POWER February 2011 - 55

INSTRUMENTATION & CONTROL
2. Golden oldies. A 1950s vintage temperature-equalizing column from Yarnall-Waring (Yarway)
on the left and Republic Flow Meters on the right. Courtesy: Southern Co.
of the reference leg can flash to steam on
a rapid pressure decrease (load increase),
and this reaction will result temporarily in a
false high-level measurement, which adds to
the boiler swell effect. The design of some
of the columns may have been less susceptible
to this flashing phenomenon because
the intent was to keep the column at a constant
temperature, meant to be somewhat
less than the drum saturation temperature.
Measurement inaccuracy can also be related
to the use of these columns because the temperature
of the reference leg requires an assumed
temperature.
Manufacturers began discontinuing sale
of these instruments in the 1960s, but they
can still be found in service in many power
plants. Temperature-equalizing columns of
two different manufacturers were found in
the piping for two of the steam drums at one
of our Southern Co. facilities. Photographs of
these devices, with the insulation removed,
are shown in Figure 2.
3. Measuring different water weights. Steam drum level measurement specific
weight differences will cause errors in boiler water elevation measurement if they are not corrected.
Source: Southern Co.
Steam at saturation temp (Wsd)
Water at saturation temp (Wwd)
Hs
Hw
H
Water at drum
saturation
pressure but
at a cooler
temperature
(Wwo)
Water at the same
temperature in
both legs
Drum level transmitter
ature-related inaccuracies in measurement
were usually addressed by calibrating the
drum level transmitter so that it was correct
for the drum normal operating pressure and
for an assumed ambient reference leg temperature
(usually 100F). This was usually good
enough for lower drum pressure applications,
but as steam drum pressures increased, mechanical
solutions had to be found to better
deal with this phenomenon.
A number of manufacturers addressed
this problem with a two-pronged approach.
To accommodate changing drum pressures,
the differential pressure measurement device
would also need to include a pressure measurement
bourdon tube or helix coil. This
additional device was connected to linkage
that used drum pressure to change the gain
applied to the spindle-induced movement of
the level measurement pointer. The Bailey
LH meter used a Class 13H helix coil comFebruary
2011 | POWER
dPT
pensator mechanism for pressure compensation.
Yarway Corp. used a bourdon tube in its
pointer gauges to perform the same function.
To address issues with changing ambient
temperatures, manufacturers developed enhanced
versions of the reference leg reservoir.
These enhanced reservoirs were known
as temperature-compensated reference columns,
temperature-equalizing columns, or
temperature-compensated constant head
chambers. Other manufacturers may have
had other names for these devices, but for the
purpose of this article they will be referred to
as temperature-equalizing columns.
Concerns About TemperatureEqualizing
Columns
Many of these temperature-equalizing columns
can be problematic because during
boiler operation they keep the reference leg
at a high temperature. As a result, a portion
www.powermag.com
Pressure Compensation Equation
For all types of steam drum differential pressure
(dP)-based level measurements, the dP
measured by the level transmitter is inversely
related to the level of water in the steam
drum. As the water level goes up, the sensing
leg water column approaches the height
of the reference leg water column. When the
water level in the steam drum reaches the
same elevation as the top of the reference
leg, the two legs will be the same height, and
there will be a differential pressure of zero
between the two legs of the transmitter if the
legs are both at the same temperature (Figure
3). But, as stated previously, these legs
are not usually at the same temperature for
steam drum level measurements, and that is
the primary source of the error related to this
measurement technique.
Using the arrangement shown in Figure
4, the differential pressure measured by the
steam drum level transmitter is represented
by the following equation:
(H x Wwo
dP =
) - [(Hw
x Wwd) + (Hs
x Wsd
)]
_______________________________
Ww
where:
dP = transmitter-measured differential (H2
Hs
Hw
Wsd
Wwd
O)
H = height of the reference leg between the
taps (inches)
= height of the column of steam in the
drum (inches)
= height of the column of water in the
drum (inches)
= specific weight of steam at drum saturation
pressure (lb/ft3
)
= specific weight of water at drum saturation
pressure (lb/ft3
)
55

http://www.powermag.com

POWER February 2011

Table of Contents for the Digital Edition of POWER February 2011