POWER June 2011 - 50

INSTRUMENTATION & CONTROL
1. Automation hierarchy. This schematic
shows the main automation structure
for the feedwater pumps. The sequence for
a feedwater system is controlled by a single
subgroup control block. The two feedwater
pumps are in turn managed by a single device
configuration overlay block. Courtesy: Siemens
Energy Inc.
SGC
Step 1
DCO
Pump1
Motor
Pump2
Motor
Notes: DCO = device configuration
overlay, SGC = subgroup control.
Step 2
■ Shutdown will stop all pumps and close all
valves in the feedwater system.
■ VFD and feedwater valve coordination as
supplied on the piping and instrumentation
diagram appears incorrect. The original operation
as stated above is an interpretation.
Proposed Control Strategies
Operators and maintenance personnel can
use a number of control strategies to improve
the operation of their facilities' feedwater
pump systems.
Open-Loop Control. There are many
possible methods of automating boiler feed
pump systems. The method chosen minimizes
the number of sequences so the operator
can easily follow the actions of the automation
system. This becomes more important as
the whole power plant is automated. The sequence
is used to place equipment in service
and place closed-loop controls in the relevant
modes, such as filling, start-up drum level,
and normal drum level control. The protection
logic is carried out directly at the device
drive level. A basic principle is that the operator
has the option to control at all times.
Any field equipment loss of status during
start-up after being placed in service (such
as the isolating valve open status being lost)
should be handled as a status discrepancy on
the device. The affected closed-loop controls
utilize the process values (feedwater flow and
isolating valve not closed) to maintain their
operation. The plant protects itself through
the process values (drum level trip).
On this basis, reversing sequences are not
recommended as a best practice. Instead, the
sequence design utilizes check-backs from
the process (such as pressures and flows) and
device check-backs as the sequence feedback
50
for overrides and step criteria. If a sequence
is restarted, fulfilled criteria or override
check-backs bring the sequence back to point
of action required immediately. As a result,
status discrepancies can be used to stop the
sequence and allow the operator to investigate
the fault and restart the sequence without
unnecessary actions on the sequence or
process.
The main automation structure for the
feedwater pumps is given in Figure 1.The sequence
for a feedwater system is controlled
by a single subgroup control (SGC) block.
The two feedwater pumps are, in turn, managed
by a single device configuration overlay
(DCO) block. Each motor has a separate motor
block, with appropriate logic for permissives,
interlocks, and automatic operation.
Sequential Logic. The sequence logic
mirrors the steps and actions of the manual
start-up sequence taken from a plant operating
procedure. The complete sequence begins
with no pumps running, first having the operator
go through the required manual checks.
Once the system is ready, the IP and HP drums
are isolated, the recirculation path is opened,
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the lube oil pumps are started, and the selected
pump is started. From this point forward,
a pump failure will cause the DCO block to
start the standby pump. In fact, the switchover
logic will remain in effect even after the main
sequence is complete. Steps match the written
sequence for this example but could easily be
consolidated for simplicity.
Shutdown. The shutdown sequence initiates
the closure of the IP and HP feedwater
and isolation valves as well as the feedwater
and lube oil pumps.
Operator Prompts. As the sequence progresses,
three single-line messages appear on
the screen. The first line displays an operator
prompt and acknowledgement button, if
required. The second line is a description of
the current step actions and conditions. The
final line lists information related to the next
step in the sequence. Figure 2 shows typical
messages and an operator prompt.
HMI Concepts for Robustness
Several concepts built into the HMI have
the goal of improving the operator interface:
A single process graphic simplifies navigaPOWER
| June 2011
3. One-click convenience. A single consolidated process graphic encompasses the major
components of the feedwater system, including the LP, IP, and HP drums. Courtesy: Siemens
Energy Inc.
2. Friendly reminders. This figure shows typical messages and operator prompts. Courtesy:
Siemens Energy Inc.
http://www.powermag.com
POWER June 2011

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