Part II: The pros and cons of velocity meters for volumetric flow measurement

Editor’s Note: This is Part II in a five-part series discussing the types of technologies that can be applied to measure the flow of raw materials.

Positive displacement flowmeters that measure the actual volume of the fluid passing through the flowmeter (volumetric flow) were discussed in Part 1 of this article series.

Some flowmeters measure the actual velocity of the fluid passing through the flowmeter from which the volumetric flow can be calculated by multiplying the velocity times the (known) cross-sectional area of the flowmeter. Turbine and vortex-shedding flowmeter technologies can be used to measure the velocity of liquids, gases and steam, however not all designs are so suited. Magnetic flowmeter technology is constrained by its principle of operation and can only measure the flow of liquids.

READ ALSO: Part I—Considering the Pros & Cons of Volumetric Flow Measurement

Fluid density can affect flow measurements in a manner similar to that exhibited by positive displacement flowmeters. Variations in gas pressure, temperature and/or composition can cause the density of a given amount of gas to vary and affect the measurement. Pressure and/or temperature compensation is often applied to correct the flow measurement for these changes. Liquid applications generally exhibit smaller such variations that may or may not be corrected.

Whereas positive displacement flowmeters utilize moving parts to entrap fluid, some flowmeters that measure velocity have no moving parts. Notable examples are magnetic and vortex shedding flowmeters. The elimination of moving parts generally increases flowmeter reliability by reducing the possibility of inaccurate operation and/or flowmeter failure due to bearing wear. The possibility of plugging the flowmeter is also reduced. However, flowmeters that measure velocity may have other failure modes (such as velocity and conductivity constraints) that positive displacement flowmeters do not exhibit.