PTQ Q1 2023 Issue

the system pressure drop remains the same. The first-stage ejector will experience performance break. Second-stage ejectors rarely have much excess capac- ity. This example demonstrates how sensitive vacuum systems are to first-stage inter-condenser performance. An increase in water equivalent load of less than 900 lb/h increased the second-stage ejector pressure enough to result in performance break of the first-stage ejector. There is roughly 70,000 lb/h of inlet vapour to this first- stage inter-condenser. The sub-cooling zone must be mechanically sealed and permit less than 1% of the inlet vapour to bypass. Conclusions Vacuum ejector systems are designed with razor-thin mar- gins to minimise installed cost and utility consumption. Inter-condensers using X-shell design with a sub-cooling zone minimise pressure drop and reduce utility consump- tion. To capture the full benefit of the X-shell design, the design and construction of the exchanger must be meticu- lous. Failure of the long air baffle to properly seal as intended will lead to vapour bypass of the sub-cooling zone, which frequently overloads the second-stage ejector and results in first-stage ejector performance break. When the vapour outlet temperature is higher than the condensate outlet, there is long air baffle bypass. This is easy to diagnose. However, the necessary instrumenta- tion is rarely available to enable a thorough analysis so that other causes can be eliminated. Often, ridiculous conclu- sions are drawn from bad or incomplete data. As part of a purchase agreement, all ejectors should be tested to generate accurate, certified performance curves and to confirm actual MDP. Design margin should be applied where appropriate to minimise the chance of per - formance break from deviations in design loads, cooling water conditions, and general fouling. In some cases, how- ever, performance break is inevitable when the sub-cooling zone is partially bypassed. References 1 Golden S, Barletta A, White S, Campbell D, Identifying the root cause of underperforming ejector systems, PTQ, Revamps 2022. 2 Hartman E, Barletta A, Solliec L, Tretzer P, Importance of testing for vacuum ejectors in refinery service, PTQ , Q2 2022. Scott Golden is President of Process Consulting Services. He has more than 40 years of experience including refinery process design, refinery technical service, and distillation equipment design. Tony Barletta is Vice President of Process Consulting Services. He has over 34 years of experience with refinery revamps and process design for heavy oil units. Steve White is a Chemical Engineer with Process Consulting Services. He handles large capital grassroot process design packages. He has more than 45 years of process design experience for refinery revamps and grassroots units.

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PTQ Q1 2023

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