|David W. Spitzer|
A thin concentric orifice plate is installed in a 2” Schedule 40 pipe flowing 0 to 100 liters per minute of a liquid with a specific gravity of 1.00 and a viscosity of 20 cP. The differential-pressure transmitter is calibrated for 0-1,000 mm of water column. What differential pressures are produced by the orifice plate at 25 percent and 50 percent of full-scale flow?
A. 63 and 250 mm of water column
B. 63 and 500 mm of water column
C. 250 and 500 mm of water column
D. None of the above
Orifice plate flowmeters generate a differential pressure that is proportional to the square of the flowrate. In other words, 50 percent of full-scale flow will generate 0.5 * 0.5, or 25 percent of the full-scale differential pressure. Similarly, 25 percent of full-scale flow generates 0.25 * 0.25, or only 6.25 percent of full-scale differential pressure.
The full-scale DP in this orifice plate application is 1,000 mm of water column. Due to the square-root relationship between flow and DP, the DP transmitter generated would be 62.5 and 250 mm of water column at 25 and 50 percent of flow respectively Answer A appears to be correct.
Additional Complicating Factors
However, this question inquires about the differential pressure produced—not the differential-pressure transmitter calibration. Reynolds number at full-scale flow is approximately 2,000. Therefore, the orifice plate is operating well into the laminar flow regime where the squared relationship between flow and differential pressure does not apply. Answer C might be somewhat applicable, but I suggest that Answer D might be more pragmatic. Regardless of the answer chosen, this flowmeter appears to be misapplied.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.