Physical Properties

Physical Properties Calculator

The Physical Properties calculator determines key gas mixture properties — heating value, specific gravity, specific heat ratio, and viscosity — from a user-defined gas composition expressed in mole percentages. These values are required inputs for most hydraulic, compressibility, and measurement calculations in GASCalc. Note: Except for heating value and specific gravity, all computed properties are referenced to a base pressure of 14.696 psi (101.35 kPa) and a base temperature of 60°F (15.55°C); slight deviations from these base conditions generally do not require adjustment.

Background

Nearly all hydraulic calculations require knowledge of certain gas properties. These properties may include viscosity, specific gravity, and specific heat ratio. Most real-life applications involve working with a mixture of gas components, and computing the properties of a mixture requires a weighted summation of the individual component properties — the specific method depending on the property being calculated.

Viscosity

The viscosity of a gas mixture is a measure of the fluid’s resistance to flow. A higher viscosity indicates greater resistance and more difficulty in moving the fluid; a lower value means less resistance and easier flow. Although an important property, viscosity appears only as a variable in some pipe flow and hydraulic equations — equations that do not use it directly typically assume a representative value.

Heating Value

The heating value of a gas mixture is the energy contained in a specific volume of the mixture — the amount of energy that can potentially be released during combustion. The heating value is not generally used in hydraulic calculations directly, but it is used by some compressibility factor methods to convert energy-based flow units to volumetric units. The Physical Properties calculator reports the dry gross heating value for the mixture.

Specific Gravity

The specific gravity value represents the weight of the gas mixture relative to the weight of dry air. It is customarily computed on an ideal gas basis. Most hydraulic calculations use the ideal gas specific gravity. Some compressibility factor and measurement calculation methods require the “real” specific gravity, which is calculated by adjusting the ideal value using the compressibility factors of the gas mixture and the reference air mixture at the specified base conditions. The air compressibility value used in this adjustment is calculated using the AGA Report No. 8 1992 method.

Specific Heat Ratio

The specific heat ratio represents the ratio of the mixture’s constant-pressure specific heat to its constant-volume specific heat. This value is used by some valve and regulator calculation routines to calculate the speed of sound.

Equations

Heating Value — Gas Processors Suppliers Association 1998

The GPSA 1998 method calculates the gross heating value of the gas mixture using the following equation (Reference 4):

HV = \sum_{i=1}^{n} x_i \, HV_i

HV = \sum_{i=1}^{n} x_i \, HV_i

Where:
HV − Gross Dry Heating Value, Btu/ft³
HV_i − Heating Value for Component i, Btu/ft³
x_i − Mole Fraction for Component i, decimal

The following component heating values are used as the basis for the GPSA 1998 method. All values are for a base condition of 14.696 psi (101.352 kPa) and 60°F (15.55°C).

Additional heating value calculation methods are available, each referencing a different standard: AGA Report No. 8 1992, GPA Standard 2172-86, GPA Standard 2172-09 (with optional C6+ handling via Method 1 or Method 2 of Appendix B.10). The details of these methods are too extensive to reproduce here; refer to the cited references for the complete calculation procedures.

Specific Gravity

GASCalc calculates the ideal gas basis specific gravity using the following equation, referenced to the molecular weight of dry air (Reference 6):

SG = \sum_{i=1}^{n} \frac{x_i \, MW_i}{MW_{AIR}}

SG = \sum_{i=1}^{n} \frac{x_i \, MW_i}{MW_{AIR}}

The real gas basis specific gravity is calculated by adjusting the ideal value using the compressibility factors of the gas mixture and the reference air mixture at base conditions (Reference 1):

SG_R = SG \times \frac{Z_{AIR}}{Z}

SG_R = SG \times \frac{Z_{AIR}}{Z}

Where:
SG − Specific Gravity (Ideal Gas Basis), dimensionless
SG_R − Specific Gravity (Real Gas Basis), dimensionless
MW_i − Molecular Weight of Component i, g/mol
MW_AIR − Molecular Weight of Dry Air, g/mol (28.9625)
x_i − Mole Fraction for Component i, decimal
Z − Compressibility Factor for the Gas Mixture at Base Conditions, dimensionless
Z_AIR − Compressibility Factor for the Reference Air Mixture at Base Conditions, dimensionless

Specific Heat Ratio

GASCalc calculates the specific heat ratio of the gas composition using the following equations (Reference 5):

k = \frac{C_P}{C_V} \qquad C_P = \sum_{i=1}^{n} x_i C_{P_i} \qquad C_V = \sum_{i=1}^{n} x_i C_{V_i}

k = \frac{C_P}{C_V} \qquad C_P = \sum_{i=1}^{n} x_i C_{P_i} \qquad C_V = \sum_{i=1}^{n} x_i C_{V_i}

Where:
k − Specific Heat Ratio, dimensionless
C_P − Specific Heat at Constant Pressure, Btu/lbm·°F
C_V − Specific Heat at Constant Volume, Btu/lbm·°F
C_Pi − Constant Pressure Specific Heat of Component i, Btu/lbm·°F
C_Vi − Constant Volume Specific Heat of Component i, Btu/lbm·°F
x_i − Mole Fraction for Component i, decimal

Viscosity

GASCalc calculates the absolute (dynamic) viscosity of the gas composition using the following equation. The individual molecular weight values are the same as those listed for the Specific Gravity equation above (Reference 8):

\mu = \frac{\displaystyle\sum_{i=1}^{n} x_i \, \mu_i \, \sqrt{MW_i}}{\displaystyle\sum_{i=1}^{n} x_i \, \sqrt{MW_i}}

\mu = \frac{\displaystyle\sum_{i=1}^{n} x_i \, \mu_i \, \sqrt{MW_i}}{\displaystyle\sum_{i=1}^{n} x_i \, \sqrt{MW_i}}

Where:
μ − Dynamic or Absolute Viscosity of the mixture, lbm/ft·sec
μ_i − Viscosity of Component i, lbm/ft·sec
MW_i − Molecular Weight of Component i, g/mol
x_i − Mole Fraction for Component i, decimal

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

Note: Except for heating value and specific gravity, all computed properties are referenced to a base pressure of 14.696 psi (101.35 kPa) and a base temperature of 60°F (15.55°C). If a gas component is missing from the available list, combine its value with a similar component or enter it in the Other field. If the missing component is a significant part of the composition, manually computing properties using the equations in the Equations section above is recommended. The number of decimal places displayed for any calculated item can be set under File > Preferences > Decimals.

References

• American Gas Association, AGA Report No. 8, Compressibility of Natural Gas and Other Related Hydrocarbon Gases, 1992, 1994 Printing.
• Gas Processors Association Standard 2172-86, “Calculation of Gross Heating Value, Relative Density, and Compressibility Factor for Natural Gas Mixtures from Compositional Analysis,” 1986.
• Gas Processors Association, GPA Standard 2172-09, Calculation of Gross Heating Value Relative, Density Compressibility, and Theoretical Hydrocarbon Liquid Content for Natural Gas Mixtures for Custody Transfer, Third Edition, 2009.
• Gas Processors Suppliers Association, Engineering Data Book, Eleventh Edition – FPS, 1998.
• Gas Processors Association, GPA Standard 2145-09, Table of Physical Properties for Hydrocarbons and Other Compounds of Interest to the Natural Gas Industry, 2009.
• Industrial Press, Gas Engineers Handbook, 1965.
• Wiley, Handbook of Engineering Fundamentals – Eshbach, Third Edition, 1975.
• American Gas Association, Gas Engineering and Operating Practices – System Design, GEOP Series Book D-1, 1990.
• Institute Of Gas Technology, Research Bulletin 23, Viscosities of Natural Gas Components, 1953.

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