Where and how should a differential pressure flow transmitter be located for liquid service?
A. Above the flow element with the impulse tubes sloping upward to the transmitter
B. Above the flow element with the impulse tubes sloping upward and then downward to the transmitter
C. Below the flow element with the impulse tubes sloping upward and then downward to the transmitter
D. Below the flow element with the impulse tubes sloping downward to the transmitter
Accurately transmitting the differential pressure generated by the flow element to the transmitter involves keeping the impulse tubing full of liquid. Locating the transmitter above the flow element enables gas to be trapped and potentially compressed, whereby it can adversely affect accurate transmission of the differential pressure signal to the transmitter — eliminating Answer A and Answer B.
Upward sloping impulse tubing also eliminates Answer C. Answer D is correct.
Additional complicating factors
In some installations, it is not possible to install impulse tubes that constantly slope downward to the transmitter, so other means may be used to effect accurate transmission of the differential pressure signal. These include diaphragm seals or pots where gas can accumulate without affecting the measurement.
David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.