Traditional relief load estimation methodologies are known to be overly conservative and can leadrnto the overdesign of flare systems or the determination that added capacity requires expandedrnflare capacity. Dynamic simulation has become an accepted methodology to more accuratelyrnestimate relief loads when traditional methodologies indicate that an existing flare is at or overrncapacity. Depew and Dessing1 reported significant reductions in peak relief load consideringrndynamic simulation as compared to traditional methods but the benefits vary on a case by casernbasis. Often the reductions are not enough to avoid additional expenditure in a flare systemrnexpansion.rnThis paper discusses a dynamic simulation study performed to evaluate the peak relief rate for anrnintegrated De-isobutanizer and De-butanizer considering:rn1. Traditional unbalanced heat load approachrn2. Rigorous dynamic simulation adhering to API 521 recommended practicesrn3. Rigorous dynamic simulation considering a SIL Rated High Integrity Protection Systemrn(HIPS) on the re-boiler steam.rnThe study shows how the different approaches to determining peak relief load complement eachrnother and how the model can be used to determine the appropriate set point for the HIPS systemrnin order to avoid nuisance trips while ensuring a significant reduction in relief load.rnThe work also verifies that in this case the designed HIPS substantially reduced the relief load ofrnthe towers under the tested failure scenarios, even leading to eliminating the relief in some cases.rnThe information such studies provide can be used to compare the cost of installing such safetyrninstrumentation systems (SIS) versus expanding the flare system. The decision of whether tornimplement the SIS system also needs to factor in the increasing pressures from the public andrnregulatory authorities to reduce flaring due to air quality and global warming concerns.
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