FIELD OF INVENTION
The present invention relates to an improved stiffening arrangement for ash hoppers to
withstand high ash load in economizer duct of large capacity CFBC boilers. More particularly,
this invention relates to an improved stiffening arrangement of pyramid shaped ash hopper
which can withstand ash load up to maximum of 4 shifts of operation (32 hrs) of ash
accumulation, so that frequent unloading of the ash content from the hopper is reduced.
BACKGROUND OF THE INVENTION
Ash hopper is a bottom portion of an economizer enclosure duct. Economizer
enclosure duct is a gas tight vertical enclosure with rectangular cross section of (15mX12m)
and height of 19m approximately. It is being utilized in Circulating Fluidized Bed Combustor
(CFBC) Boiler in Thermal Power Plants. It is located in the back pass of CFBC boiler.
The Economizer enclosure duct per se constitutes horizontal coils which are
placed to transfer the remaining heat from flue gases after passing through the evaporator
and super heater heat transfer sections. Bottom portion of Enclosure duct acts like a ash
hopper to accumulate the fly ash from the flue gas with bulk density of ranging from 800 to
1000 kg/m3. This accumulated fly ash is disposed to external ash handling system at frequent
intervals (Approx. for every 8 hours). Economizer enclosure duct is externally stiffened with
backstays and levellers against the flue gas design pressure of ± 1050Mbar. All the loading
exerted on Economizer Enclosure Duct is to be positively transferred to the nearby structural
columns.
The complete structure of Economizer enclosure duct basically comprises of four
main parts Upper Carbon Portion (1), Below the Junction Box (2), Ash Hopper (3), Duct Outlet
(4), as shown in figure 2.

Economizer Enclosure Duct is subjected to various load such as self-weight of the
structure, incremental ash loadings according to ash accumulation in each shift of boiler
operation (refer figure 7), horizontal and lateral loads from wind loadings, differential thermal
loads due to heated ash collection within the ash hopper of the system.
Thus, it has become necessary to do precise Finite Element Analysis to check the
adequacy of all members to withstand all the imposed loads and moments. In other words, to
assure structural integrity against damage due to ash accumulation. Accordingly, a complete
model has been simulated in static environment to study the structural behavior during actual
operation.
It was observed that upon solving incremental load cases of ash load starting
from 120MT (refer fig 7), hopper walls were getting higher stresses mainly at bottom portion
due to accumulation of ashes having differential temperature distribution from top to bottom.
Thus adding inertia or stiffeners at appropriate locations in ash hopper's wall would be a viable
solution to the persisting structural problem.
Excessive accumulation of fly ash below the economizer can have a significant
impact on the structural integrity of the entire lower casing enclosure, economizer hoppers
and convection pass support system. In addition to the overload, excessive ash accumulation
can lead to significant temperature differentials within the structural system, causing
potentially damaging thermal loads and load redistribution. Damage from fatigue may not
show up for years, since thermal loads may be time related. These temperature-and ash-
related loads can be significant and must be avoided.
As shown in Figure 3, the economizer ash-collection hoppers are the pyramidal-
shaped cavities located below the economizer outlet flue. The structure between the
economizer and the top of the ash-collection hoppers is the economizer outlet flue. Structural
failure may result in the economizer outlet flue or hoppers separating from the steam-cooled
convection pass enclosure due to the following:
■ Erosion
■ Corrosion
■ Operation with excessive gas temperatures
■ Temperature differentials due to ash deposits
■ Improper structural modifications or repairs

■ Excessive ash accumulation
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved stiffening
arrangement for ash hoppers to withstand high ash load in economizer duct of large capacity
CFBC boilers which is capable of withstanding higher ash loads upto four shift of operation (32
hrs.)
Another object of the invention is to propose an improved stiffening arrangement
for ash hoppers to withstand high ash load in economizer duct of large capacity CFBC boilers
which can delimit the structural deflection and thermal stress up to acceptable limits.
Still another object of the invention is to propose an improved stiffening
arrangement for ash hoppers to withstand high ash load in economizer duct of large capacity
CFBC boilers which is economical to build, maintain and operate in four shift of operation
without any failure.
Further object of the invention is to propose an improved stiffening arrangement
for ash hoppers to withstand high ash load in economizer duct of large capacity CFBC boilers
which assures structural integrity against damage.
Still further object of the invention is to propose an improved stiffening
arrangement for ash hoppers to withstand high ash load in economizer duct of large capacity
CFBC boilers which is able to minimize physical deterioration of the complete structure and
results an efficient performance of the thermal Power Plant.
SUMMARY OF INVENTION
According to the invention, several iterations have been performed to achieve
the best configuration of stiffening arrangement of ash hopper, which could be easily
implemented and can loaded with ash up to max. of 4 shift of operation (32hrs). It has done
by identifying the appropriate locations through simulation and then stiffen them.
Static Analysis is carried out for entire structure subjected to appropriate load
accompanied with incremented ash load of 120 tonnes each time. It is observed that most

flexible part is hopper walls due to ash accumulation, hoppers walls are subjected to
significant temperature gradient from top to bottom portion of hoppers around the wall. Thus
experiencing load redistribution on the hopper walls causing higher thermal stresses and
deflection.
Thus it became vital to provide new pattern of stiffening arrangement over
hopper wall at identified appropriate locations. Thus selection of stiffeners have been
optimized to maintain deflection and stresses.
According to the invention, the new configuration of the ash hopper's stiffening
arrangement is indicated in Table-I.



BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - Shows the economizer ash hopper according to prior art.
Fig. 2 - Shows components of a Economizer Duct structure accompanied with existing
stiffeners according to the present invention.
Fig. 3 - Shows an isometric view of an ash hopper wall along with stiffeners indicating view-p
and view-s pertaining to the invention.
Fig. 4 - New configuration of Stiffening Arrangement of Ash Hopper's wall along view-P
according to the present invention.
Fig. 5 - New configuration of Stiffening Arrangement over Ash Hopper's wall along view-S
according to the present invention.
Fig. 6 - Plan of Ash hopper along with new configuration of stiffening arrangement according
to the present invention.
Fig. 7 - Typical loading Pattern for each shift of boiler operation according to the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 2 - Shows an elevation of an Economizer Enclosure Duct accompanied with provided
stiffeners according to the invention.

Economizer Enclosure Duct as shown in figure 2, which comprises of upper carbon steel
portion (1), Junction box below economizer (2), Economizer bottom Hopper (3),
Economizer Duct outlet (4).
Figure 3 - Shows an enlarged isometric view of an ash hopper with modified stiffeners, arrows
indicating view-P and view-S
Along view-P the ash hopper's wall stiffening arrangement are identical.
Along view-S the ash hopper's wall stiffening arrangement are identical.
Figure 4- shows New configuration of Stiffening Arrangement of ash hopper along view-P
7 horizontal stiffeners have been incorporated over each of the 8 faces of the hopper wall,
which are grouped by horizontal stiffeners (5). So total 56 (7X8 faces of hopper's wall)
horizontal stiffeners are incorporated over hopper's wall along view-P.
13 vertical stiffeners have been incorporated over each of the 8 faces of the hopper wall,
which are grouped by vertical stiffeners(7). So total 104 (13X8 faces of hopper's wall)
vertical stiffeners are incorporated over hopper's wall along view-P
Figure 5 shows New configuration of Stiffening Arrangement of ash hopper along view-S
7 horizontal stiffeners have been incorporated over each of the 8 faces of the hopper wall,
which are grouped by horizontal stiffeners (8). So total 56 (7X8 faces of hoppers wall)
horizontal stiffeners are incorporated over hopper's wall along view-S.
Same as above, 13 vertical stiffeners have been incorporated over each of the 8 faces of
the hopper wall, which are grouped by vertical stiffeners (9). So total 104 (13X8 faces of
hopper's wall) vertical stiffeners are incorporated over hopper's wall along view-S
To strengthen corners of the pyramid shaped hopper, 4 Nos. corner beams (6) have been
added over each hopper. So total 16 Nos. (4X4 faces) are added.
Figure 6 - Plan of Ash hopper along with new configuration of stiffening arrangement
Figure 7, Typical incremental loading pattern of ash loading at different heights.

As shown in figure 7, the incremental ash loading i.e. 120 Tonnes of ash weight
(one shift of operation), 240 tonnes of ash weight (two shifts of operation) and so on. By
considering modified stiffening arrangement in to an account maximum ash loading condition
is for 4 shift of operation resistible by the ash hopper without any failure.
Entire structure is supported on structural columns, all the loading exerted is
positively transferred to the structural columns. The static Analysis is done to find out the
structural adequacy of the system. It was observed that after two shifts of operation /16 hrs,
some of the ash hopper's corner stiffeners and some of the horizontal and vertical beam
members were failing. A plurality of horizontal and vertical stiffeners have been incorporated
at a appropriate location based on study of the deflection pattern while simulation of the
model in static environment.
Horizontal and vertical stiffeners are of ISMB 250 and ISMB 200 around hopper wall.
After incorporating the modifications, reanalysis has been done for the same load cases
i.e., 120 tonnes of ash (one shift of operation), 240 tonnes of ash weight (two shifts of
operation) simultaneously. It was observed from the static results, that the number of failed
members were drastically reduced as compared to the existing arrangement. The reason
behind this reduction in number of failed members is because of the stiffening of hopper area
in appropriate locations.
As mentioned earlier an improved method of stiffening and supporting arrangement of
pyramid shaped ash hopper of economizer duct is made to troubleshoot structural problems.
The invention pertains to stiffeners and supporting arrangement for ash hopper which can
take up high ash loads (up to a maximum of 4 shifts of operation/32 hrs). This arrangement
comprises of plurality of practicable horizontal and vertical stiffeners along with corner beams.
At least 56 Horizontal stiffeners (ISMB beams) have been welded over hopper wall along
view p.
At least 104 vertical stiffeners (ISMB beams) have been welded over hopper wall along
view p.

At least 56 Horizontal stiffeners (ISMB beams) have been welded over hopper wall along
view S
At least 104 vertical stiffeners (ISMB beams) have been welded over hopper wall along
view S.
At least 16 corner beams (ISA angle) have been welded over corners of each hopper.
According to the members selected as tabulated in Table-I, a consolidated static
results have been achieved. Table-II shown summarized result contains deflection and stress
values in ash hopper stiffeners at ash loading of 4 shift. It is clearly seen from the results that
stresses and deflection of beam members being maintained under the allowable limit. Ash
should be removed frequently (up to a maximum of four shift of operation/32 hrs of ash
accumulation) for safe operation of the duct.


WE CLAIM
1. An improved stiffening arrangement for ash hoppers to withstand high ash load in
economizer duct of large capacity CFBC boilers comprising;
incorporating a plurality of horizontal stiffeners (5) over each of the 8 faces of the
hopper wall disposing the stiffeners along view-P;
incorporating a plurality of vertical stiffeners (7) over each of the 8 faces of the
hopper wall disposing the stiffeners along view - 'P';
incorporating a plurality horizontal stiffeners (8) over each of the 8 faces of the
hopper wall disposing the stiffeners along view 'S';
incorporating a plurality of vertical stiffeners (9) over each of the 8 faces of the
hopper wall disposing the stiffeners along view 'S' wherein a plurality of corner
beams (6) are arranged over each of the hopper when the corner beams are
disposed over the corners of all 4 hoppers improving the supporting arrangement for
ash hopper.
2. The stiffening arrangement as claimed in claim 1, wherein at least 7 numbers of
horizontal stiffeners are incorporated over each of the 8 faces of the hopper wall so
that a total of at least 56 numbers of horizontal stiffeners are arranged over
hopper's wall along view-P.
3. The stiffening arrangement as claimed in claim 1, wherein at least 13 numbers of
vertical stiffeners are incorporated over each of the 8 faces of the hopper wall so
that a total of at least 104 numbers of vertical stiffeners are arranged over hopper's
wall along view-P.
4. The stiffening arrangement as claimed in claim 1, wherein at least 7 numbers of
horizontal stiffeners are incorporated over each of the 8 faces of the hopper wall so
that a total of at least 56 numbers of vertical stiffeners are arranged over hopper's
wall along view-S.

5. The stiffening arrangement as claimed in claim 1, wherein at least 13 numbers of
vertical stiffeners are incorporated over each of the 8 faces of the hopper wall so
that a total of at least 104 numbers of vertical stiffeners are arranged over hopper's
wall along view-S.
6. The stiffening arrangement as claimed in claim 1, wherein at least 4 number of
corner beam are incorporated over each hopper so that at least 16 numbers of
corner beams are arranged to strengthen the corner of the hoppers along view P.
7. The stiffening arrangement as claimed in claim 1, wherein at least 56 numbers of
horizontal stiffeners are welded over hopper wall along view P.
8. The stiffening arrangement as claimed in claim 1, wherein at least 104 numbers of
vertical stiffeners are welded over hopper's wall along view-P.
9. The stiffening arrangement as claimed in claim 1, wherein at least 56 numbers of
horizontal stiffeners are welded over hopper's wall along view-S.
10. The stiffening arrangement as claimed in claim 1, wherein at least 104 numbers of
vertical stiffeners are welded over hopper's wall along view-S.
11. The stiffening arrangement as claimed in claim 1, wherein at least 16 corner beams
are welded over corners of each hopper.