Fluid Flow Measurement Explained in Detail

​Measurement of fluid flow is important in process industries for various reasons, including quality control, asset management and to ensure safety during the process, among other reasons. Fluid flow can be measured in terms of mass, i.e. kilograms per second, or in terms of volume, i.e. liters per second. Fluid flow is mostly measured using the method of positive displacement. This simply means counting the number of times a fluid passes through a pipe of known volume. The device used to measure fluid flow is known as a fluid flow meter. Different flow meters are used for different applications. Some flow meter applications include:

Building automation as in HVAC systems and water heaters Measurement of fluids in chemical processes such as production of sulfuric acid and measurement of hydrofluoric acid in the production of Teflon Food and beverage processing, for example, measurement of fluid flow in breweries, measurement of fluid flow in fermentation processes, and measurement of fluid flow in bakeries Measurement of fluid flow in oil and gas production

How is it possible to measure gas flow?

Gas flow can be measured because all gases are compressible and the volume of a gas can therefore be accurately measured. Pressure and temperature affect the volume of gases. Gas flow measurement is therefore made in reference to the prevailing conditions. Some of the flow meters that can be used to measure gas flow include thermal mass flow meters and Coriolis mass flow meters, among others. For gases, it is better to measure flow in terms of mass as opposed to volume, as mass is not affected by changes in pressure and temperature. If a device that measures volumetric flow, such as a turbine flow meter or a variable area meter, is used, the results may be inaccurate as it will not give allowances for the temperature and pressure variations.

Mass flow meters versus volumetric flow meters

As explained above, the volume of a fluid is affected by changes in temperature and pressure. Therefore, when a volumetric flow meter is used, additional equipment is needed to adjust the readings for the temperature and pressure variations. This means it is more expensive and cumbersome to use a volumetric flow meter compared to a mass flow meter. Secondly, the readings taken from a volumetric flow meter must be converted to mass flow. It is for this reason that mass flow meters are also referred to as direct flow meters.

However, volumetric flow meters work just fine for applications where the pressure and temperature are known and stable or regulated. Apart from the Coriolis mass flow meter and the thermal mass flow meter, other examples of mass flow meters are ultrasonic meters and rotometers. Apart from turbine flow meters and variable area meters, other examples of volumetric flow meters include orifice plates, Pitot tubes and vortex meters.

Choice of fluid flow meters

There are many factors that need to be considered in choosing a flow meter for a given application. The best way to go about it is to do thorough research and seek guidance from the experts in case you are not particularly knowledgeable on the subject.