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 non-horizontal 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.
This article originally appeared in Flow Control magazine.