Inspectioneering Journal

Is Furnace Tube Temperature Measurement an Impossible Task?

By Mark Badrick, Senior Reliability Engineer at Bahrain Petroleum Company. This article appears in the March/April 2004 issue of Inspectioneering Journal.
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The title question is often asked and more often than not, impossible to answer. This paper follows on from a previous discussion (Inspectioneering Journal Volume 4 Issue 1 Jan/Feb 1998) relating to the difficulties arising whilst attempting to carry out temperature surveys of furnace tubes using a thermal imager. The issues discussed then i.e. calculation of emissivity and ambient temperatures, reflected heat etc, are still current concerns, but since the writing of that article, an additional equally important challenge has become apparent - “how do we measure the temperature of an externally scaled or fouled tube”? Where the external scale or some other external deposit, such as refractory dust etc. may mask the true tube temperature.

The process and technical reasons for the hot spots discussed in this paper are subjects of internal investigations and will not be examined here.


Hot Spot

If we agree, that in order to maintain integrity, each piece of equipment has to have a specific set of conditions that must not be exceeded, e.g. furnace tube free scaling temperature. And with regard to furnace tubes, material selection is such that free scaling temperatures will not be reached in normal operation. We can therefore say, that any tube, or part thereof, that exceeds their free scaling temperature, shall be classed as a hot “spot”. The hot spot may, or may not be visible to the naked eye.

Free Scaling Temperature

Oxidation occurs when the tube material reacts at elevated temperatures with oxygen in the surrounding furnace atmosphere and becomes scaled. In refineries high temperature oxidation is primarily limited to the outside of furnace tubes, to furnace tube hangers and other internal furnace components, which are exposed to combustion gases containing excess air. For example the maximum metal free scaling temperature for carbon steel, which will result in acceptable scaling rates in the presence of air, is around 1000°F.

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