Quiz Corner: Orifice plate flowmeter transmitter calibration

An orifice plate is sized to produce a differential pressure of 0 to 100 inches of water column (WC) corresponding to zero to full-scale flow respectively to measure steam flow through a nominal 200# steam header where the flowmeter is located approximately 25 feet above grade. The differential pressure transmitter will be mounted on a pipe stand for convenient access. What is the approximate bench calibration of the differential pressure transmitter?

A.   -2.5 to 97.5 inches of water column

B.   0 to 100 inches of water column

C.   2.5 to 102.5 inches of water column

D.   240 to 340 inches of water column

The steam header is located approximately 25 feet above grade. Transmitters are typically mounted approximately 4.5 feet above grade. It was previously shown that the pressure at the transmitter is higher than the steam header pressure due to a fixed condensate leg that forms during operation. However, this increase will not affect the differential pressure measurement because the same added pressure will appear on both taps of the differential pressure transmitter and be subtracted out by the transmitter. Answer D is not correct.

Answer B would be correct if the condensate legs are of equal height and equal temperature.

Additional complicating factors

The differential pressure transmitter can shift if it is rotated such that its taps are not at the same elevation. A 90-degree rotation causes approximately 2.5 inches of shift as suggested by Answer A and Answer C. The direction of the shift depends on the direction of rotation.

The above analysis presumes that the height of the condensate legs is the same. However, if the steam header is vertical, the flowmeter taps will be at different elevations, thereby forming condensate legs with different condensate heights. In some installations, an inverted U-tube is used to physically make the two condensate legs of equal height and eliminate the shift.