induced damage. It is important to note here that high temperature alone may not be the sole reason for ITS failure, as generally perceived. Rather, the temperature gradient can also play a major role in such failure incidents. Assessing variations in furnace operating conditions can go a long way if over-firing or extracting more from existing asset is not intended Conclusion Troubleshooting fired heaters can be an intriguing task. However, not latching on to a premeditated solution can be helpful, as a multitude of factors can combine to cause the anomaly. Working out a specific solution, depending on the gravity of the problem and the state of the fired heater's health, can actually ease the limitations to a large extent. To achieve this, a detailed study of the fired heater and a possible high-technology assessment using advanced tools may be required. On the other hand, seemingly complex reasons leading to unsafe operation, such as high firebox pressure, can be identified through apparently simpler fired heater site audits and regular checking of the systems through digital tools. Through this article, an attempt was made to connect with the refining and downstream oil processing community and share incidents and remedial solutions commonly encountered in the daily operation of fired heaters. Every refiner may face a unique set of issues. Nevertheless, the common problems, as elaborated, may help the refiner in deciding the course of action. EngRT-Htr is a trademark of Engineers India Limited for operational analysis of fired heaters. Acknowledgement The authors would like to wholeheartedly thank the Ben Leviton is a senior process engineer with Fluor Canada Ltd. He has more than 10 years of experience in refining technologies with a primary focus on process simulation and relief analysis. Leviton holds a BSc in engineering chemistry from Queen’s University in Kingston, Ontario. Email: ben.leviton@fluor.com management of Engineers India Limited for their support and encouragement in the publication of this work. VIEW REFERENCES Shilpa Singh shilpa@eil.co.in Rupam Mukherjee rupam.mukherjee@eil.co.in systems. Typically, this method will yield conservative results with an intrinsic safety margin, but a thoughtful approach is required to evaluate all assumptions employed. To avoid underestimating column relief loads, consider the following recommendations: • When employing UBH on a grassroots design, con- sider if the system has characteristics that might make steady-state or dynamic simulation the preferred method. Examples include systems that are sensitive to changes in product composition, such as strippers and stabilisers, sys- tems with near-critical relief conditions, and systems with unique configurations or complexities. Refer to guidance offered in ‘dynamic simulation to estimate tower relief’⁴ as a starting point. • Carefully review the basic control system at a P&ID level to determine if it is possible for the normal instrumented response of a system to worsen relief loads by increasing feed rates, increasing heat input, or reducing cooling duty. • Avoid taking credit for LMTD (‘reboiler pinch’) to reduce reboiler duties, except in simple systems where it is self- evident that the compositional changes over time will make the process stream heavier and hotter. In the case where the upset will potentially make the column bottoms hotter, consider using the normal feed composition as the basis for reboiler pinch rather than the normal bottoms composition as a measure of conservatism. • When accounting for residual heat input from an offline fired heater, consider performing a side calculation to con - firm the refractory will shed most of its heat before the col - umn reaches relief pressure. • When in doubt, consider employing dynamic simulation to evaluate these assumptions. Dynamic modelling is the most rigorous method and will typically result in the most accurate relief loads. References 1 Sengupta, M., Staats, F. Y., A new approach to relief valve load calculations, 43rd Proceedings of the Refining Section of American Petroleum Institute, Toronto, Canada, 1978. 2 API Standard 521. Pressure-relieving and Depressuring Systems, American Petroleum Institute, 6th Edition, 2014. 3 Ha, H. Z., Harji, A., Webber, J., Accurate prediction of tower relief, PTQ , Q2 2014. 4 Ha, H. Z., Leviton, B., Dynamic simulation to estimate tower relief, PTQ , Q4 2018. Harry Z Ha is a Process Technical Fellow with Fluor Canada Ltd. He has more than 30 years of experience in R&D in the petrochemical industry. In addition to process design, he focuses on data and meth- ods development to support process modelling and simulations. Dr Ha holds a MSc in environmental engineering from Hong Kong University of Science and Technology and a PhD in chemical engineering from the University of Alberta in Edmonton, Alberta. Email: harry.ha@fluor.com
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