POWER June 2011 - 52

INSTRUMENTATION & CONTROL
tion as well as monitoring of the feedwater
system; linked pop-up displays provide easy
access to more detailed information; and a
dedicated alarm summary displays a prefiltered
set of alarms related to the feedwater
system for faster reference.
Single Process Graphic. A single consolidated
process graphic encompasses the
major components of the feedwater system,
including the LP, IP, and HP drums. Figure
3 shows typical messages and an operator
prompt.
Control is accessed by faceplates linked to
each dynamic symbol. The sequence (SGC)
and group (DCO) controls are linked here
as well. A space on the page is reserved for
operator messages and prompts during the
sequence. Access to control settings, alarms,
and the linked control diagram is given by the
icons below the title bar. Linking to the online
control diagram permits one-click checking
of logic or permissive/interlock conditions.
Pop-up Displays. For major equipment
such as a feedwater pump, it is sometimes
useful to have a dedicated set of small popup
displays, each linked to a button on the
process display. Figure 4 shows several examples.
The permissives listed in the EPRI
case study have been assembled into a small
pop-up overlay, along with a similar list of
interlocks for each pump. Bearing temperatures
and other pump/motor data are available
from the data button.
Regulatory (Closed-Loop) Control
Single push-button start-up is not possible
without stable regulatory controls. This stability
is required not only during normal operation
but also during start-up and throughout
the full operating range of the unit. With this
in mind, the coordination of the VFD and
feedwater valves is revisited.
VFD Control Strategies
Provided stable operation is maintained, reducing
pump load can result in significant
energy savings. Assuming a plant electrical
load of approximately 1.5 MW for the feed
pump, a 10% reduction in pump load would
save over 1,300 MWh annually.
The strategy as described for the case
study uses the VFD in combination with
the feedwater valves in an overlapping split
range. The VFD starts at minimum speed (a
35% turndown from maximum). It increases
as needed once the valves are at the upper
end of their control range and only modulates
while the IP valve is less than 85% open. One
consequence of this strategy is a somewhat
disjointed control response and an undesirable
coupling of the IP and HP drum levels.
For example, as the VFD speed increases,
both the IP and HP valves throttle back in re52
4.
Pop-ups in a flash. For major equipment such as a feedwater pump, it is sometimes
useful to have a dedicated set of small pop-up displays, each linked to a button on the process
display. Courtesy: Siemens Energy Inc.
5. Easy overrider. Once the IP valve throttles below 85%, the variable-frequency drive
(VFD) could remain above minimum speed indefinitely. An alternative approach is to change the
VFD strategy by introducing an override controller. Courtesy: Siemens Energy Inc.
HP drum level
LI
-
+
PI
HP steam
flow
FI
-
+

PI
3-element
No
T
f(X)
Yes
f(X)
+
PI
HP feedwater
flow
FI
A
IP valve output
-
+
-
A
PI
85%
A
T
A
>
f(X)
A
HP valve
output
f(X)
Pump master
speed control
sponse to increased flow. Once the IP valve
throttles below 85%, the VFD could remain
above minimum speed indefinitely.
An alternative approach is to change the
VFD strategy by introducing an override conwww.powermag.com
troller,
as illustrated in Figure 5. For simplicity,
a single output is used for the HP drum.
This output could in turn be sent to each
valve via a balancing control or simple split
range. The IP drum level controls also are not
POWER | June 2011
T
A

http://www.powermag.com

POWER June 2011

Table of Contents for the Digital Edition of POWER June 2011