Two methods used for calculation of the correction factor for pressure effects were standardized by the American Petroleum Institute: MPMS Chapter 11.2.1-1984 (now superseded by Chapter 11.1-2004) and MPMS Chapter 11.2.2-1986.

These methods require a knowledge of the equilibrium bubble point pressure (vapor pressure) at the measured conditions.

However, the vapor pressure of the process liquid is generally not measured. The vapor pressure can also be calculated from compositional information, but the composition is not always measured for natural gas liquids, NGLs. Therefore, a correlation for the vapor pressure of NGLs based upon normally measured properties is required and is documented in this publication.

This publication is an updated version of API MPMS Addendum to Chapter 11.2.2-1994.