Normal, Standard and Actual Flow Rates for Gases

They are often used terms, but what is the difference between normal flow rate, standard flow rate and actual flow rate when measuring gas flows?

Actual Flow Rate

Actual flow rate is the actual volume of fluid that passes a given point in a pipe per unit time e.g. m3/hr. This can be a useful measurement to have, however since gases are compressible the volume of gas will vary depending on its pressure and temperature. Therefore it is generally more useful to have a flow rate referenced to a set pressure and temperature, hence the use of Standard flow rate, and Normal flow rate. This allows us to compare different flows existing at different flowing conditions of pressure and temperature. Standard and Normal flow rate are corrections applied to an actual flow measurement based on a given temperature and pressure. The correction is applied using the ideal gas law.

ideal gas law

However, the problem with Standard and Normal conditions is that they have several different definitions depending on the industry you work in, and in the country you work. See below for the most commonly used conversion references.

 

 

Standard Flow Conditions

For standard conditions, the most commonly accepted definition used by engineers is the ISO definition, i.e. 1 atmosphere at sea level (101.3 kPa, 14.696 psia) and 59 oF (15 oC). However don't be surprised to see others used e.g.

standard conditions for gases

 

Normal Flow Conditions

Another reference flow condition is called Normal flow, and this is more commonly used by engineers using the Standard International (SI) metric system of measurement, and 0 oC (32 oF.) as a reference temperature i.e. 101.3 kPa at 0 oC.

normal conditions for gases

 

Avoiding Confusion

The only safe way to guarantee there is no confusion when using normal and standard flow rates is to state what conditions you are using, e.g. 314 standard m3/hr at 15 oC and 101.3 kPa.

Alternatively, consider using mass instead of volume when measuring gas flow rate.

 

Converting Actual Flow to Standard Flow

It is easy to convert between Standard and Actual flows if the densities of the fluid at the actual and the standard conditions you are using are known. It is just a matter of multiplying by the ratio of densities;

actual to standard flow formula

 

Technical Library

The following pages on Control and Instrumentation.com give more of an insight in to the many and various techniques used in flow measurement:

 

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Technical Bookshop

For those who want to read further about the theory of flow measurement and the differing types of flow instrumentation, then the following books from Amazon will be of interest: