POWER October 2011 - 20

3. Intrex design improvements. In the fall of 2009, a number of
additional design improvements were made to the Intrex. Source: JEA
All weir walls
modified to
Hoover Dam
design
Raised return
channel floor
Tube support moved
to mid span
Heavy refractory
Light refractory
Lowest
pressure
Partitioned air supply
Middle
pressure
Acts as
support for
modified
tube shelf
4. Good results from Intrex upgrades. On the left is Intrex
2C in as-found condition before the 2009 outage. Note the agglomeration
formation above the tube shelf. Agglomeration forms above the
tube shelf due to poor fluidization. This buildup causes load reduction
and cyclone plugging. Targeted modifications made in 2009 eliminated
this problem, as shown in the photo on the right. Courtesy: JEA
Highest
pressure
high-performance butterfly valves. By testing various butterfly valve
disk designs in their testing laboratory, they created a disk with an
inherent equal percentage flow characteristic. This means that the
percentage flow through the valve was proportional to the percentage
valve opening, so there were more predictable flows over a much
wider valve travel range, in this case up to 70% open.
Following successful field trials, the new 4-inch control disk
valve system was introduced early in 2009, just in time for the
new technology to be adopted by JEA. A Fisher spring and diaphragm
actuator operates each of the new valves. Post-upgrade
tests confirmed that during normal Intrex operation, the desired
pressures in each of the three sections of the air supply plenum
were achieved, which improved the flow of bed material. A dozen
of the 10-inch damper valves were replaced by 36 of the 4-inch
control disk valves on each of the two units. Another advantage
was the control disc valve design is the complete shutoff of the air
supply to the bed during start-up, avoiding Intrex tube damage.
Final Test Results
The redesign and modifications completed in 2009 greatly improved
control of the airflow into each of the three Intrex heat exchangers in
each of the two units. Ash flow through the Intrex heaters was also
improved, reducing tube fouling and producing better heat transfer.
The stability of bed levels in each furnace resulted in lower air pressures
required to fluidize the bed material. The improved fluidization
also prevents plugging of the Intrex heat exchangers. Other observed
benefits have been reduced emissions of SO2
and NOx
and increased
load output. The valves can also control accurately at lower flows,
making it easier to adjust to lower power loads at night.
Figure 4 shows before and after views of the superheater tubes
desirable inflow at start-up caused overheating of the heat exchanger
tubes, contributing to metallurgical damage and premature tube failures.
Those 10-inch valves also tended to " hunt, " meaning that they
were unstable, especially in the range of 10% to 30% open.
The designers realized well-controlled airflow was essential to
ensure good circulation within the Intrex units to prevent agglomeration
of solids on the tubes and for improved combustion
in the CFB. As a result, the Intrex units were redesigned.
Three modifications were implemented during the fall 2009 shutdown:
moving the superheater tube support, replacing weir walls with
a new design to aid circulation, and using a three-pipe air supply
inside of the single, 10-inch-diameter pipe (Figure 3). The original
10-inch air supply line was replaced by three pipes, each equipped
with a new 4-inch-high performance butterfly valve designed to pass
the same amount of air, but with improved accuracy.
A New Butterfly Valve
High-performance butterfly valves are traditionally used in throttling
applications requiring close flow control, but their accurate operating
range is usually limited to a very narrow control range, about 30% to
50% open. When pushed to open wider, operation usually becomes
sluggish and unpredictable. Perhaps this is why high-performance
butterfly valves were not used in the original Intrex design, which
had airflow controlled by the 10-inch damper valves.
Meanwhile, engineers at the Fisher Controls Division of Emerson
Process Management were working to broaden the control range of
20
in one Intrex. One year after the valve modifications were made,
the Intrex tubes remain clear and problem-free. Recently, the
distributed control system (DCS) monitor of one CFB showed very
precise valve opening of 63.69%, 70.02%, and 66.79% on each
of three control disk valves serving one Intrex heater. These valve
opening readings represent gas flows of 2,056 lb/hr, 1,999 lb/
hour, and 1,997 lb/hour, respectively, indicating that the Intrex
heaters are operating as designed.
Also shown on the DCS screen was the power output of 308.37
MW for that unit, confirming that the upgraded Intrex system
and other plant modifications have produced improved furnace
operation and increased power output.
Looking Ahead
JEA intends to continue its leadership in CFB boiler design and
operation with further improvements to the Northside plant. Today,
JEA management believes that more fine-tuning will further
increase plant output and efficiency. The next step is restoration of
a number of Intrex tube bundles, already scheduled during the fall
2011 shutdown. Other plans call for improvement of secondary and
primary air control, limiting Intrex tube temperatures on start-up
to 1,050F, and analyzing start-up control factors such as feedwater
flow, primary and secondary airflow, bed pressures, and above-bed
burner uses to improve the responsiveness of each unit.
-Contributed by Frank Thomas (thomfh@jea.com), a reliability
engineer, and John Kang (kangjj@jea.com), an electric generation
optimization specialist, for JEA.
Applying Acoustic Pulse Reflectometry
in a Geothermal Plant
Acoustic pulse reflectometry (APR) is a tube inspection method
that has been gradually gaining acceptance as a tool for heat exchanger
inspection. Different types of heat exchangers operating
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POWER | October 2011
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POWER October 2011

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