Flow Measurement & Control


Ultrasonic flowmeters as accurate as mag meters

June 2001 Flow Measurement & Control

Calibration tests were carried out at Eskom Enterprises - Technology Services International (TSI) Flow Laboratory (SANAS Accredited Calibration Laboratory, No.901). Nine readings were taken on a 600 mm line at various flow rates, with errors varying from +0,21 to +0,93% of measured value. The measurements were consistent, reflecting a systematic error. As shown on the graph, a straight line has been fitted through the errors and the systematic error has been subtracted from the results obtained at the flow laboratory. The resultant errors as stated in the calibration certificate now fall in a tight band well within the uncertainty of calibration of ±0,23% as stated in Eskom's certificate. N&Z's conclusion is that had this correction been done (and the Fuji meter has a built-in facility to do just this by removing the offset and slope errors) measurement accuracies of ±0,23% could have been obtained.

This clearly demonstrates that with a spool piece of known parameters (material, outside diameter and pipe thickness), the Fuji ultrasonic flowmeter is accurate. Of course, for on-site applications there is uncertainty of pipe parameters and the possibility of the pipe being corroded or coated internally. The solution to this application uncertainty is to carefully measure the pipe parameters where possible; alternatively insert a suitable spool piece of known parameters. (The spool piece approach is inherent in mag flowmeters, which normally consist of a spool piece on which the coils and electrodes are mounted.)

A major advantage of the Fuji ultrasonic flowmeter is that the sensors are completely nonintrusive and can be removed at any time without interrupting the flow. All Fuji's sensors and converters are interchangeable, so if there is a failure due to, for example, a lightning strike, the damaged component can simply be exchanged. If laboratory calibration is required, the sensors and converter can easily and inexpensively be removed without interrupting the flow.

N&Z points out that this flowmeter also has major price advantages especially for larger lines. For example, the company says that in the case of a 900 mm line, the Fuji ultrasonic meter would be less than half the cost of a typical mag meter.

Ultrasonic flowmeters have been proven to be as accurate as mag flow meters if installed with the necessary caution, and offer great benefits in terms of maintainability and plant downtime.

N&Z Instrumentation & Control

(011) 435 1080

[email protected]

www.nz.co.za



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