POWER February 2011 - 57

INSTRUMENTATION & CONTROL
firms the assumption that the Republic columns
will be near the saturation temperature
when they are properly insulated. (Note that
the Figure 2 pictures were taken before the
thermocouples were added and before the Republic
column was re-insulated.)
The data also show that when the unit with
the single Yarway column is operating near
its 1,900 psig normal operating pressure, the
reference leg associated with that column
(left side of Figure 2) is about 126F below
the saturation temperature. This shows that
the assumption of the Yarway being halfway
between saturation and ambient is not always
correct, because our measurements show the
reference leg in this column to be operating
three-quarters of the way between ambient
and saturation.
Recommendations for Dealing with
Older Columns Still in Operation
The type of temperature-equalizing columns
plant personnel encounter at their
facilities determine what course of action
should be taken in order to ensure optimum
steam drum level measurements. Here are
our recommendations:
■ Because temperature-equalizing column
designs similar to the Bailey and Republic
columns are prone to flashing, it is recommended
that these and similar columns be
removed from service and replaced with
an arrangement similar to that depicted in
Figure 1, minus the reservoir if the transmitter
has negligible displacement.
■ Because temperature-equalizing column
designs similar to the Yarway columns have
a less-predictable reference leg temperature,
they should either be retrofitted with a
reference leg temperature measurement or
be removed from service and replaced with
an arrangement similar to that in Figure 1.
■ For arrangements similar to Figure 1, the
assumed reference leg temperature, which
is usually 100F, should be investigated and
verified. If that assumption is not reasonable,
then the assumed temperature should
be modified. When the reference leg is outdoors
or if that leg is indoors but the ambient
temperature is expected to vary widely,
consideration should be given to measuring
the reference leg temperature and bringing
that temperature into the compensation
calculation in the plant controls.
This article is based on a paper presented
by the author at the 15th Annual POWID/EPRI
Controls and Instrumentation Symposium. ■
-Dale P. Evely, PE (dpevely@
southernco.com) is principal engineer,
I&C Technical Support, at Southern Co.
in Birmingham, Alabama.
February 2011 | POWER
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