Waterwall tube metal temperatures are highest on the fire side of the tube. Usually on a clean tube, the mean wall temperature is less than 75°F above saturation temperature. Supercritical boiler furnace wall tubes are usually within similar temperature differences of about 50-75°F between the fluid temperature and the “crown” of the tube on the fire side. However, if circulation is obstructed or more commonly, if the water side of the tubes accumulate deposits, the metal temperature can climb to the point of metal overheating.
Chordal thermocouples are a useful tool for predicting waterwall metal temperature distress. The readings from the mean wall temperature of a chordal thermocouple should be recorded weekly, after installation. If a previous problem of water side deposits caused overheating to the extent that acid cleaning of the boiler was required, then use of chordal thermocouples are useful tools to record temperatures on a weekly basis. The chordal thermocouple readings are directly related to heat flow through the tube wall. Therefore, the recording of temperatures should always be at the same high load, with the same burners in operation. Recording the mean wall temperature, similar to the graph shown below, will provide a trend of water side deposits. It only takes a thin coating of water side deposits to increase the temperature of the tube metal. Iron oxide deposits have a heat transfer coefficient of about 1/70th of steel. Mineral constituents have even lower heat transfer coefficients.
If the tube metal temperature of a chordal thermocouple continues to gradually climb, then an adjacent tube sample should be removed for internal deposit analysis. This is one way to maximize boiler reliability and prevent surprise waterwall failures. STORM® can design and manufacture these chordal thermocouple tube sections to monitor the mean wall temperature of waterwalls.