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Chapter 1:
Temperature is perhaps the most critical parameter in the operation of a petrochemical furnace. Accurate temperature measurements of process tubes inside steam reformers, platformers, crude heaters, vacuum distillation units, etc., are essential for managing the overall performance of the plant. The benefits are numerous. First, life assessment is critically dependent on good quality temperature information. The ability to accurately predict the remnant life of furnace tubes allows furnace operators to manage shutdown cycles without the risk and expense of unplanned outages due to unexpected tube failures. Metallurgical damage in tube material accumulates over time with exposure to high temperature. A typical HP alloy reformer tube may be designed for a 100,000 hour life at a temperature of 930 °C. Excursions from this temperature will alter the tube's life expectancy. Models used to assess the remnant life at any given time require a good thermal history of the tube. Exceeding the design temperature by just 15 °C will halve the life expectancy of a tube [1]. This places quite strict requirements on the accuracy of temperature measurements if they are to be meaningful in life assessment models. Figure 1.1 shows the consequence of tubes being exposed to too high a temperature, and Figure 1.2 demonstrates the catastrophic effects of a single extreme temperature excursion. The cost of such an event, including materials replacement, lost production and increased insurance premiums, can be in the vicinity of 100 million dollars. Secondly, the plant efficiency and reliability can by maximized through an understanding of the temperature distribution within the furnace. In a reformer, chemical reaction rates and output increase as the temperature increases, so efficiency is governed by the average tube temperature. On the other hand, the reliability of the furnace is dependent on the hottest tube since it is most at risk of failure. Ideally, the average tube temperature should be the same as the highest tube temperature, so that each tube is working equally hard and no tube is more at risk that any other. In practice, this is never achievable because of inevitable measurement uncertainties, but there is significant advantage to be gained by ensuring that all tubes are operating as close to the same temperature as possible. Identification of areas of hot and cold tubes allows the furnace to be adjusted to smooth out any temperature non-uniformities.
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