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

Commentary

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.