Flow

There are several different flow terminology conventions in common use. The following discussion is presented in order to eliminate any confusion this may cause.

The most important thing to remember in compressor calculations is that compressor flow is a volumetric value based on the flowing conditions of pressure, temperature, relative humidity (if moisture is present), and gas composition at the compressor inlet nozzle. The flow units are inlet cubic feet per minute (icfm).

Process calculations, where material balances are performed, normally produce flow values in terms of a weight flow. The flow is generally stated as pounds per hour. Equation 2.10 can be used either with a single-component gas or with a mixture.

Pipeline engineers use the flow value stated as standard cubic feet per day. This is an artificial weight flow because flowing conditions are referred to a standard pressure and temperature. The balance of the flow specification is then stated in terms of specific gravity.

A common method of stating flow is standard cubic feet per minute where the flowing conditions are referred to an arbitrary set of standard conditions. Unfortunately, standard conditions are anything but standard. Of the many used, two are more common. The ASME standard uses 68°F and 14.7 psia. The relative humidity is given as 36%. The other standard that is used by the gas transmission industry and the API Mechanical Equipment Standards is 60°F at 14.7 psia. As can be seen from this short discussion, a flow value must be carefully evaluated before it can be used in a compressor calculation.

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