(141a) Stratification during Vapor or Gas Freeing of Storage Tanks

API RP 2016 provides guidelines for vapor freeing and degassing of tanks. The instructions include warnings and methods to recognize and avoid stratification or channeling during vapor freeing, and to control and prevent ignition sources from occurring in the tank during vapor freeing. A recent incident occurred where vapor in a tank ignited shortly after vapor freeing. The investigation revealed that the vapor freeing method used most likely resulted in stratification of the vapor in the tank, and the atmosphere testing method used to test the vapor level in the tank was not capable of detecting stratification. The low vapor reading resulted in shutting down the vapor freeing operation after only 5 hours when the planned duration was 48 hours. By following API RP 2016, this event would likely not have occurred. This paper discusses methods for estimating when stratification is likely to occur during vapor freeing of tanks based on jet mixing correlations, and uses a CFD model to examine the likelihood of stratification in several cases. These methods can be used to develop criteria for vapor freeing of tanks and for investigating incidents involving stratification of flammable vapors in an enclosed space.