Simplified Model of the Internal Atmosphere of Flammable Liquid Tanks in Case of Air Inlet from a Pressure Safety Valve

Storage of flammable liquids is a common activity in many industrial domains. A history of accidents shows that liquid storage has been involved in several critical accidents due to the large amount of hazardous substances potentially involved in the incident. Safe storage of flammable liquids is often guaranteed through blanketing of the internal atmosphere of the tank through the introduction of an inert gas, usually nitrogen. A double action pressure safety valve is often installed on the tank to protect the tank from damage in the event of overpressure or depression. In case of depression, an inert gas, usually nitrogen, is fed to the vapor space of the tank to maintain the vapor composition outside of the flammability limits. In case of lack of nitrogen, the opening of the pressure safety valve allows air to enter. The entry of air, especially if prolonged, can bring the atmosphere inside the tank to explosive conditions. This paper presents a simplified model for the estimation of the internal composition of the tank following the entry of air due to the opening of the pressure safety valve, following the process of fluid removal in case of lack of nitrogen. The model also allows the estimation of how much liquid can be safely removed. The simplified model can analyze both the case of a single tank and a tank farm.