Comparison between Steady State and Dynamic Pressure Relief Calculations

Traditional pressure relief sizing calculations have typically used steady state equations to determine both the required relief rate and the pressure relief system capacity, for every applicable overpressure scenario. These equations are well accepted and understood and are available in API Standards 520 and 521.

API Standard 521 is open to the application of dynamic simulations when considering pressure relief system calculations. Some overpressure scenarios are very dynamic in their nature, such as external fire exposure, abnormal heat input, vapor breakthrough and runaway reaction, to name a few.

Modeling overpressure scenarios dynamically can allow for a more accurate, less conservative, representation of the affected system; often resulting in a lower relief requirement as well as an understanding how the system pressure and temperature changes over time.

This paper compares an external fire overpressure scenario which is modeled using steady state calculations versus the same system modeled dynamically. It is intended to give the user a better understanding of the differences in the two approaches, as well as the benefits of applying dynamic simulation for specific scenarios.

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