Manual of Petroleum Measurement Standards Chapter 11.2.5

A Simplified Vapor Pressure Correlation for Commercial NGLs

GPA 8117

SECOND EDITION, DECEMBER 2020

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Contents

Page

1. Implementation Guidelines 1

2. Background 1

3. Vapor Pressure Correlation for Commercial NGLs 2

4. Correlation Development 3

   1. Propanes 6

   2. Butanes 10

   3. Natural Gasolines 13

   4. Light Ends 18

5. Ad Hoc Improvement of the Correlation For Specific Situations 22

Bibliography 24

Figures

1. Vapor Pressures from Correlations 4

2. “A” Parameter [Equation (2) and Table 1]. 5

3. “B” Parameter [Equation (2) and Table 1]. 5

4. Maximum Temperature vs Relative Density 19

Tables

1. Parameters for Vapor Pressure Correlation [Use in Equation (2)] 3

2. GPA Liquefied Petroleum Gas Specifications: GPA Standard 2140-88 6

3. Correlation Parameters for Propanes and Butanes 8

4. SRK Interaction Parameters for Propanes and Butanes 8

5. Compositions and Relative Densities of Propane Samples 8

6. Comparison of Vapor Pressure Correlations for Commercial Propanes 9

7. Compositions and Relative Densities of Butane Samples Data Regression 11

8. Comparison of Vapor Pressure Correlations for Commercial 12

9. GPA Standard 3132-84, Natural Gasoline Specifications and Test Methods 13

10. Grades of Natural Gasoline as Specified by the GPA 14

11. Correlation Constants for Natural Gasolines 14

12. SRK Interaction Parameters for Natural Gasolines 15

13. Compositions and Relative Densities of Natural Gasolines 15

14. Comparison of Vapor Pressure Correlations for Natural Gasolines 16

15. Compositions of Components Used to Generate Data for Light Ends Correlation 19

16. Representative Comparison of Vapor Pressures Obtained from HYSYS

    with Those from NGLCALC 20

17. Representative Comparison Between HYSYS SRK Vapor Pressures

and Vapor Pressures from the Correlation for Light End Fluids 21

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A Simplified Vapor Pressure Correlation for Commercial NGLs

1. Implementation Guidelines

   
   

   This 2nd edition of API MPMS Chapter 11.2.5/GPA 8117 replaces the 1st edition, which was published in 2007. This 2nd edition was changed to correct the examples in the 1st edition to agree with the published equations.

   
   

   Please note that this 2nd edition is effective upon the date of publication and supersedes all previous revisions of the standard/technical publication, including API MPMS 11.2.2A/GPA TP-15.

   
   

   Once this edition is implemented in a particular application, the previous edition will no longer be used in that application.

   
   

   However, the use of API standards 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.

   
   

2. 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 (USC) 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 

(1)

Where:

60 is the relative density at 60 °F and the greater of one atmosphere pressure or the equilibrium vapor pressure of the liquid;

 is the relative density at measured conditions;

Ft is the correction factor for temperature effects; and

Fp is the correction factor for pressure effects.

Two methods used for calculation of the Fp term were standardized by API: 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). Therefore, a correlation for the vapor pressure of NGLs based upon normally measured properties is required.