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Calibration of orifice meters at low Reynolds numbers

University of British Columbia

Seventeen square-edged orifice plates were calibrated to study the effect of varying throat length, both in the absence and in the presence of geometrical similarity, on the coefficient of discharge for pipe Reynolds numbers from 20 to1 10,000. The experiments involved the use of hydraulically smooth pipe in test lines of 1 - inch, 1.5 - inch and 2 - inch nominal I.D., corner pressure taps and Beta ratios (orifice diameter/pipe diameter) of 0.2, 0.4 and 0.6. Six of the plates were built using design criteria recommended by the A.S.M.E. in "Fluid Meters" (1), and with Beta ratios specified at 0.4. Each test line was used with two plates, which were identical except for a variation in throat length within the range of the given design recommendations. A comparison of discharge coefficients as a function of pipe Reynolds number indicated the following: (1) Reynolds numbers 30 - 3,000. Consistent differences of 2-6% were observed in the calibration curves due to the variation in throat length allowed by "Fluid Meters".(1). (2) Reynolds number 3,000 - 10,000. Geometry appeared to be much less important, as all calibration curves tended to coincide. Two further plates, for the 1.5 - inch pipe only, were built using the A.S.M.E. "Power Test Code" (2) as a design reference. They were designed to have respectively the minimum and maximum throat length allowable under the code. The results, when plotted, indicated that in a pipe Reynolds number range of 40 - 1,500, plates built identically except for small differences in throat length, still gave distinguishably different calibration curves. The remaining nine plates were divided into three groups of three plates each, encompassing the three pipe diameters and three Beta ratios. All plates within a group were geometrically similar. The results, for pipe Reynolds numbers from 20 to 2,000, indicated that a total specification of orifice shape gave consistent calibration curves with no apparent absolute size effects. Thus, both "Fluid Meters" *and the "Power Test Code" recommended design criteria for standard orifice plates which do not completely take into account the effect of geometry on the coefficient of discharge at low Reynolds numbers. In particular, the tolerances allowed on throat length are too large.

Text

2011-10-13

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http://hdl.handle.net/2429/37937

Chemical and Biological Engineering

University of British Columbia

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