True and Reid Vapor Pressure

Lili Lyddon

January 16, 2007

The Vapor Pressure Analysis in ProMax returns values for both Reid and True vapor pressures. The calculated True Vapor Pressure of the stream is based on ASTM D2889-95a(2000) Standard Test Method for Calculation of True Vapor Pressures of Petroleum Distillate Fuels. True Vapor Pressure is the pressure of the vapor in equilibrium with the liquid at 100 F (it is equal to the bubble point pressure at 100 F). The calculated Reid Vapor Pressure of the stream is based on ASTM D323-99a Standard Test Method for Vapor Pressure of Petroleum Products. Reid Vapor Pressure is the vapor pressure of a chilled sample of gasoline or other fuel as measured in a test bomb at 100 F. The Reid Vapor Pressure differs from the True Vapor Pressure of the sample due to sample vaporization and the presence of water vapor and air in the confined space resulting from the Reid Vapor Pressure test method. To presaturate the sample for the Reid Vapor Pressure test, the streams are flashed at 1 atm and 33 F, and the resulting liquid product is then combined with air at the rate of 4 parts air and 1 part liquid. Next the system is flashed at constant volume at 100 F. The resulting gauge pressure is the Reid vapor pressure. Because of this procedure, the Reid Vapor Pressure may be significantly different from the "True Vapor Pressure" if the Reid vapor pressure exceeds 26 psi. The Reid vapor pressure is applicable only for gasoline, volatile crude oil, and other volatile petroleum products. It is not applicable for liquefied petroleum gases. (The vapor pressure test method for LPG is ASTM D1267 which is not currently implemented in ProMax).

Authored by Lili Lyddon (BR&E Technical Support and Help Author)