(106k) 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.