POWER June 2011 - 48

INSTRUMENTATION & CONTROL
Fully Automating HRSG
Feedwater Pumps
Modern distributed control system platforms can provide many tools to capture
best operating practices and automate them. This case study shows
the steps taken to automate a hypothetical simplified feedwater pump
system for a combined-cycle power plant. It describes a combination of
controls automation strategies and human-machine interface techniques
designed to increase the overall level of automation while improving
ease of use.
By Steven Leibbrandt and Bill Thackston, Siemens Energy Inc.
M
odern distributed control system
(DCS) platforms offer capabilities
that were unavailable just a
few years ago. Features such as integrated
graphical engineering environments, simplified
sequencing controls, and improved
human-machine interfaces (HMI) make
higher levels of automation more practical
from the standpoints of implementation,
maintenance, and ease of use. The timing of
these advances couldn't be better-critical
operating personnel throughout the power
industry are approaching retirement age,
and there are insufficient numbers of skilled
younger personnel to replace them. Leveraging
the existing plant knowledge base to
design automation that reduces the burden
on plant operators will be essential to meeting
tomorrow's plant demands.
As an example, the following case study
describes automating a simplified feedwater
system for a combined-cycle power plant.
The existing legacy DCS controls are proven
and reliable; however, the sequence of operations
and coordination of regulatory controls
is not automated, therefore, it requires
a high degree of knowledge and attention
on the part of the operator. This case study
describes a combination of controls automation
strategies and HMI techniques designed
to increase the overall level of automation
while improving ease of use by operators
and maintenance personnel. (Note: DCS examples
were developed using the Siemens
SPPA-T3000 DCS platform. A detailed description
of this control system was included
in " Upgraded Control System Adds to Merchant
Plant's Bottom Line, " January 2009,
available at www.powermag.com.)
The " as-found " DCS graphics and controls
strategy for this case study form the basis for
comparison with newer strategies, so a brief
discussion of the existing automation base48
line
is in order. The reference information
used in this case study was provided by the
Electrical Power Research Institute (EPRI)
but also incorporates information taken from
one or more operating plants.
The Hypothetical Plant's Layout
The hypothetical plant consists of two combined-cycle
combustion turbines, each with a
heat-recovery steam generator (HRSG). Both
HRSG units are coupled to a single steam turbine
with feedwater supplied from a common
condenser and the hotwell, the receptacle for
the hot water drawn from the condenser by
the air pump.
Looking at a single HRSG, the feedwater
train consists of a pair of 100% capacity
feedwater pumps and a set of three drums
for low-pressure (LP), intermediate-pressure
(IP), and high-pressure (HP) steam headers.
The feedwater pumps transfer feedwater from
the LP drum to the IP and HP drums. Minimum
flow through the pump is maintained
by a recirculation line back to the LP drum
with a modulating valve and a variable-flow
setpoint calculated from the pump manufacturer's
operating curves. Both pumps share
supply lines and valves to the IP and HP
drums. Each pump is equipped with a variable
frequency drive (VFD) for speed control
and has a dedicated lube oil pump.
Existing Drum Level Control
Feedwater control to IP and HP drums consists
of both single- and three-element drum level
control. Single-element controls modulate the
supply valve to each drum based on the level.
Three-element control uses steam flow from
the drum as a feedforward signal to a feedwater
flow controller whose setpoint is modulated
to maintain desired drum level. Pump speed is
modulated to adjust IP and HP flows in coordination
with the feedwater valves as follows:
www.powermag.com
■ Control of VFD and feedwater valves applies
regardless of single- or three-element
level control.
■ The VFD runs at minimum speed, modulating
the IP valve to maintain IP drum
level and the HP start-up valve to maintain
HP drum level until the HP start-up valve
is fully open.
■ Once the HP start-up valve is fully open,
the VFD will modulate to keep the IP
valve within its control range (<85%).
■ If the IP valve is <85% open, modulate the IP
valve to maintain IP drum level and modulate
the VFD to maintain HP drum level.
■ If the IP valve is >85% open and the HP
start-up valve is fully open, increase the
VFD speed until the IP valve output is
<85%. Modulate the HP main feedwater
valve to maintain the HP drum level.
Existing Baseline HMI
The existing operator interface for each
feedwater train consists of three full-screen
graphics. The first is a partial schematic of
the feedwater system, showing pumps and
discharge lines, but not the drums. The second
graphic is a collection of control faceplates
for use in setting up discrete devices
and regulatory controls. The last display is a
tabular collection of relevant plant operating
data, including feedwater pumps and drum
conditions. Each feedwater pump has numerous
process interlocks that are not immediately
accessible from the HMI.
Case Study Assumptions
The assumptions for this case study are as
follows:
■ Only normal operation is considered (no
power augmentation).
■ Pump failure will start the standby device
without cycling common discharge valves.
POWER | June 2011
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POWER June 2011

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