meter failure, possibly compromise
safety of plant personnel and
equipment, and risk the environment.
Similarly, some fluids are chemically
harmless, but can become corrosive
when small amounts of other
materials are present, perhaps only
contamination. Therefore care must
be exercised. The variety of possible
materials is especially restricted
where complicated and difficult to
manufacture parts are required. A
meter removed from operation with
one process media and placed into
service with a second media that is
incompatible with the first can cause
unwanted chemical reactions that
may result in contamination of process
media or even a catastrophic explosion.
Corrosion of the temperature element
assembly in the first installation may
also be sufficient to cause early failure in
the second installation.
Therefore is it not advisable that
temperature meters be moved from
one application to another. Should it
be necessary, however, then it is vital to
establish the chemical compatibility of
the two media. For detailed information
refer to NEMA-12 specifications for
more information on protection against
corrosion.
T emperature range P lant history
Range represents a vital selection criterion
because it stipulates the maximum
allowable and minimum allowable
operating temperatures the gauge can
detect. Therefore, any temperature
meter that does not meet the required
temperature range requirements for an
application should not be considered. For
instance, thermocouples typically have a
range of -200 to 1600°C. The plant history of the temperature
meter in the same or similar scenarios is
a good measure to use when specifying
a particular instrument. The end user
should evaluate such plant history such
as lifetime cost and ease of maintenance
and interchange. Note that at this stage,
the expertise of the temperature meter
manufacturer does not really affect the
final decision of the meter design to use in
the application .
Issue 32 PECM
11