Metering of Industrial Gases
and Flow Computations
Part 1
General-
Industrial gases are normally measured in terms of equivalent volume units of a specific gas at stated reference conditions of temperature and absolute pressure. One example would be in units of standard cubic feet at 14.696 psia and
60 de­grees F.
Gas metering systems typically include volumetric flow metering devices, pressure and temperature sensors and a
computational device, such as a flow computer or volume corrector.
A variety of flow metering devices can be used to measure the flow rate of industrial gases. Volumetric flow devices
include positive displacement, turbine, ultrasonic, orifice and similar differential pressure meters. Mass flow meters such
as thermal and vortex shedding instruments are also common in industrial flow metering applications.
Some industrial gases can be distributed and stored in a liquid form, then vaporized on-site and distributed under
regulated line pressures within a facility. Here the gas is usually measured in a line at a regulated pressure.
Other gases, such as compressed air, are produced locally on site and distributed under regulated line pressures
within a facility. The compressed air is also measured in a line at a regulated pressure.
Corrected
Gas Volume
Illustration
TOTAL
1
RATE
2
PRE 1
3
TEMP
4
PRINT
5
GRAND
6
SCROLL
7
PRE 2
8
DENS
9
TIME
Pressure
Transmitter
Flowmeter
0
–
CLEAR
MENU
HELP
•
ENTER
Temperature
Transmitter
Behavior of Gases-
“Fluids” is a general term used to describe both liquids and gases. Liquids, such as water, are essentially not com­
pressible. Gases can be compressed by increased pressure, and rarified by increased temperature.
Ideal Gas Law
Early physicists discovered the basic general relationships between volume, temperature and pressure of a gas.
These “laws” are really general relationships and were named after their discoverer as Charles’s Law, Boyle’s Law and
the combined gas law (or ideal gas law).
These general equations, or relationships, are used today to measure gases at low and moderate pressures and varying temperatures and used to compute the equivalent volume (or standard volume) at some stated reference condi­tion
of temperature and pressure.
06/13/11
Page 1
Metering of Industrial Gases
and Flow Computations
Part 1
Real Gases
It has been observed that all real industrial gases deviate from the ideal behavior predicted by these laws. An addi­
tional term, known as the Z-factor (which is a fudge factor), was added to the ideal gas law to enable more accurate
compu­tations to be performed. An ideal gas would have a Z-Factor with a value of 1.0000.
Published values exist for the Z-Factor for all common industrial gases and allow the user to determine the varia­tion
from the ideal gas law as the conditions of line pressure and temperature vary.
Note: It is essential when using gas law basic relationships with a manually entered average Z-Factor that the value of
the assumed Z-Factor in the calculation has been verified as being applicable for the variations in line temperature and
pressure actually to be encountered in the application. Be sure to check the validity of your assumptions!
In most common industrial gas metering applications the line pressure is reasonably regulated and the operating temperature range does not vary over wide extremes. This enables the user to use an average value of Z-Factor to obtain
the desired degree of accuracy.
Consider an example of compressed air where the desired range is -10 to 80 degrees F and the pressure is regulated
between 60 and 100 psig. The Z-Factor in this case would only vary between 0.99352 and 0.99879. If an average value
of 0.996155 were to be assumed, a systematic error would be less than 0.3%.
The non-ideal behavior, as indicated by the Z-Factor for a given gas, becomes more pronounced at higher pressures
and lower temperatures.
Gases Under High Pressure
A number of industrial gases are sold as cylinders of compressed gases. Such a cylinder is normally supplied under
very high pressures (perhaps 3000 psi). The customer will typically use a pressure regulator and use the gas at much
lower pressure. Actual gas metering is most often performed at these lower pressures.
For argon gas at room temperature, the Z-Factor changes by less than 0.8 % from 14.696 psia to 150 psia. However,
the Z-Factor changes by 5.5% at 1500 psi at room temperature. Clearly a more elaborate gas calculation than the ideal
gas law with an average Z-Factor is needed at higher pressures encountered in the actual cylinder filling operations or if
the gas is being metered at cylinder pressure. In these applications, calculations called “Equations of State” are sometimes used.
Gases Near the Liquid/Vapor Boundary
The ideal gas law also poorly represents the behavior of a gas as temperature and pressures approach the liquid-gas
boundary, or two-phase interface. For this reason it is important to know the normal boiling point of an industrial gas
when applying the ideal gas law to a gas measurement. Propane and carbon dioxide are two commercial gases with
boiling points near room temperature.
Gas Mixtures
A gas mixture can sometimes be considered as being composed of a series of individual gas measurements being
computed independently. Such a calculation would assume that the line pressure may represent the sum of the partial
pressures of the various gases present as indicated by their Mole % in the mixture.
Air is a mixture of several component gases including nitrogen, oxygen, argon, carbon dioxide, and a small percent­
age of rare gases. At lower pressures air is treated as a dry gas with its own reference density and Z-Factor representing the actual gas mixture in the atmosphere. A more exact computation would allow for the interaction of the various
gas molecules with each other and would be needed at higher pressures.
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Metering of Industrial Gases
and Flow Computations
Part 1
Humid Gases
Most industrial gases are normally assumed to be metered as “dry” gases with minimal moisture content. The flow
computers or volume correctors similarly make the same assumptions.
In compressed air systems, atmospheric moisture will be drawn in and is normally removed so that the output of the
system can be considered as being “dry”. If humidity is present, and must be considered in the computation of the gas
properties, then more complex computa­tions must be performed. In such cases, the gas may be considered as a mixture of water vapor and the industrial gas.
Humid gases with significant water content are frequently found in landfill gases. In engine research, test cells may
also document atmospheric conditions moisture (or relative humidity) in the air inlet to the engine.
Metering of Natural Gas
Natural gas is a very common energy commodity for domestic and industrial use. Natural gas is actually a mixture of
several gases which typically include methane, heavier hydrocarbons and traces of inert gases such as CO2 and nitrogen.
A variety of very specific computations have been developed for measuring this gas by the natural gas industry. Some
of these AGA (American Gas Association) Standard computations are specific to the type of flow meter being used,
where others are generally applied to the fluid property computation for the Z-Factor or Combustion Heating Value for
the gas. These standardized calculations typi­cally are identified by an AGA reference number.
At the point of use, natural gas is frequently metered at lower pressures. In such conditions basic flow computations
and volume correctors are used. More advanced calculations and requirements for on-line gas analysis are often required into and out of gas pipeline transports, whereas more simplified devices are used at point-of-use. While transiting
pipelines, natural gas is at much higher pressures and more elaborate computations and gas analysis is commonplace.
Some AGA methods include “Gross Characterization Methods” for computation of Z-Factor. Here only the specific
gravity, Mole % CO2 and Mole % nitrogen are required to perform the computation. NX-19 and AGA-8 can, and are,
used for this purpose. AGA-8 allows a greater range of high pressure and more significant Mole % of inert components
then does NX-19 but the two agree within +/- 0.25% for a wide range of temperatures and pressures generally encoun­
tered at low and intermediate pressures.
More detailed methods are termed “Detailed Characterization Methods” for the computation of the Z-Factor and Heat­
ing Value of the Gas. Here the Mole % of each component gas in the natural gas mixture is required. Once again AGA-8
calculation methods are included.
Conclusions
Gas metering computations are much more complex than simple liquid computations. There are a wide variety of
industrial gases and each one brings its own challenges. To achieve accurate flow measurement and flow computations
one must challenge the assumptions based on the gas type to be metered, as well as the line pressure and temperatures encountered at the site.
Published Gas Property Tables can be consulted to examine the actual variations in Z-Factor over the range of line
conditions of temperature and pressure that may be present. An average value of Z-Factor can then be determined and
an estimate made of the errors that will result from assuming an average value of Z-Factor. More advanced calculations
are used by specific industries and for gases at higher pressures to more accurately compute gas flow rates.
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