Chemical & Process Technology: Dynamic Simulation of Relief and Controlled Blowdown Cases

Sunday, May 23, 2010

Dynamic Simulation of Relief and Controlled Blowdown Cases

Pressure Relief load estimation is commonly estimated by taking system is approximately at "steady state condition". The estimated relief load could be excessive due to conservative assumption, unrealistic external energy inputs, etc. Current trend is to utilize Dynamic simulation in order to derive realistic but still preserve conservatism, integrity and safety of plant pressure relief and overpressure protection systems. This approach inline with API std 521 recommendation and probably the way to go in near future.

This study presented the dynamic simulation of a gas compression system, proving the viability of operational philosophy and emergency shutdown logic with quantitative process responses in various situations. To avoid unnecessarily high peak in an initial stage of blowdown, this study employed the controlled blowdown and investigated its safety level.

This study concluded that dynamic simulation of start-up and emergency operation improved the operability of the whole process. The revealed transient behavior demonstrated that PSVs sized to API standard led to chattering because the standard gives excessive size. Choice of properly sized PSVs eliminated the chattering with a decrease in relief loads by 40%. The blowdown valves and PSVs are likely to be oversized if the API RP 521 is observed. The dynamic simulation gave precise estimates, consequently decreased the flare loads, and better safety. The controlled blowdown system mitigated the flare load to about 60% of the conventional blowdown system. Its safety was more reliable than that of the conventional, satisfying SIL 2 of IEC 61508.

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