POWER February 2011 - 56

INSTRUMENTATION & CONTROL
Wwo
Ww
= specific weight of water in the reference
leg between the taps (lb/ft3
)
= specific weight of water at 68F that is
used to calibrate the transmitter (lb/ft3
)
This formula, the basis for the boiler steam
drum pressure compensation algorithms that
are available for use in most digital control
systems, illustrates that the specific weight
of the water in the reference leg (Wwo
) is an
important part of the compensation calculation.
Figure 4 illustrates the actual water and
insulate the lines connecting the temperatureequalizing
column to the drum. However,
if they were to do a really good job of insulating
the column itself, the steam would
probably never condense to form the desired
reference leg, or the height of the reference
leg would be constantly changing with pressure
and would become unpredictable. Each
manufacturer of these columns had its own
specific requirements for column insulation.
Because the temperature-equalizing column
is somewhat cooler than the drum, the
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.
steam conditions when a particular type of
temperature-equalizing column is used to
develop the reference leg for the differential
pressure measurement. Figure 4 is representative
of past temperature-equalizing column
designs of Bailey Meter Co. and Republic
Flow Meters.
In Figure 4, the water in the temperatureequalizing
column that surrounds the lower
part of the reference leg is in balance with
the water in the steam drum. Water may seek
its own level; however, in this specific application
the water in the column is somewhat
cooler than the water in the drum, so it will
take a shorter column of this cooler water to
be in balance with the water in the drum.
Of course, plant operators would want to
steam that sits over the balancing water in the
figure is wet steam (of a low steam quality).
The steam is wet because that steam's pressure
is the same as the pressure in the drum,
but the temperature is less than saturation
temperature; basically, it is raining in the column
in the space above the balancing water.
This wet steam condition doesn't have
much impact on the pressure compensation
equation because the water and steam in the
column is in balance with the water and steam
in the drum, so the sensing leg pressure at the
drum level transmitter should still be representative
of the level of water in the steam drum.
The unknown in the pressure compensation
equation is the temperature of the reference
leg. An assumption must be made as to what
4. Getting a leg up on pressure measurement. Temperature-equalizing column
steam and water conditions. Source: Southern Co.
this reference leg temperature is, and that assumption
must be made based on the design
of the temperature-equalizing column.
Reducing the Measurement Error
Bailey Meter Co. and Republic Flow Meter
column designs and insulation approaches
were to keep the reference leg as close to
saturation temperature as practical. In contrast,
the Yarway Corp. design and insulation
approaches were quite a bit different, as that
company's philosophy was to keep the reference
leg temperature approximately halfway
between saturation and ambient temperature,
according to David A. Kalix, PE, who works
for Yarway and covered the topic in his 1995
ISA paper, " Density Error and Its Correction
in Boiler Drum Level Indication. "
A reference leg temperature measurement
can be retrofitted into an existing installation
like that shown in Figure 1 by adding
a thermocouple that is in contact with the
tubing skin of the reference leg. To carry
out this retrofit, however, this thermocouple
would need to be attached at an elevation
that is midway between the top and bottom
connections of the steam drum. If the reference
leg is heat-traced for freeze protection
purposes, it would be important to place the
thermocouple on the opposite side of the reference
leg tubing from the heat tracing. This
thermocouple and the reference leg tubing
adjacent to it should then be insulated and
weatherproofed. The thermocouple can then
be brought into the control system and incorporated
into the drum level measurement
pressure compensation algorithm.
A reference leg temperature measurement
Reference leg
Wet steam
Steam
Water
Balancing water
Temperature-equalizing column
can also be retrofitted to an existing temperature-equalizing
column installation (see Figure
4 and the right side of Figure 2). In this
case, the thermocouple would be attached to
the column skin midway between the top and
bottom column connections, and insulation
would be placed over the top of it. In the case
of a Yarway temperature-compensated constant
head chamber such as the one shown on
the left side of Figure 2, a thermocouple must
be placed in direct contact with the reference
leg (also called the constant head pipe) that is
underneath the stainless steel jacket.
This exact situation occurred last year durSensing
leg
Reference leg
dPT
56
Drum level transmitter
www.powermag.com
ing a scheduled outage at two of our steam
plants. In those two cases, plant managers decided
to add reference leg thermocouple measurements
and to bring those signals into the
control system. The data collected since these
two units were brought back online show that
when the units are operating near their 1,900
psig normal operating pressure, the three Republic
columns that are in place between these
two units (right side of Figure 2) are within
15F of the saturation temperature. This conPOWER
| February 2011
http://www.powermag.com
POWER February 2011

Table of Contents for the Digital Edition of POWER February 2011
