Page 443 - Boiler_Operators_Handbook,_Second_Edition

428 Boiler Operator’s Handbook

cause they have to heat up before the thermal element so mitters is they are not designed to be mounted on hot
there’s no reason to install them where they aren’t neces- ductwork and piping. Unless I’m certain the fluid in the
sary. Some process applications don’t use thermowells piping will not be too hot and the transmitter will not be
to achieve faster response time. Many thermowells are heated by another source I insist on mounting the tem-
filled with a grease or other compound to improve heat perature transmitter away from the probe on another
transfer between the well and the element. support attached to the building structure.
I prefer temperature transmitters to recorders or That requires the temperature element be fitted
controllers that are directly connected to the sensing with extension leads long enough to reach the transmit-
element. Both RTDs and thermocouples require more ter. I have long specified three feet as a requirement
expensive wiring than the typical twisted shielded pair for extension leads (except stack temperature elements
required for a transmitter. Exposing that wiring to elec- where I double that) so there’s enough lead to conve-
tromagnetic fields in the plant can also produce errone- niently locate the transmitter at a platform or grade
ous outputs. where it’s readily accessible, four foot above just like for
By installing local transmitters you eliminate an pressure and flow transmitters.
inventory of special wire and a lot of running back and There are other stories in this book that address
forth when trying to check the calibration of the instru- problems with instrumentation. These comments will,
ment. A local reading of what the transmitter is sensing hopefully, give you the ability to know when the infor-
can be provided by adding a relatively inexpensive mation you are looking at is flawed and what you might
meter on a transmitter. The only caveat with local trans- do about it.