395709 Credibility of Designing Refrigerated Atmospheric Storage Tank Pressure Relief Systems for a Tornado

Monday, April 27, 2015
Exhibit Hall 5 (Austin Convention Center)
Syed Usman Ahmed, Process Safety, Agrium Wholesale, Fort Saskatchewan, AB, Canada

Atmospheric pressure changes occur due to varying weather patterns. As per API 2000, a change in barometric pressure is a possible cause of overpressure or vacuum in refrigerated storage tanks due to the potential for a substantial amount of vaporization or condensation. To serve as a valid layer of protection, relief systems are required to be sized to handle the worst case scenario with the largest required relief area. This paper will examine an extreme case of a drastic change (sudden decrease followed by a sudden increase) in barometric pressure; the passing of a tornado causing an overpressure scenario due to the generation of vapor to reestablish the vapor-liquid equilibrium within the storage tank.

Determining accurate barometric pressure measurements in tornado cores is difficult due to very high wind velocities, heavy rain, and wind driven debris. API 2000 suggests using 2000 pascals/hr as the absolute value of rate of variation in atmospheric pressure if local atmospheric pressure change data is not available. However, this typically is not an issue as weather networks have historical weather data that is monitored and collected by local airports.

The paper will delve into the reasons a tornado would not cause a realistic overpressure scenario for the storage tank’s pressure relief devices, namely:

  • The unlikely probability (10-7 chances per year) of encountering such a tornado which exceeds the intensity of Region II (200 mph winds)
  • The near impossibility of the system (within the storage tank) to reestablish vapor-liquid equilibrium in a few seconds (i.e. time span of pressure drop) thus resulting in an overpressure or  vacuum scenario
  • The more likely damage mechanism of very high wind velocities, heavy rain, and wind driven debris renders overpressure protection a moot point.

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