Understanding Gas Blowby Scenario Calculations

Gas blowby overpressure scenarios are routinely identified when analyzing vessels with liquid-level inlet control valves. Blowby can occur when this type of inlet control valve fails open, emptying the upstream vessel liquid inventory into the downstream vessel of interest, with subsequent vapor or two-phase flow into the downstream vessel. While it is easy to determine if this scenario leads to a credible overpressure scenario, based on the operating pressure in the upstream vessel compared to the downstream vessel MAWP, the ensuing calculations are often not well understood, even by engineers regularly involved in pressure relief analysis. This paper provides guidance on determining the relief phase (liquid, vapor or two-phase), calculating the required relief rate, and taking credits where appropriate.