Real vs. Ideal: Compressibility is the Key
In the domestic natural gas industry, "real" standard cubic feet is the volumetric unit referenced by the American Gas Association (AGA) and most natural gas contracts. A real gas volume is distinguished from its counterpart, an "ideal" gas volume, by the compressibility factor (Z), which is the basis for converting between the two units of measurement. Although the Z factor is an adjustment in flow computer calculations, many other important applications beyond the flow computer also require compressibility correction. As a result, a practical understanding of real gas volumes is essential for accurately evaluating your system and quantifying production.
One notable application is plant balancing. In order to accurately quantify liquid volumes produced from a gas stream, physical constants from GPA 2145 (Table of Physical Properties of Hydrocarbons and Other Compounds — from the GPA Midstream Association, formerly the Gas Processors Association) are used in conjunction with ideal rather than real gas volumes. Applying the physical constants to real volumes would result in inaccurate product quantities.
In addition, formulas from GPA 2172 (Calculation of Gross Heating Value, Relative Density, Compressibility and Theoretical Hydrocarbon Liquid Content for Natural Gas Mixtures for Custody Transfer) often require a compressibility correction. These include:
- Relative Density
- Heating Value
- Liquefiable Contents (GPM, the abbreviation for gallons of liquid per thousand cubic feet of gas)
In each of these cases, failing to account for Z results in misstated physical quantities.
Understanding when and how to convert between real and ideal gas volumes is critical for a range of industry applications. Although flow computers are programmed to automatically calculate and apply the compressibility factor, incorporating Z into plant balances and other more standardized gas quality calculations is often required.
Industry standards offer guidance, but may not specifically address compressibility for every application. For additional insights and guidance related to real gas volumes and the compressibility factor, be sure to consult with your company’s measurement experts, whether in-house or third-party.
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