When Wrong Is Right

David W. Spitzer, P.E. In many situations, there is a right way to do something and a wrong way to do the same thing. In other situations, the right way is incorrect, and the wrong way is correct. This may sound confusing, but bear with me as we examine the process of taking a differential pressure transmitter in and out of service. Differential pressure flowmeters, such as an orifice plate, Venturi, flow nozzle, and the like, are commonly applied in many applications. Operating conditions can vary from low to high pressure and low to high temperature. However, the majority of installations operate at low and medium pressures and low and medium temperatures. Most differential pressure transmitters are installed with some sort of three-valve manifold configuration (see figure below). An integral three-valve manifold is a common option offered by differential pressure transmitter suppliers.

Assuming the transmitter is operating and needs to be removed from service, the bypass valve in the three-valve manifold is opened before the block valves are closed. This process creates a hydraulic jumper across the differential pressure transmitter that protects the transmitter from an overpressure condition. If one of the shutoff valves were to be closed before the bypass valve was opened, pressure would be trapped on one side of the transmitter, and a line pressure change could create an excessively large differential pressure across the transmitter that could cause damage. Putting the transmitter back in service involves being sure the bypass valve is open (to again create a hydraulic jumper across the transmitter) before opening the block valves. Failure to open the bypass valve prior to opening a block valve could expose only one side of the transmitter to full line pressure and could create an excessively large differential pressure across the transmitter that could cause damage. For the overwhelming majority of differential pressure flowmeter installations, the above descriptions represents the correct way to take a transmitter in and out of service. However, the same procedure can be quite dangerous (and hence incorrect) in some installations. The problem is that opening the bypass valve while the transmitter is in service allows fluid to flow in the impulse tubing. This flow results from the differential pressure developed across the flowmeter primary element. This would not generally cause a problem in typical installations, such as water service. However, consider a fluid that operates at high pressure and/or at high temperature, such as steam service. If the transmitter were taken out of service as described above, live steam would flow through the impulse tubing to the transmitter and return to the pipe. This would remove the liquid seal in the tubing and expose the transmitter to live steam that could damage the transmitter due to its high temperature. In addition, the sudden introduction of high temperature fluid into the relatively cool impulse tubing could damage the tubing and cause it to leak. In high-pressure/high-temperature steam service, even a small leak can be extremely dangerous. Further, if a leak does occur for the reasons cited above, it is highly likely the instrument technician will be present and could be injured. Therefore, in this (and similar) service, the block valves should be closed first when removing the transmitter from service. This procedure may seem incorrect and may damage the transmitter on occasion, but it will also help protect the technician from potential harm. About the Author David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830. For More Information: www.spitzerandboyes.com