|David W. Spitzer|
Flowmeters are geometric devices. By this I mean that flowmeter operation is dependent upon the geometry of the installation. Measurement error can occur by relaxing or ignoring virtually any constraint of any aspect of the flowmeter geometry.
What if the flowmeter is installed backwards? The flowmeter may or may not work. If the flowmeter works, the measurement it provides is suspect and will likely not be accurate because most flowmeters present a different geometry to the flow stream when installed backwards. Bidirectional flowmeters, such as certain turbine and magnetic flowmeters, are exceptions in that they are designed to measure forward and reverse flow. However, it is usually recommended that these flowmeters be calibrated in both directions to meet their accuracy specifications.
What if the flowmeter is located immediately downstream of a control valve? The flowmeter will likely be inaccurate unless the flowmeter is immune to velocity profile effects. Finding a control valve located upstream of a flowmeter may indicate that fundamental problems exist and suggest a need for investigation of the entire process, application and installation.
What if the flowmeter is not located in the center of the pipe? Again, the flowmeter may exhibit measurement error due to the relaxation of a geometrical constraint. For example, the differential pressure produced by a concentric-orifice plate that is installed with an offset from the center of the pipe may be different than if the orifice plate were installed within acceptable tolerances of center.
The flow measurement system includes the upstream straight run, flowmeter, downstream straight run, and their appurtenances. The flowmeter itself is only one component in the flow measurement system that, if installed in a proper flow measurement system, should measure accurately.
How well have you checked your flowmeter installations?
David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.