POWER February 2013 - 35

INSTRUMENTATION & CONTROL
test of the HP FWH energy control applying
partial closure of both parallel high-pressure
feedwater heater extraction isolation valves.
Specifically, the HP FWH energy control
system pinched back the extraction valves
one at a time to produce a 50 psi drop across
each steam extraction line. The objectives of
the test were to confirm the:
■ Expected drop in feedwater outlet temperature.
■
Predicted increase in superheater steam
temperature.
■ Predicted increase in reheat steam flow.
■ Predicted reduction in superheated
steam flow and associated turbine throttle
valve position while maintaining the
same load generation.
The trend lines illustrate the positive
response of the 1A and 1B feedwater
heater (parallel heater trains) outlet water
temperature, the economizer water inlet
temperature, the A/B superheater steam
temperatures (two superheater sections)
and the Unit 4 turbine valve position demand
signal. Test results confirmed the
program predictions.
A second test was conducted with full
isolation of the HP feedwater heaters (1B
and 2B). Figure 5 illustrates the trend results
for 1A and 1B feedwater heater outlet
water temperature, economizer water inlet
temperature, A/B burner tilt positions, percentage
of NOx
and A/B air heater gas outlet temperature.
In this test the burner tilts were adjusted
downward to maintain superheat steam
temperatures in the prior range. The test
encompassed four hours to allow sufficient
settling time for the process. Sootblowing
and other normal controls were maintained
throughout the test (Figure 6).
The test results illustrate the key variables
before and after isolation of HP feedwater
heater 1B:
■ Confirmed the predicted drop in economizer
inlet temperature, 46F.
, A/B superheater steam
temperatures, Unit 3 and Unit 4 turbine
valve position demands, reheat spray flow,
5. Test 2 results. The second test fully isolated the first and second HP FWHs. Source:
Invensys Operations Management
Economizer inlet temp (F)
FWH 1A outlet temp (F)
FWH 1B outlet temp (F)
Burner tilt A (degrees)
Burner tilt B (degrees)
NOx (%)
SH A setpoint (F)
SH A measured (F)
SH B measured (F)
TCV 4 position (%)
TCV 3 position (%)
RH spray flow (%)
AH A gas exit temp (F)
AH B gas exit temp (F)
Notes: AH = air heater, FWH = feedwater heater, HP = high-pressure, SH = superheater, TCV = turbine control valve.
6. Good operating data. A screen grab of the plant control system shows the plant performance
with and without the HP FWH 1B in service. Note that superheat temperatures are
up, NOx is down, and desuperheater spray water is down-all indications of more efficient unit
operation. Source: Invensys Operations Management
■ Lowered the burner tilts required to maintain
superheat steam temperatures by 12F.
emissions by 12%.
■ Lowered NOx
■ Lowered the furnace exit gas temperatures
by 14F.
■ Increased the reheat steam flow.
■ Lowered the reheat spray flow by 25,000
lb/hr.
■ Reduced the superheat steam flow and
corresponding turbine throttle valve
position.
■ Increased the load generation margin as
related to turbine throttle valve position.
HP FWH energy control can compensate
for the adverse impact of higher-Btu
coal, using less-than-ideal coal mills due
to equipment outages, boiler component
replacement, load cycling, and other conditions
averse to attaining design steam
temperatures. Tests also confirm that peak
load generation capability can be significantly
increased through HP FWH energy
control with little, if any, consequence on
the unit heat rate. The estimated potential
increase in load generation is up to 7%,
depending on cycle design and equipment
limitations.
For units that do not have a steam
temperature deficiency, including oncethrough
units, the efficiency impact of
cooler feedwater is mitigated by the increased
boiler efficiency and reheat steam
flow through the LP turbine.
HP FWH energy control can be a vital
component of a multivariable boiler, load
generation, emissions, and steam temperature
control system. With the additional advantage
of increased sustainable power generation
and reduced furnace NOx
emissions, the
potential benefits are substantial. ■
-Donald Labbe, PE (donald.labbe@
invensys.com) is a consulting control
engineer for Invensys Operations
Management, based in Foxboro, Mass.
He is an ISA Fellow and director for ISA
POWID. This article is derived from a
technical paper presented at the 2012
ISA POWID Conference.
February 2013 | POWER
www.powermag.com
35

http://www.powermag.com

POWER February 2013

Table of Contents for the Digital Edition of POWER February 2013