|David W. Spitzer|
A differential pressure flow element measuring liquid flow is located 3 meters above grade. The impulse tubing should be:
A. Constantly sloped down to the top of the transmitter at grade
B. Sloped down and then up into the bottom of the transmitter at grade
C. Constantly sloped up to the bottom of the transmitter on the floor above
D. Sloped up and then down into the top of the transmitter on the floor above
The impulse tubing in liquid service should be installed so as to be maintained full of liquid at all times. Sloping upward will allow gas to accumulate in the impulse tubing. Therefore, Answer B, Answer C, and Answer D can be eliminated. Answer A is the correct answer.
Additional Complicating Factors
Other factors can significantly complicate this seemingly simple answer. For example, if the liquid is dirty, Answer A will not be acceptable because dirt will likely accumulate in the transmitter. In this case, consideration should be given to directly coupling the transmitter to the flow element, purging the impulse tubes, or eliminating the impulse tubing by using diaphragm seals. Similar techniques can be considered if the transmitter must be located above the flow element because of accessibility issues with the transmitter at grade.
Most impulse tubing can be designed in a straightforward manner—but there are numerous exceptions and complications to be considered.
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.