|David W. Spitzer, P.E.|
Flow measurement systems are often composed of a number of components that can have different performance specifications. This is especially applicable for flowmeters in which the flow measurement system consists of components that are designed and manufactured separately.
Differential-pressure flowmeters are a case in point. Primary flow elements — such as orifice plates, Venturi tubes and flow nozzles — are typically designed and manufactured by one supplier. Another supplier (or a different division of the first supplier) typically designs and manufactures the associated differential-pressure transmitter. The flow measurement system consists of these two components plus other components, such as flanges, taps, impulse tubing and the like. This is in contrast to a thermal flowmeter that is designed and manufactured as one component.
The primary element and transmitter have specifications that describe their performance. Specifications for the primary element tend to be expressed as a percentage of the actual flow measurement, while the transmitter specifications tend to be expressed as a percentage of full scale. In general, the primary element specifications tend to dominate system performance when the flow is high in the flow range, while the differential pressure transmitter tends to dominate system performance when the flow is low in the flow range. Overall flow measurement system performance should take both the primary element and the transmitter specifications into account at the flowrate in question. Therefore, system performance is not the same at all flowrates.
Flow element and differential-pressure specifications can give the user a false sense of the expected performance of the flow measurement system. Flow element performance is often better than one percent of the actual flowrate, but it may be predicated on flowmeter operation above a certain Reynolds number. Failure to comply with this requirement can result in a measurement error that might exceed a few percent depending upon the technology and flowmeter operation. A differential-pressure transmitter touted as having 0.05 percent accuracy can have a measurement error that exceeds two percent of the actual flowrate when the flow measurement system is operated at 10 percent of flow.
There is nothing wrong with purchasing and installing flow measurement systems that consist of a number of components that may be designed, manufactured and purchased from a number of sources. However, the designer should be aware of the issues surrounding the performance of each component as it relates to the entire flow measurement system.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” 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 845 623-1830.