Effect of operating parameters on fired heater tube skin temperature measurement accuracy & the development of new improved tube skin thermocouple

Refineries are often faced with challenges to meet production target, improve efficiency, process opportunity crudes with limited information on fouling characteristics etc. The common parameter that often becomes constraining are the tube skin temperatures which are critical in maintaining the integrity and reliability of process fired heaters. Tube skin temperature is an important monitoring parameter in applications that have continuous fouling / coking inside the tube or that operate with tube skins very close to the creep. It is a given that an inaccurate temperature reading (lower / higher) would either lead to operation in the unsafe zone, leading to rapid deterioration of the tube or at conservative levels, underutilizing the asset. Products installed on tubes to measure skin temperatures are composites comprising thermocouple, insulation, shield etc. designed to meet accuracy and reliability. The reliability of tube skins is quantifiable through years of operation, recorded data etc. however the accuracy is often assumed to be within a certain range with no validation data available using a true reference. The paper discusses the development and use of reference thermocouple for measuring true tube skin temperature in fired heaters with the prime objective to be used for evaluating tube skin thermocouple accuracy. Since heaters in various process applications have varying fireside and tube side conditions, it is important to understand the impact of the operating condition on the accuracy of the tube skin temperature measurement thermocouples. The effect of operating parameters on the accuracy of most commonly used thermocouple designs in the industry were evaluated in the test furnace to gain insight into products characteristics. The learning from the test results are currently being used to develop new design concepts and improve upon the existing.