An orifice plate flow element has shut-off valves located at each pressure tap. Its differential pressure transmitter has a three-valve manifold. The transmitter is to be removed for shop calibration. In what sequence should the valves be opened and closed to safely remove the transmitter and manifold?
A. Close upstream and then downstream shut-off valves
B. Close downstream and then upstream shut-off valves
C. Close high and then low manifold valves, then close shut-off valves
D. Close low and then high manifold valves, then close shut-off valves
E. Open manifold bypass valve then close shut-off valves
First, closing one of the shut-off valves or the low or high three-valve manifold valve will isolate one side of the transmitter while the other side is still connected to the process. If the process pressure changes when one of these valves is closed, a potentially large differential pressure can be created across the transmitter that can damage the transmitter, especially if a process upset causes the line pressure to change dramatically (on only one side of the differential pressure diaphragm).
To avoid this possibility, it would be prudent to protect the transmitter by applying a "hydraulic jumper" across the transmitter by opening the manifold bypass valve. This will cause some fluid to flow from the high tap to the low tap and generate a relatively small differential pressure across the transmitter that can be tolerated.
Additional complicating factors
Opening the bypass first in high-pressure steam service will cause live steam to remove the liquid seal and flow through the transmitter. Not only can this damage the transmitter (because of the presence of live steam in the transmitter), but it also creates a potential hazard for the instrument technician. In this service, Answers A, B, C or D would be safer alternatives but Answers C and D would usually be even safer and more practical.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control.
Spitzer may be reached at 845-623-1830 or via spitzerandboyes.com. Click on the "Products" tab to find his Consumer Guides to various flow and level measurement technologies.