scope:

A simplified correlation for the equilibrium bubble point pressure, vapor pressure, of commercial natural gas liquids, NGLs, has been developed. This correlation was developed in response to the industry's need for a simplified correlation to be used when adjusting densities from flowing conditions to a set of standard conditions. Methods for improving the accuracy and extending the correlation are also discussed.

The purpose of this procedure is to provide a simplified means of estimating flowing equilibrium vapor pressures of various Natural Gas Liquids, using relative density (60°F/60°F) and flowing temperature inputs. This procedure is proposed for use on GPA Standard Specification 2140, commercial HD-5 Propane, Iso-Butane, Normal Butane, and 12-14 psia Natural Gasoline between −50°F and +140°F. The intended application of this procedure is for determining the value of Pe (equilibrium vapor pressure at flowing conditions) required for determining pressure volume correction factors as specified in API MPMS Chapter 11.2.1 and 11.2.2.

Three primary correlations (commercial HD-5 Propane, Butanes, and Natural Gasoline mixtures) were developed from data. To provide continuity, two secondary correlations or extrapolations were derived which connect or bridge the three primary correlations.

Minor variations from the predicted vapor pressure to actual are to be expected in the primary equation due to the possible combinations of product compositions which may result in the same relative density product. The magnitude of these variations, however, is expected to have minimal overall impact on the volume correction factors determined from its use.

Due to the unconstrained range of compositions possible in non-specification mixtures, greater variations may occur between an actual sample and the correlation values in the extrapolated regions, and accuracy should be checked by the user.

These bridge correlations may be utilized for settlement purposes if agreed to between the contracting parties. However, additional accuracy may be obtained by developing a modified correlation as indicated in the procedure, based upon test data.

API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publication may conflict.

Suggested revisions are invited and should be submitted to Measurement Coordination, Exploration and Production Department, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005.