A pressure transmitter with a span of 300 psi will be mounted on a pipe stand for convenient access to measure the pressure of a nominal 200-pound steam header that is located approximately 25 feet above grade. What is the approximate bench calibration of the pressure transmitter?
A. 0 to 300 psig
B. 0 to 300 psia
C. 10 to 310 psig
D. 10 to 310 psia
Steam header pressures in this range are traditionally measured and expressed as gauge pressures so Answer B and Answer D would likely not be correct.
In operation, steam will condense and form a condensate leg in the impulse tube. The transmitter should be calibrated to take this leg of liquid into account to accurately measure the steam header pressure — not the pressure at the transmitter tap.
The steam header is approximately 25 feet above grade. Transmitters are typically mounted approximately 4.5 feet above grade. Therefore, the transmitter calibration at zero steam header pressure should account for a fixed condensate leg of approximately 20 feet, or approximately 10 psi where 2 feet of water column roughly corresponds to 1 psi. Answer C is correct.
Additional complicating factors
Mechanical factors can come into play when the condensate leg is not approximately 20 feet such as can occur when the transmitter is installed at a different height above grade and/or when the pressure tap is located on the top of the pipe that would increase the height of the condensate leg.
In addition, the density of the condensate leg can vary with temperature and affect the accuracy of the pressure measurement.