Thermal Inbreathing Requirements of Low Pressure Storage Tanks At Elevated Temperatures

ISO 28300 (API 2000) “Petroleum, petrochemical and natural gas industries – Venting of atmospheric and low pressure storage tanks” provides guidance on determining the required thermal inbreathing flow rates to protect atmospheric and low pressure storage tanks against vacuum due to weather changes (e.g. a decrease in atmospheric temperature).  For heated, un-insulated tanks with vapor space temperatures above 58.9 °C (120 °F), ISO 28300 recommends an engineering review of thermal inbreathing requirements.

This paper describes one approach to evaluating thermal inbreathing requirements for un-insulated storage tanks with vapor space temperatures exceeding 58.8 °C (120 °F).  The tank is assumed to be exposed to the cooling effects of a sudden rain.  A simplified unsteady state heat transfer analysis is performed to determine the tank wall and vapor space temperatures versus time.  Initial gas mass and pressure are assumed to remain constant in the tank.  Gas volume in the tank is calculated as a function of vapor space temperature using the ideal gas law.  The rate of volume change is calculated for each time increment.  The rate of volume change goes through a maximum value.  This maximum value is selected as the thermal inbreathing requirement of the tank.

The calculation methodology described in this paper can be programmed in a spreadsheet for ease of use.