Q: When considering performance statements, what are the key metrics end-users should be monitoring to ensure the flowmeter is going to provide a level of performance consistent with the needs of their application?
A: Flowmeter performance is usually expressed as a percentage accuracy related to flowrate (reading), calibration span, upper range limit (URL), or full-scale value. These all have very different meanings and can skew the customer’s expectations and view of meter performance. So the end-user needs to beware – flowmeter manufacturers express performance accuracy differently. But the required performance accuracy, just like other parameters, should be dictated by the application requirements. The end-user just needs to make sure that flowmeter accuracy is expressed in the same terms when comparing manufacturers’ products.
Another metric worth mentioning when considering performance statements is repeatability. Consistency of performance of any measured variable allows the end-user to “calibrate out” system inaccuracy as long as that error is repeatable.
Q: Why is straight-run piping such a key requirement when it comes to flow measurement applications? How might the availability of straight-run piping affect the flowmeter selection process?
A: The two predominant types of fluid disturbance within a flowing pipe are distortion and swirl. Distortion is usually caused by obstructions within the pipe, such as a partially closed valve or a flange gasket protruding into the pipe. (Obstructions can cause errors of 50 percent or more and greatly affect the flowmeter’s flow coeffiecients.) Swirl is usually caused by pipe bends of various planes. Swirl, of course, is difficult to correct because it is determined by the piping design and installation. Distortion can often be corrected by removing obstructions that are upstream and/or close to the flowmeter.
The location where the flow measurement needs to be made is usually nonnegotiable when determining the type of flowmeter selected. If a flow straightener or flow conditioner cannot be used, generally the flow technology will dictate how many straight-runs upstream and downstream are required for a particular type of flowmeter. The flowmeter manufacturer usually indicates what those requirements are and the application dictates whether the meter needs to be a magnetic, vortex, Coriolis, target, DP, ultrasonic, variable-area, thermal mass, or other type.
The response to this question was provided by David Matherly, product manager of Advanced Product Solutions with Yokogawa Corporation of America. He holds a bachelor’s degree in Applied Science from Arizona State University in Engineering Technology and an associate’s degree in Applied Science from New River Community College in Instrumentation Technology. Mr. Matherly can be contacted via email at: [email protected].
