POWER June 2011 - 53

INSTRUMENTATION & CONTROL
6. Curbing disturbances. Changing the override to a bias function and adding a decoupling
function to this control strategy minimizes disturbances to HP drum levels during override
conditions. Courtesy: Siemens Energy Inc.
HP drum level
LI
-
+
PI
HP steam
flow
FI
+
-
+

PI
3-element
No
T
f(X)
f(X)
+
+
-

A
T
A
HP valve output
f(X)
f(X)
Pump master
speed control
shown in this scheme for simplicity but could
also be added as an additional override.
The level control demand output for the
HP drum, whether single- or three-element,
is passed to both the HP valve and the pump
VFD. As long as the IP valve stays below
85% open, this output is a simple split range,
with the valve opening from 0% to 100%,
then increasing the pump from minimum
to maximum from 100% to 200%. A hidden
direct-acting controller (error = process
variable - setpoint) monitors the IP valve demand
with a fixed setpoint. The output of this
controller passes through the " high select "
function to the VFD. As the valves respond
to the increased flow by throttling, the override
controller output returns to zero, which
allows the HP valve to open completely and
the VFD to modulate to maintain level. The
master speed control drives the speed controls
for both pumps.
This control scheme has the advantage of
being simple yet efficient; however, it does
June 2011 | POWER
retain some coupling between IP and HP
drums. As a further modification, changing
the override to a bias function and adding a
decoupling function to this control strategy
would minimize disturbances to HP drum
level during override conditions. This modified
strategy is illustrated in Figure 6. As in
the previous diagram, the IP drum level is
omitted for simplicity. To account for this, a
second proportional integral controller would
be added to the scheme with IP feedwater
valve position as its controlled variable. The
two positioning controllers would pass to the
VFD via a high select, with decoupling biases
to each valve.
Process Aspects: Pressure Variations,
Drum Swell, and Stability
During start-up, the drum level controls can
be a challenge for any operator. The operator
and controls have to contend with drum level
shrink/swell; the drum level swings due to
the interaction of the steam pressures and the
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A

T
A
+
Yes
f(X)
+
+
PI
HP feedwater
flow
FI
A
IP valve output
-
+
-
A
PI
85%
drum levels. Providing stable drum levels is a
key part of any full start-up automation.
Operators use different strategies to deal
with the so-called " drum-level rodeo. " This
sometimes entails continuous blowdown of
the drums to ensure that the feedwater is
continuously flowing, mitigating the effect
of feedwater cycling and temperature
differential between the water entering the
drum and drum water/steam temperatures.
However, this requires continuous attention
by the operators.
Pressure variations also affect the drum
level through the compressibility of the
water-steam mix, and cause steam flow
variation leaving the drum. This means any
instabilities or cycling of the bypass systems
are also reflected in the drum level. In
addition, variations in the rate of pressure
change affect the drum level. This effect is
also utilized by operators when manually
influencing the drum level.
Bypass strategies have been implemented
that stabilize drum level controls, and
pressure-raising strategies have been used
to stabilize drum level for cold through hot
starts. In addition, including the pressure
dynamics in the drum level controls has
proven to be a factor in stabilizing drum
level controls during start-up. Proper tuning
of the controls is also a must.
Because the boiler feed pumps usually
supply water for attemperation of steam
supply and bypass lines, changing the supply
pressure may affect these temperature
controls, requiring further coordination.
One example might be a pressure control
feedforward to the temperature controls;
decreasing boiler feed pump discharge
pressure increases spray water demand.
Simple, Yet Complex
Even a relatively simple start-up process
can have many steps and delays for operator
cross-checks. Automation can simplify
the procedure for the operator while reducing
start-up time. In addition, more sophisticated
controls techniques may be applied to improve
process stability, reduce energy usage,
and increase operating flexibility.
This article was written based on case
study guidelines provided by the Electric
Power Research Institute. The control diagrams
created for this case study are available
in PDF format from the authors, via
email request (george.thackston@siemens
.com). Portions of this article were presented
at the 2010 ISA POWID Symposium. ■
-Steven Leibbrandt (steven.leibbrandt@
siemens.com), chief technology officer,
and Bill Thackston (george.thackston@
siemens.com), principal engineer, work for
Siemens Energy Inc.
53
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POWER June 2011

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