Monitoring tube skin temperatures can show conditions of firing and interior coking that could lead to tube failure and heater fires.
Tube-skin thermocouples should be installed to measure the tube wall temperatures on selected radiant section tubes. Selection of thermocouple locations is covered in Section 2126.
With RefractoPad skin points properly installed and leads suitably routed, you can limit the temperature margin used to compensate for thermocouple inaccuracies to the inherent manufacturing tolerance of the thermocouple junction and lead wires (e.g., 0.75% for type K).
Most thermocouple failures are failures of the thermocouple sheath from either overheating or fatigue. Overheating occurs when a span of sheath is not properly clipped against the tube and is exposed to direct flame radiation and/or high flue gas velocity. Fatigue can occur if an unanchored span of thermocouple sheath is repeatedly flexed by local flue gas currents or by tube movement due to thermal expansion. Other problems can occur because of improper welds or positioning of the thermocouple tip and insulated shield. Appendix E contains a typical thermocouple installation procedure.
The operating life of a thermocouple on a furnace tube is largely dependent on the care exercised in installing the component part. In addition to careful welding to the tube, and insulation, the following points are very important:
• The sheath should be cold bent 180 degrees to match the contour of the tube.
• Expansion loops are used to allow for furnace tube and sheath expansion.
• Minimum radius bends should be done with a tubing bender, not by hand.
• The heat shield and thermocouple should be spotlessly clean, because the presence of even the smallest amount of foreign material will impair the accuracy of the thermocouple.
• The thermocouple sheath should be secured to the tubes with welded clips to avoid unanchored spans that could repeatedly flex or vibrate.
• The clips that support the thermocouple sheath should be welded to the tube, not the sheath.
• The thermocouple sheath should be routed to avoid direct radiation and flame impingement. Hot spots burn through the sheath.
• The thermocouple sheath should leave the furnace as close as possible to where the tubes are anchored so that tube movement on start-up and shut-down will not flex the sheath.
Note that a material lab report (File E-517) indicates Type K thermocouple life expectancy may be no more than 4 years if exposed to operating temperatures of 1000°F or higher.