> why gas metering package is in necessary while we can use only flow
> meter to measure quantity?
You don't say what gas but natural gas is a good example.
Gas is a compressible fluid. A flowmeter will report the gas flow at the process conditions. If those conditions change the volume will change and hence the volume measured.
In utility gas metering for domestic users the temperature effects are neglected and a pressure regulator used to ensure a constant gas pressure.
This allows a low cost meter installation.
However, in industrial natural gas use not only do temperature and pressure effects have to be considered but also the gas calorific value or quality.
So the first requirement is to meter the volume and then convert the measured volume to the volume at standard conditions. The conversion could use PTZ or it could require a densitometer. For quality measurements a gas specific gravity sensor might also be used.
A gas metering package may include a variety of equipment designed to ensure relatively stable temperatures and pressures, provide temperature and pressure measurement and possible density and relative density (gas SG) sensors.
For a good idea of the calculations involved and the various standards you could have a look at the Emerson 7950 gas flow computer manual which will describe the various meter options and corrections.
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