Condensible vapor applications (such as steam flow measurement) have traditionally been treated
similarly to liquid measurement applications. Here, condensed liquid will collect in the transmitter’s
impulse lines so long as the impulse lines are cooler than the vapor flowing through the pipe (which
is typically the case). Placing the transmitter below the pipe allows vapors to condense and fill the
impulse lines with liquid (condensate), which then acts as a natural seal protecting the transmitter
from exposure to hot process vapors.
In such applications it is important for the technician to pre-fill both impulse lines with condensed
liquid prior to placing the flowmeter into service. “Tee” fittings with removable plugs or fill valves
are provided to do this. Failure to pre-fill the impulse lines will likely result in measurement errors
during initial operation, as condensed vapors will inevitably fill the impulse lines at slightly different
rates and cause a difference in vertical liquid column heights within those lines.
It should be noted that some steam flow element installations, however, will work well if the
impulse lines are above the pipe. If such an installation is possible, the advantage of not having to
deal with pre-filling impulse lines (or waiting for steam to condense to equal levels in both lines) are
significant. For more information, I recommend consulting the Rosemount whitepaper entitled “Top
Mount Installation for DP Flowmeters in Steam Service” (document 00870-0200-4809 first published
August 2009).
If tap holes must be drilled into the pipe (or flanges) at the process site, great care must be
taken to properly drill and de-burr the holes. A pressure-sensing tap hole should be flush with the
inner pipe wall, with no rough edges or burrs to create turbulence. Also, there should be no reliefs
or countersinking near the hole on the inside of the pipe. Even small irregularities at the tap holes
may generate surprisingly large flow-measurement errors