This article is about observations by us shared for the benefit of power plant engineers.

1. CRITICAL OBSERVATIONS DURING SHUT DOWN AUDIT OF A 330 MW
PULVERIZED COAL FIRED BOILER
By K.K.Parthiban, B.Tech ( IIT-M), M.E Thermal Engineering- RECT Trichy
Introduction
Boiler maintenance cost can be reduced, if every failure & shortcoming are analysed and addressed.
In a power plant, engineers are stressed and hard-pressed for early restart of the unit and hence they
do not have time to analyze the failures. However,
employing specialists for unit inspection during
shut down would help to identify defects in boiler
& reasons for past failures. Actions could be taken
to stop repetitive failures. In future, unscheduled
outages can be avoided. Performance of the plant
can be greatly improved. There have been cases
wherein two or three annual shut down audits have
led to much better understanding of the boilers.
There have been cases where the plant engineers
stopped calling us when they became clear of what
to inspect in the annual shut downs. The value
addition of a shutdown audit by experienced boiler
consultants can be known only if it is experienced. We bring below the value addition by us in a
tower type PF boiler outside India.
Features of the unit
The boiler is a once through unit with a centrifugal separator. The furnace is made of spiral wall rifle
tube construction. All the tube banks of superheaters, reheaters and economizers are horizontally
arranged in a single pass directly above the furnace. The screen tubes are arranged above the furnace.
The screen outlet tubes act as support tubes for all the tube banks except for the low temperature
economiser. The tower type configuration is a bold configuration selected by the designers. It
certainly improves the free board for better burn out of the coal particles. The lateral pitch of bank
assemblies decrease, as the gas moves up. Thus, required gas side velocities are maintained for good
heat transfer. The low temperature economiser is constructed out of square finned tube. The unit is
provided with two rotary airheaters. Gas recirculation fans are provided for startup purpose and for
low load operation. 5 mills are provided. Electrostatic precipitator is provided for dust capture.
Condensate polishing units are provided for ensuring the feed water / return condensate quality.
Detailed observations at shut down
We bring out our observations in the unit which was inspected by us. We feel this technical paper
would help the unit operators to understand about methodical approach to be taken for inspection of
the boiler and correlate to the failures being experienced in the unit. We have compiled below the
important observations in this unit.
Photo 1: Distant view of the unit

2. Final superheater stub cracks
It was learnt from plant engineers, that there had been cracks of the stubs at final SH outlet since
long. It had become a routine to inspect the stub weldment at headers by DP test. The stub
weldments are repaired when the cracks are observed. On review of detailed drawing and installation
of SH inlet & outlet headers, it became clear that there had been design mistake. The header supports
were restricting the thermal expansion and hence the short stubs were subject to force.
Photo 2: Location of stub cracks in superheater outlet
header
Photo 3: DP test done at header stubs show cracks.
These cracks are not deep routed.
Photo 4: The SH outlet header moves to the right by 42
mm as per calculation. Available slot is 30 mm only.
Photo 5: The SH outlet header is required to move
away from Waterwall by 16.5 mm. But space
available is less.
Photo 6: The above
drawing shows SH inlet
header support detail. The
header moves by 31 mm
away from waterwall. The
slot provision is 40 mm.
Since the details of SH
outlet header support was
not available, this drawing
is referred here for
comparison purpose.

3. Photo 7: A true to scale drawing was made to find the
fillet length of SH stubs.
Photo 8: The actual weld lengths were found to be
25-27 mm. It should have been around 12.8 mm.
High steam temperature headers are subject to creep and thermal fatigue. The stubs should be with
full penetration welds. At the same time, the weld reinforcements should be within limits.
Low temperature superheater spacer support failures
In low temperature superheater bottom bank is of T2 material and top bank is of T22 material. The
tubes are supported by hanger tubes. However vertical pitch is maintained by sliding spacer of SS
304 material. The sliding supports welding with T22 tube material had cracked at 295 locations.
Photo 9: The above drawing shows sliding support detail. The insert photo shows the dislodging of the sliding
support from the tube by weld crack. This is a dissimilar weld. The reason is the wrong choice of electrode.

4. Photo 10: Photo shows the weld failure of sliding spacer
supports between LTSH tubes.
Photo 11: The photo shows the LTSH supporting
(binding) by hanger tubes.
Table 1: T22 material comes under P5A group. P8 is the Stainless grouping.
Photo 12: The extreme left photo shows
the LTSH tubes spaced by an attachment
bar welded to top & bottom tubes at the
location shown. The support is found
slipped. This is because of the width of
the support plate was inadequate by
design.
Photo 13 & 14: Instead of removing and
rewelding a new support plate, Alternate
arrangement is suggested to prevent the
slipping. The elevation & sectional views
are shown here on the right-side
drawings.

5. Observations in Deaerator tower
For many years, the deaerator tower was not inspected. It was the first time when we had inspected.
The deaerator spray nozzles had come off completely. Fortunately, the dislodged components did not
trouble the boiler feed pump.
Photo 15 & 16: The above photographs show the dislodged spray nozzles from the deaerator tower.
Drag chain feeders
The unit is provided with drag chain feeders to feed coals to different mills. Of late, belt weigh
feeders are used to feed coal. By VFD it is possible to achieve uniform coal feed rate between
feeders. In drag chain feeder a drop test is a must, to ensure that the rate of feeding is uniform in all
feeders. This depends on the dam plate (coal bed height control) levels at all feeders. It was observed
that as the dam gates were worn out, this aspect was ignored.
Photo 17 & 18: The above photographs show the different type of dam arrangement between feeders.

6. Coal nozzles
Coal nozzles distortion in service has been there in some units. This has to do with the differential
temperature between the inside and outside surfaces of the outer shell. The outer shell would distort
when the tilt is such that there is no sweeping of air over the outer surface. The space between the
inner and outer shell is also critical and can cause accumulation of coal.
Photo 19: Distortion of outer shell only at bottom side
of the coal nozzle.
Photo 20: Distortion of outer shell only at bottom
side at all levels.
Photo 21: The photo shows the distortion of outer shell
due to differential temperature between inner & outer
surface in a different boiler. But then the space between
inner & outer shell is large enough that no coal ash
slags are seen.
Photo 22: The new coal nozzle developed by OEM
now addresses the problem. The air sweep is
maintained at all tilts and at the same time the outer
shell does not support the coal ash because the
outer shell is now trimmed to 115 mm. This to
overcome the problem shown in photo 19 & 20.
Secondary / OFA nozzles
The distortion of SA and OFA nozzles are also common for the same reason that the inside surface is

7. at different temperature as compared to outside surface temperature when the tilt is done. However,
seeing the different profiles, we can infer what could be right design to prevent distortion of the
nozzles. See photos below.
Photo 23 & 24: The top photo shows the distortion
of SA nozzle. It is seen that the flat top has suffered
distortion, while the corrugated profile is not
distorted. The bottom photo shows the tendency of
flat plate to distort.
Photo 25: The corrugated nozzles do not exhibit any
distortion. The internal cross stiffeners hold the shape.
It is seen that the plain plate is distorted.
High temperature economiser
This assembly is of staggered construction and is of plain tube. We have come across erosion of end
tubes in many cases wherein a tube bank is arranged in staggered pitch in elevation. The resistance
Photo 26: Erosion of bends near rearwall. Photo 27: Erosion of HT eco inlet tubes at front wall.

8. Photo 28: Filter baffles placed in the tube
bank below has caused preferential flow of
flue gas along the front wall in the upper
bank.
Photo 29 & 30: The photos show the end tube to panel
clearance. This had led to the erosion of end tube.
Staggered pitch banks are sensitive to such preferential
flow patterns.
for flow through the staggered tube bank, leads to end tube erosion along the four boundaries. Some
filter baffles have been used in the tube bank below. This has resulted in additional gas flow along
the front wall and resulted in erosion of the bottom tubes.
Furnace panel erosion around the wall blowers
The furnace is found free of ash slagging, as the coal used is a good one. However, there is distortion
of the panels in the upper part of the spiral walls. Due to this, the wall blower has been causing
frequent erosion of panel tubes over a large perimeter. We recommended that the soot blower tip is
advanced suitably to reduce the erosion.
Photo 31: The photo shows the erosion of furnace wall
panels over a large perimeter around.
Photo 32: The photo shows the distortion of spiral
wall panels at upper levels.

9. Mills
The important observations in mills were the plugging of classifier vanes & wear of the top plate,
which would affect the separation efficiency. The wear pattern of the rolls show that the rolls are not
advanced after the gradual wearing off. This would affect the output from the mills.
Photo 33: The photo shows the erosion of the plate
above the classifier
Photo 34: The photo shows the rags clogging the
classifier vanes.
Photo 35: The rolls of mills were found to be rolling of
ridge at the end of bowl.
Photo 36: The damage pattern indicates the rolls
are not adjusted forward after the wearing off.
Every boiler has some scope for improvement. Critical shut down inspection would benefit the users
in improving the PLF and in reducing loss due to unforeseen shut down. For comments write to
parthi2006@gmail.com / venus1@venus-boiler.com. Visit www.venus-boiler.com for more articles.
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