A lawyer recently hired me to investigate the "health" of differential-pressure flow measurement system in high-pressure steam service. The flowmeter elements were in operation for about 50 years, whereas parts of the secondary equipment were modified over the last few decades. There were "issues" associated with the system, but the impulse tubing was surprisingly correct.
In hot-condensing vapor service such as steam, the transmitter should be located below a liquid seal. The liquid seal forms a barrier between the hot vapor and the transmitter that protects the transmitter by not allowing hot vapor to contact the transmitter.
Typically, the taps are located on the side of the pipe and the impulse tubing is constantly sloped downward to the transmitter. This configuration allows a liquid seal to form in the impulse tubing without pockets of non-condensable gas that can affect the measurement. Alternatively, the tap can be located on the top of a horizontal pipe with the impulse tubing rising and then constantly sloping downward to the transmitter to form the seal.
In addition, the temperature of both impulse tubes should be the same in nonhorizontal segments of the impulse tubing runs. Different impulse tubing temperatures can result in different liquid densities in each seal and can cause measurement error. The error associated with relatively short impulse tubes will generally be small. However, the error associated with a flowmeter element located 15-30 meters above its transmitter (typical of older installations) can cause significant measurement error.
The two impulse tubes are typically routed to the transmitter in parallel with each other. If they are installed separately, somewhat different routing, supports, and degradation of their insulation can result in measurement error caused by temperature differences between the liquids in the tubing. As a practical matter, impulse tubing of significant length is typically installed in a tubing bundle with common insulation and heat tracing (if needed). This tends to reduce the temperature difference between the liquids in the impulse tubing and hence the measurement errors that the difference can cause. This installation also tends to reduce the installed cost of the impulse tubing.
The impulse tubes that I inspected were installed together in tubing bundles. I did note that there were some slight dips in relatively short horizontal runs where someone appeared to have stepped on the impulse tubing. This is not bad considering the age of the facility.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” 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.
