Page 441 - Boiler_Operators_Handbook,_Second_Edition

426 Boiler Operator’s Handbook

elevation four feet above a floor or platform and readily removed by blowing down the line through the drain
accessible to a person standing on that floor or platform. valve. It’s almost impossible for the dirt to get up into
Sometimes it requires extra piping and installations the transmitter (it will if the transmitter is vented too
where the operator may have to blow down the sensing fast) and, despite some arguments to the contrary, steam
lines a little more frequently. That’s okay though, I don’t will not get into the transmitter when a steam pressure
mind doing something a little more frequently if I don’t sensing unit is blown down.
have to climb all over things to do it. Where the transmitter is located and the fluid
Pressure and differential flow transmitters require sensed has a lot to do with how a transmitter is piped.
piping connecting them to the process line. Some of The diagram in Figure 11-41 is recommended for dirty
those lines require long runs of sensing lines and they liquid systems making it more difficult for solids and
should be installed in a manner that limits problems debris in the system getting into the transmitter.
with the instruments. The most common problem I The piping routed to the process sensing connection
encounter (Figure 11-39) is a transmitter installed at the should always run vertically or at least slope up to the
bottom of a sensing line. Any scale, rust, or sediment connection so any gas that may form in the sensing pip-
that comes drifting down the line ends up inside the ing will naturally rise to the process connection and be
transmitter. replaced by liquid. A little air in a liquid sensing line for
Liquid pressure and differential pressure transmit- flow measurement will introduce a considerable error.
ters should be installed as shown in Figure 11-40 so the If the transmitter is sensing a non-condensing gas
only thing that flows to the transmitter is liquid; the rust (just about anything but steam) the transmitter should
and sediment ends up in the drop leg where it can be be mounted above the process sensing connection and
run in such a manner that anything condensing out of
the gas will run back out of the sensing lines into the
process line. When it’s absolutely necessary to install

Figure 11-39. Improper transmitter installation Figure 11-40. Proper transmitter installation