Manual of Petroleum Measurement Standards

Chapter 11—Physical Properties Data

Section 2, Part 5—A Simplified Vapor Pressure

Correlation for Commercial NGLs

GPA 8117

SEPTEMBER 2007

REAFFIRMED, AUGUST 2017

Manual of Petroleum Measurement Standards

Chapter 11—Physical Properties Data

Section 2, Part 5—A Simplified Vapor Pressure

Correlation for Commercial NGLs

GPA 8117

Measurement Coordination

SEPTEMBER 2007

REAFFIRMED, AUGUST 2017

Prepared for

American Petroleum Institute

200 Massachusetts Avenue, NW, Suite 1100

Washington, DC 20001

GPA Midstream Association

6060 S. American Plaza St. E, No. 700

Tulsa, OK 74135

Foreword

The purpose of this procedure is to provide a simplified means of estimating equilibrium vapor pressures of various natural gas liquids (NGLs) from a knowledge of the fluid’s relative density (60°F/60°F) and process temperature. The intended application of this procedure is to provide the values of Pe (equilibrium vapor pressure) required to determine the pressure effect contributions to volume correction factors as specified in the American Petroleum Institute Manual of Petroleum Measurement Standards (MPMS) Chapter 11.1-2004[1] (which superseded Chapter 11.2.1-1984[2]) and Chapter 11.2.2[3]. It is realized that other equations of state are currently in use for specific custody transfer applications and that such methods will continue to be used as acceptable for both buyer and seller.

This procedure is applicable to four major classifications of petroleum fluid mixtures: commercial propanes, commercial butanes, natural gasolines, and light end fluids.The latter consists of EP mixes and high ethane content fluids. It covers the relative density range of 0.350 to 0.675 over a temperature range of –50°F through 140°F. This procedure is an extension of GPA Technical Publication TP-15 (1988)[9]/API MPMS Addendum to Chapter 11.2.2-1994[4] to include light end fluids in the relative density range of 0.350 to 0.490.

Variations from the computed vapor pressures to the actual values are to be expected because of the infinite number of possible compositions that can result in the same relative density product. Representative and extreme compositions were selected to develop the correlations, but it is realized that additional streams with compositions from among the infinite potential may well behave differently. This potential for variation is especially true at relative densities in the neighborhood of 0.500. For example, at a relative density of 0.505 the fluid could be propane or Y-grade mix, each having significantly different compositions and vapor pressure behaviors.

As is always the case in correlations published for custody transfer and settlement purposes, additional accuracy may be obtained by developing a modified correlation for certain specific applications if agreed to by all contracting parties. An equation to improve the accuracy of the generalized correlation at 100°F is also included.

It is important to note that the application of the correlations presented in this document to conditions or fluids not specified, will result in untested and unknown results which could contain significant errors.

Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent.

This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is

designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director.

Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published annually and updated quarterly by API, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001.

Suggested revisions are invited and should be submitted to the Standards and Publications Department, API, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001, standards@api.org.

API Special Notes

API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed.

Neither API nor any of API's employees, subcontractors, consultants, committees, or other assignees make any warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API nor any of API's employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights.

API publications may be used by anyone desiring to do so. This publication is an updated version of MPMS Addendum to Chapter 11.2.2. Previous editions of this publication were numbered MPMS Addendum to Chapter 11.2.2. Users of this standard should take efforts to ensure they are using the most current version of this publication. 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 authorities having jurisdiction with which this publication may conflict.

API publications are published to facilitate the broad availability of proven, sound engineering and operating practices. These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should be utilized. The formulation and publication of API publications is not intended in any way to inhibit anyone from using any other practices.

Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such products do in fact conform to the applicable API standard.

All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher.

Copyright © 2007 American Petroleum Institute, GPA Midstream Association

GPA Disclaimer

Neither the GPA nor any person acting on behalf of the GPA makes any warranty, guarantee, or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report. The GPA hereby expressly disclaims any liability or responsibility for loss or damage resulting from the use of any apparatus, method, or process disclosed in this report; and for the infringement of any patent or the violation of any federal, state, or municipal law or regulation arising from the use of, any information, apparatus, method, or process disclosed in this report.

All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher.

Copyright © 2007 American Petroleum Institute, GPA MIdstream Association

Table of Contents

Foreword ii API Special Notes iii ASTM Note v

GPA Disclaimer vi

Table of Contents vii List of Tables viii List of Figures viii

0 Implementation Guidelines 1

1. Background 1

2. Vapor Pressure Correlation for Commercial NGLs 2

3. Correlation Development 3

   1. Propanes 7

      1. Product Specifications 7

      2. Data Collection 8

      3. Data Regression 8

      4. Error Analysis 8

   2. Butanes 11

      1. Product Specifications 11

      2. Data Collection 11

      3. Data Regression 12

      4. Error Analysis 12

   3. Natural Gasolines 15

      1. Product Specifications 15

      2. Data Collection 15

      3. Data Regression 17

      4. Error Analysis 17

   4. Light Ends 19

      1. Product Specifications 19

      2. Data Collection and Validation 20

      3. Data Regression 22

      4. Error Analysis 25

4. Ad Hoc Improvement of the Correlation For Specific Situations 25

5. List of References 26

List of Tables

Table 1: Parameters for Vapor Pressure Correlation (Use in Equation 2) 3

Table 2: GPA Liquefied Petroleum Gas Specifications: GPA Standard 2140-88[7] 7

Table 3: Correlation Parameters for Propanes and Butanes 9

Table 4: SRK Interaction Parameters for Propanes and Butanes 9

Table 5: Compositions and Relative Densities of Propane Samples 9

Table 6: Comparison of Vapor Pressure Correlations for Commercial Propanes 10

Table 7: Compositions and Relative Densities of Butane Samples Data Regression 12

Table 8: Comparison of Vapor Pressure Correlations for Commercial Butanes 13

Table 9: GPA Standard 3132-84, “Natural Gasoline Specifications and Test Methods” 15

Table 10: Grades of Natural Gasoline as specified by the GPA 15

Table 11: Correlation Constants for Natural Gasolines 16

Table 12: SRK Interaction Parameters for Natural Gasolines 16

Table 13: Compositions and Relative Densities of Natural Gasolines 17

Table 14: Comparison of Vapor Pressure Correlations for Natural Gasolines 18

Table 15: Compositions of Components Used to Generate Data for Light Ends Correlation 21

Table 16: Representative Comparison of Vapor Pressures Obtained from HYSYS with those from NGLCALC 22

Table 17: Representative Comparison Between HYSYS SRK Vapor Pressures and Vapor Pressures from the Correlation for Light End Fluids 23

List of Figures

Figure 1: Vapor Pressures from Correlations 5

Figure 2: “A” Parameter (Equation 2 & Table 1)… 6

Figure 3: “B” Parameter (Equation 2 & Table 1). 6

Figure 4: Maximum Temperature vs Relative Density… 21

A Simplified Vapor Pressure Correlation for Commercial NGLs

0 Implementation Guidelines

This Revised Standard/Technical Publication is effective upon the date of publication and supersedes all previous revisions of the Standard/Technical Publication and API MPMS 11.2.2A/GPA TP-15. However, due to the nature of the changes in this Revised Standard/Technical Publication and the fact that it is or may be incorporated by reference in various regulations, it is recognized that guidance concerning an implementation period may be needed in order to avoid disruptions within the industry and ensure proper application. As a result, it is recommended that this Revised Standard/Technical Publication be utilized on all new and existing applications no later than TWO YEARS after the publication date. An application, for this purpose, is defined as the point where the calculation is applied.

Once the Revised Standard/Technical Publication is implemented in a particular application, the Previous Standard/Technical Publication will no longer be used in that application.

However, the use of API standards and ASTM and GPA technical publications remains voluntary and the decision on when to utilize a standard/technical publication is an issue that is subject to the negotiations between the parties involved in the transaction.

1 Background

The transfer of ownership of liquids is usually based on the volume of liquid at agreed upon standard conditions, usually 60°F for the U.S. customary system of units and the greater of one atmosphere pressure or the equilibrium vapor pressure of the liquid. Actual measurement of the liquid volumes and the their associated densities occurs at flowing or process conditions. Thus these measurements must be converted to equivalent values at the standard conditions. Once the liquid densities are converted, the conversion of the volumes becomes a trivial exercise.

Densities are normally converted from measured conditions to standard conditions by equations of the form:

60 = Ft × Fp ×  Equation 1

Where:

60 Relative Density at 60°F and the greater of one atmosphere pressure or the equilibrium vapor pressure of the liquid

 Relative Density at measured conditions Ft Correction factor for temperature effects Fp Correction factor for pressure effects

Two methods used for calculation of the Fp term were standardized by the American Petroleum Institute: MPMS Chapter 11.2.1-1984[2] (now superseded by Chapter 11.1-2004[1]) and MPMS Chapter 11.2.2-1986[3]. 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.