. Each might look identical, but you must select the right model for your application. Each pyrometer model is set at a different “wavelength” and as we know as thermographers, wavelength affects temperature especially when looking through flames.

Using a spot pyrometer on reformer tubes in my opinion is beneficial to plant inspectors and operators in order to “Trend” temperature changes. But if you intend to use an IR thermometer for temperature accuracy for your tubes, you must go with a MW imaging radiometer AKA IR Camera with 3.9-micron filter. That way you will have the benefits of a smaller spot size, see the cool edges and know to avoid them and able to measure and compensate for emissivity, background, and other atmospheric variables. You will still need the gold cup pyrometer as your reference to adjust your IR camera, but keep in mind that this is only good for the first few tubes. You still have to find a way to get accurate contact reference temperatures with the tubes in the middle.

There is much more to this, but there is so much I can write before I turn this post into an actual technical paper

So beware of your inspection equipment and procedure! Know your equipment limitations.

Regards,

This is an old thread, but it is still a topic of great interest for some.

Check out a paper I wrote and presented at IRInfo 2008. Maybe it might help in some way or another.

Reformer Inspection