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. 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 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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