Difference Between Actual, Standard And Normal Flows

Gas density changes with pressure and temperature, so the use of standardized volumes when referring to quantities of gas is mandatory.

Standardized volumes are commonly used in different types of industries which uses gas as raw material. The volume of gas at actual pressure and temperature must be converted to standardized volumes.

The two common standardized volumes are standard cubic feet (scf) and normal cubic meters (Nm3).

A standard cubic foot of gas corresponds to 1 cubic foot of gas at 32 °F (0 °C) and 14.6959 PSI, and a normal cubic meter of gas corresponds to 1 cubic meter at 20°C at 101.325 kPa (NIST Reference). We will see later that it is important to list the temperature and pressure being used as standard, since many standards are used worldwide.

Why we use the standardized flows instead of simple volumetric flow?

We use standardized flows to make calculations easy.

For example, if we are compressing 125 Nm3/min air from 1 bar to 25 bar pressure then the volumetric flow through the compressor will remains same in Nm3/min but this is not the case for the volumetric flow in actual conditions.

The gas being compressible, it is possible to change a volume of the same amount of gas by compressing or changing its temperature.

It becomes very difficult to define a quantity of gas volume without relating this value with the pressure and temperature when the volume was measured.

Usually is not easy to define the temperature and pressure of a volume of gas, this is why normally we use standardized volumes referred to a define set of temperature and pressure measurements. We call this conditions normalized or standard.

1.1. STP vs NTP

• STP means Standard Temperature and Pressure.
• NTP means Normal Temperature and Pressure.