by Ron Lucier, ASNT NDT Level III
(From the IRInformIR.blogspot.com, September 27, 2017 with format altered for easier reading online – all text and images from IRInformIR)
“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.
“Since this is a very complex subject it is probably appropriate to start from the beginning.”
“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.”
“In the simple drawing above we illustrate 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.
“Therefore, the odds are stacked against you from the start!”
“Why are heater tubes of interest anyway?”
“There are several reasons for inspecting tubes. Qualitatively slag (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 slag or coke prevents the transfer of heat into the process fluid. In the case of slag 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.
“In fossil boilers this is called “DNB” – Departure from Nucleate Boiling and is usually caused by flame impingement, which initiates a layer of steam on the inside of the tube. The external tube surface, unable to conduct its heat to the water, increases dramatically, causing a failure (opening) in the tube.”
“Read more »”
ED NOTE: The SPIE has published a very useful and detailed book in its Tutorial Text Series entitled
Radiation Thermometry: Fundamentals and Applications in the Petrochemical Industry
Author(s): Peter Saunders (August 2007) that deals with this topic in depth from the point of view of non-contact temperature measurement (radiation thermometry). It contains a wealth of detail about the issues of slag and reflected thermal radiation as well as a useful tutorial on infrared temperature measurement.
It is available online at the SPIE bookstore at a modest price as both a softcover book and a pdf download.
The link is: https://spie.org/Publications/Book/741687.
Here’s some details from the (above) linked SPIE web page:
This tutorial text provides an introduction to the subject of radiation thermometry, focusing on sources of measurement error and giving advice on methods for minimizing or eliminating these errors. Topics covered include: blackbody radiation, emissivity, reflection errors, and atmospheric absorption and emission; commonly used radiation thermometer types; uncertainty calculation; and procedures for in-house calibration of radiation thermometers. Included is a chapter containing detailed measurement examples for a variety of furnace types and operating conditions found in the methanol, ammonia, and refining industries.
Date Published: 3 August 2007