|David W. Spitzer, P.E.|
You are instructed to purchase a positive-displacement flowmeter for a liquid flowing continuously at approximately one gallon per minute (approx. four liters per minute). What size flowmeter would you purchase?
As is often the case, the answer is dependent upon a number of factors, many of which were not presented in the question. The most important factor that was not presented is the viscosity of the flowing stream.
If the viscosity is low, such as the case of water with a viscosity of approximately 1 cP, it is likely that the 0.5-inch flowmeter (Answer A) would pass the flow with a reasonable pressure drop.
Note that the key concept to consider is the pressure drop across the flowmeter. This is because as viscosity increases, the pressure drop across the flowmeter tends to increase. Increasing the pressure drop increases stress on the bearings and can reduce their life. When the pressure drop becomes excessive, flowmeter size may have to be increased to reduce the pressure drop to a level that is within the specified limits of an acceptable pressure drop across the flowmeter.
The flowmeter size is also influenced by the geometry of the flowmeter because some designs tend to exhibit a lower pressure drop at a given viscosity than other designs.
Increasing liquid viscosity also has the effect of decreasing slippage through the flowmeter at low flowrates. Whereas the performance of most flowmeters degrades at low flowrates, positive-displacement flowmeters maintain their accuracy in high viscosity applications due to reduced slippage at these flowrates.
Therefore, liquid viscosity considerations can increase the size of the flowmeter so that any of the answers could be correct.
Additional Complicating Factors
Further impacting positive-displacement flowmeter capacity, and hence sizing, is the lubricity of the liquid. Liquids that lubricate well tend to extend bearing life so the flowmeter can tolerate higher continuous flowrates as compared to “dry” liquids that do not lubricate well. Information about the lubricity of various liquids is often difficult to locate such that bearing life is often determined by actual operating experience.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at email@example.com or 845 623-1830.