The wingwall SH is common in many CFBC installations. This discusses about the solution

1. THERMAL EXPANSION OF WINGWALL SUPERHEATER AND ITS DISTORTION
By K.K.Parthiban, B. Tech (IIT-Madras), M.E Thermal Engineering- RECT Trichy
Distortion of wingwall superheater is experienced in many CFBC boilers. This article is about the
cause and care to be taken in design,
installation and operation of CFBC boilers, to
prevent distortion. Photo 1 shows the
distortion experienced in a CFBC installation.
The below drawing (photo 2) shows the
expansion movements given in thermal
expansion diagram by the boiler manufacturer
for this case. Plant engineers may obtain this
information from the manufacturer for their
boilers.
In this illustrated case, the wing wall
superheater is anchored at the front waterwall
panel at the bottom. The working point is at
17.851 m. The wing wall superheater outlet header is supported at 32.894 m. The absolute expansion
of the wingwall SH outlet header is 110 mm upward. The downward expansion of the anchor point is
66.7 mm. The net expansion at the
wingwall SH outlet header is 110 -
66.7 = 43.3 mm upward. The spring
movement of the constant level
hanger provided for supporting the
wing wall superheater is designed
for the upward travel of 43.3 mm.
The length of the wing wall
superheater panel is 15 m.
Most CFBC boilers are provided
with coal feed points in the front
panel. During start-up of the plant,
the combustion takes place in the
upper furnace due to the inadequate
bed material in the furnace. The
required upper bed is formed only
after sufficient fine ash is generated
in the combustion process. Only
after the sufficient fine ash is
generated, the waterwall starts
absorbing the design heat, at the
design heat flux. Without the fine
ash, the combustion takes place at
the upper furnace. This is because
Photo 1: Distortion of wingwall SH
Photo 2: Expansion movement diagram

2. there is no resistance available to prevent the transport
of burning coal to the top. Hence the wingwall
superheater receives high heat input and is subjected to
dry heating up without adequate cooling steam. The
thermal expansion of the wing wall superheater at start-
up would now exceed the normal expected thermal
expansion. This start-up criteria may not be envisaged
by the designer, while procurement for the constant
spring support for wingwall SH.
There is no thermocouple provided in the upper furnace,
below wingwall superheater, to monitor the gas side
temperature during start up. This is not provided, as it
could cause furnace wall tube failure due to preferential
flow of furnace inventory during normal operation. One
must observe the metal temperature of wingwall superheater outlet tubes only. The metal
temperature can go to the maximum allowable metal temperature for the SH material, which is
usually 585°C. We may note the rated pressure is
achieved gradually after the furnace inventory increases
in start-up process.
During start-up, the waterwall is cold. The rated
pressure is not achieved in the beginning. The saturation
temperature currently would be less than the rated
temperature. The downward thermal expansion at
anchor point could be much less. However, the
wingwall superheater must expand upwards as per the
maximum permitted metal temperature - 585°C. This
will cause a good amount of upward thermal expansion
of the wing wall superheater outlet header. This would
be in the order of 150 mm. However, the constant load
hanger selected may not accommodate this much
amount of expansion encountered during start up. The
seal box must also be designed to accommodate an
expansion of 150 mm and generally not designed so.
The thermal expansion of the wingwall superheater must
take place against gravity. The shape of the wing wall
superheater is such that, it can easily buckle because of
its less cross-sectional dimensions versus the height
ratio. Hence it doesn’t expand as rigid member. Given a
resistance to prevent thermal expansion, the panel
buckles. This leads to a permanent deformation.
The worst distortion can be seen in photo 3. This is from
another boiler. For this case, replacement is a must.
During every start-up, the distorted panel flexes at the
Photo 5: Spring hanger – large travel
Photo 4: Constant load hanger -limited
travel
Photo 3: Worst distortion of wingwall SH

3. location where the permanent deformation had taken place at the
first time. This flexing of panel is now elastic in nature. In case
of replacement wing wall superheater panels, the deformation
would take place again at some location naturally. If this must be
prevented, a constant load hanger, which can accommodate large
deflections would be required. The current model selected by
boiler maker does not take up large deflection. See photo 4
above. The Lisega (well-known manufacturer of spring supports)
make spring supports are available for large deflections. See
photo 5.
What to do with the present distortion?
In most cases, it is advisable to leave the wing wall superheater
with the deformation depending on the severity at the flex point. It is this deformation which now
accommodates the large thermal expansion, which take place during every start-up. Further relative
thermal expansion is accommodated by the constant load hangers. This can be checked during
operation.
Distorted wingwall SH panels would be subject to higher erosion rate. Regular monitoring of erosion
and thickness logging is a must. The risk carried would be similar to CFBC furnace panel erosion.
The other type of damage expected would be the cracks at the header stubs. The thermal forces
would be partly transferred to the stubs at both inlet and outlet header. Dye penetrant test is advised
to be carried out on every annual shutdown. There are cases wherein the thermal expansion forces
act on the header stubs and caused leaks. See photo 6. The number of startup & shut down cycles
would decide the initiation of stub / weldment crack at headers. This could occur after a long service.
A tube failure in wingwall SH would lead to furnace pressurization. The furnace trip protection on
high furnace pressure should not never be bypassed. Regular inspection of header stubs is advised.
Conclusion
The wingwall SH distortion can be controlled by avoiding rapid start-up during cold and hot restart
times. Even if the right type spring support is provided by the boiler maker, it is possible that the
distortion can be caused by improper operation. In the case of the worst distortion seen in photo 3
immediate replacement of the SH is required. A retractable thermocouple can be arranged below
wingwall SH. It would help the operator to monitor and prevent high gas side temperatures during
cold start up and hot restarts.
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Photo 6: SH header stub weldment
crack