Furnace and Heater Tube Inspections

by Ron Lucier, ASNT NDT Level III
One of the more challenging applications of infrared thermography is in the measurement of process heater and furnace tubes. In fact, we get dozens of inquiries each year from our clients on this very subject. Quantitative thermography is the practice of measuring temperatures accurately and with furnace tubes this requires experience and knowledge.  Success in this science is gained by being properly trained and certified as a Level I Furnace Inspection Thermographer. Certification training from the Infrared Training Center (http://www.infraredtraining.com) teaches you the techniques required for determining these values and will provide a complete understanding of temperature measurement and heat transfer.

Process Heaters
There are as many uses for process heaters as there are designs. The basic configuration consists of a shell (outer casing),   tubes (where the process fluid flows) and a heat source. These units are both thermodynamically and hydraulically complex.

The simple drawing illustrates convective gas flow, which is turbulent, and radiant heat from the flame, refractory and other tubes – all non-uniform and time varying. When you view tube from an access port typically you can only see a portion of the tube or the tube at an oblique angle. Sometimes tubes are in rows, difficult if not impossible to image.

Why are heater tubes of interest anyway?

Tubes in a fired heater

There are several reasons for inspecting tubes. Qualitatively scale buildup on the outside of the tube can be readily identified. Buildup on the inside of the tube (coking) is a bit more difficult but commonly performed. In both cases the scale or coke prevents the transfer of heat into the process fluid. In the case of scale buildup, the process fluid may not be sufficiently heated, affecting downstream processing. The case of coking on the inside of the tube is more serious. Since the coke has an increased resistance to heat transfer, the tube surface temperature increases. After all it is the flow of the process fluid that is keeping the tube “cool” in the first place. The external tube surface, unable to conduct its heat to the water, increases dramatically, causing a failure (opening) in the tube. Metallurgists use the measured temperature to calculate the life of the tube so accurate measurements are critical.

Sample Thermal Images

Tube with restricted flow

Coking on bottom of tube

Conclusion 
IR offers the operators of process heaters the ability to visualize the heat transfer and provide vital temperature data to help determine remaining tube life.  Certification training from the Infrared Training Center (http://www.infraredtraining.com) teaches you the techniques required to successfully implement this technology at your facility.

Thermography Certification Dates and Locations

• Training Dates and Locations in the US and Canada: http://www.infraredtraining.com/schedule
• Training Dates and Locations in Europe, Middle East and Africa: http://www.irtraining.eu

To learn more about these certification classes, as well as upcoming training dates and locations, please visit the Infrared Training Center online at http://www.infraredtraining.com

Why Would I Need a Mid-Wave Infrared Camera?

We are often asked this question in our infrared certification classes.  There are a number of important reasons...to detect certain gasses including methane, to better measure the temperature of glass (it has a high emissivity, >.90 in MWIR) and to minimize reflections when inspecting certain low-slope roof membranes.

It also allows us to successfully inspect inside a furnace because mid-wave cameras can see clearly through flame at specific mid-IR wavelengths, something that a long-wave imaging system does not provide due to the radiation transmission properties of fire.  This capability improves the accuracy of temperature readings and allows for better analysis of thermal patterns when evaluating the efficiency of burners, capturing the temperatures of tubes or detecting the build-up of coking.

Check out this short video clip from last year's InfraMation conference which nicely demonstrates the transmission differences between long-wave and mid-wave cameras when inspecting furnace tubes:

The presenter, Andy Whitcher, is actually returning to this year's conference in Las Vegas to speak about Optical Gas Imaging this time around.  More on that in another post.  For now, enjoy this video and then make plans to join us at the Rio Las Vegas in September for InfraMation2016; the industry’s largest thermal imaging applications and networking conference.  I hope to see you there! 

Ghost in the…Refractory?

With Halloween coming up in a few weeks, we’ve got a fun “ghost hunter” video that we’re putting the final touches on which I think you’ll enjoy.  Keep an eye out for it right before the end of the month. Until then, on a related note, a thermographer from a recent ITC training course sent in this gem of a find.

ITC Instructor John Waggoner received these images from Craig Dickey.  Craig was conducting a furnace inspection at the time when he came across this unusual thermal pattern while scanning around the access door.  I guess IR really CAN be used to find ghosts or, in this case, perhaps some type of thermal spirit of refractory inspections past?

It got me thinking, have any of you found similar? Perhaps an interesting IR pattern that caught you by surprise, maybe something that wasn’t obvious visually, but jumped right out at you thermally? Share it on our Facebook page and I’ll post it to the InformIR blog as well with a photo credit.