FIELD OF THE INVENTION
The present invention relates generally to foundations of electrostatic precipitator
in thermal power plants. More particularly, the invention relates to a partially
unsupported raft foundation for a Retrofit Electrostatic Precipitators with Ash
pipe trench.
BACKGROUND OF THE INVENTION
Electrostatic precipitator (ESP) is an equipment in a thermal power plant which
extracts the ash which comes along with the flue gas which is the product of
burning pulverized coal in a boiler in the thermal power plant.
The Electrostatic precipitator uses the electrical phenomenon called "Corona"
discharge which under high voltage turns the flue gas into an ionized state
where the negatively charged ash particles are attracted towards the positive
electrodes called collecting electrodes which is rapped at intervals and collected
in the ash hoppers at the bottom of the electrostatic precipitators.
The weight of the collected ash and the weight of the collecting electrodes
constitute the major vertical loads on the foundations of the supporting structure
of the electrostatic precipitators, apart from the wind and seismic loads.
The electrostatic precipitators are designed for a particular value of permissible
emission levels i.e (The amount of dust particles present in the flue gas that is
let into the atmosphere through the chimney). These emission levels are
normally fixed by the statutory Authorities.

The currently permitted emission levels call for modification of the already
installed electrostatic precipitators initially designed for a higher emission levels
to bring down the emission level to the current norms. The modification is likely
to increase the vertical load on the foundation of the electrostatic precipitator as
both the amount of collected ash and size of the collecting electrodes are
destined to increase considerably.
A straight forward solution to this problem is to replace the existing electrostatic
precipitators with higher capacity equipment to satisfy the current emission
norms.
This process of dismantling the existing electrostatic precipitator and installing a
new bigger electrostatic precipitator in the same location i.e. between the boiler
and the induced draft fan is called Retrofitting.
Different types of foundations are known for the retrofit ESP foundation like
isolated, combined, raft, pile foundations.
Raft foundation and pile foundations are provided in poor soil conditions. Raft
foundations for ESP retrofit foundations are preferred over pile foundations since
special type of machinery are required for providing pile foundation.
Raft foundation normally is a uniform thick reinforced concrete slab provided
interconnecting all the columns of an ESP.
Providing raft foundations for retrofit ESPs foundations where there is a Ash pipe
trench running across the series of ESP posses a problem in giving single raft
interconnecting all the columns of an ESP.

The present invention seeks to overcome the drawbacks of the prior art.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench, which
interconnects the rafts at either side of the ash trench.
Another object of the invention is to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench which
connects all the columns of retrofit electrostatic precipitator with a common raft.
A still another object of the invention is to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench, which
allows the interconnections to cross-over "no-pipe" zone of the ash trench.
Yet another object of the invention is to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench, in
which interconnection between the two portions of the raft on both side of the
trench is made at a spacing to enable removal of pipes in the ash trench for
maintenance.
A further object of the invention is to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench, in
which a reduced thickness for the interconnection compared to the raft thickness
is selected so as to maintain sufficient gap between the interconnection bottom
to the top most pipe in the trench.

A still further object of the invention is to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench, in
which portion of the raft is interconnected so that the bearing pressure at the
edges are reduced below the safe bearing capacity of the soil.
Yet further object of the invention is to propose a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench, which
reduces the differential settlement of the raft foundation by interconnecting the
two portions of the raft.
SUMMARY OF THE INVENTION
According to the invention, there is provided a partially unsupported raft
foundation for the retrofit electrostatic precipitators with ash pipe trench. The
raft foundation is provided at the grade level of the Electrostatic precipitator,
which uses a "Dry Ash evacuation system", which does not have any trench
below the ESP except a common trench which carries the Ash pipes across a
series of ESP's to the Ash disposal pump house.
The top of the raft foundation itself acts as a flooring below the ESP whereas the
normal raft foundation top is selected between 750mm to 200mm below the
graded level. After the back filling of soil, a separate 200mm thick reinforced
cement concrete flooring is provided at the graded level.
The raft on both sides of the Ash pipe trench which runs across the Electrostatic
precipitator is interconnected with reinforced concrete slabs.

The reinforced concrete interconnections are provided at intervals between the
two parts of the raft foundation on both side of the Ash pipe trench in such a
way that there remains sufficient space between the interconnection to facilitate
Ash pipe maintenance.
Since the raft is interconnected, the raft becomes a single raft which connects
all the columns of an Electrostatic precipitator which is a better technical solution
than two separate rafts for a single ESP.
The interconnection is not supported at the bottom whereas the rest of the raft
is supported on soil.
BRIEF DESCRIPTION OF THE ACCOMPANYIG DRAWING:
Figure 1 shows a sectional elevation and plan view of the partially unsupported
raft foundation for retrofit electrostatic precipitator according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figure 1, the raft foundation (1) for a retrofit electrostatic
precipitator with Ash pipe trenches (4) is made as a single unit by supporting all
the columns of the retrofit electrostatic precipitators by an interconnection (2)
across the ash pipe trench. (4)

This raft foundation (1) with interconnection (2) is provided at graded level so
that the interconnection (2) passes above the pipes (3) in the Ash pipe trench.
(4)
The interconnection (2) between the two portions of the raft on either side of the
Ash pipe trench is constructed with openings which facilitates the removal of
pipes (3) from the Ash pipe trench (4).
The interconnection (2) reduces the bearing pressure at the bottom of the
foundation when the Electrostatic precipitator columns (5) are close to the Ash
pipe trench (4).
The middle columns of an retrofit ESP carries almost double the load of a side
column (5) and at times the edge bearing pressure near a middle column (8)
near the Ash pipe trench (4) without an interconnection becomes so high that it
exceeds the allowable bearing pressure in poor soil conditions.
The interconnections (2) thus reducing the settlement of the raft foundation near
the middle ESP columns (8) near the Ash pipe trench. (4)
Though the interconnection (2) is unsupported at the bottom, it effectively
transfer moments and shear from one portion of the raft to the another side of
the raft and reduces the bending moment and shear forces in the raft near the
middle columns (8) of the retrofit electrostatic precipitator which are closer to
the ash pipe trench (4).

The interconnection (2) reduces an unequal settlement of the raft foundation (1)
compared to two rafts configuration of prior art, on both side of the Ash trench
(4) in a retrofit electrostatic precipitator.
The interconnection (2) thickness is selected in such a way that there is enough
gap between the bottom of the interconnection and the top of the top most pipe
(3) in the Ash pipe trench. (4)
The interconnections (2) are located near the/in line with the columns so that
the loads from the columns (5) are transferred from one side of the raft to the
other side of the raft there by reducing the bearing pressure near the column
The sizing of the interconnection (2) and reinforcing are chosen in such a way
that the loads, moments and shears are carried from one side of the raft to the
other side of the raft effectively.

WE CLAIM :
1. A partially unsupported raft foundation (1) for Retrofit Electrostatic
Precipitators with ash pipe trench, comprising.
- an interconnection (2) between two portions of the raft foundation on
either side of the Ash pipe trench, wherein all the columns of the Electro
static precipitator are supported on a single raft, and wherein the
interconnection (2) between the two portion of the raft is not supported in
soil.
2. The raft foundation (1) as claimed in claim (1), wherein the
interconnection (2) is made of Reinforced cement concrete.
3. The raft foundation (1) as claimed in claim 1, wherein the interconnection
(2) is provided at graded level.
4. The raft foundation (1), as claimed in claim 1, wherein the interconnection
(2) is provided over the pipes (3) of the ash trench (4).
5. The raft foundation as claimed in any of the proceeding claims, wherein
the interconnections (2) are disposed at a spacing to allow removal of the
pipes in the Ash pipe trench for maintenance.
6. The raft foundation as claimed in any of the proceeding claims, wherein
the interconnection (2) is enabled to reduce the bearing pressures under
the middle columns (8) which are closer to the Ash trench (4).

7. The raft foundation as claimed in any of the preceding claims, wherein the
interconnection is capable to transfer the moment and shear from one
portion of the raft to the other portion of the raft to reduce the moment
and shear in the raft near the middle columns (8).
8. The raft foundation as claimed any of the preceding claims, wherein the
interconnection (2) is enabled to reduce the settlement below the middle
columns (8) near the Ash pipe trench. (4), including the differential
settlement of the entire raft foundation (1).
9. The raft foundation as claimed in claim (1), wherein the raft foundation is
provided at grade level to eliminate provisions of pedestals and a separate
grade slab.
10. A partially unsupported raft foundation (1) for Retrofit Electrostatic
Precipitators with ash pipe trench, as substantially described and
illustrated herein with reference to the accompanying drawings.

The invention relates to a partially unsupported raft foundation (1) for
Retrofit Electrostatic Precipitators with ash pipe trench, comprising an
interconnection (2) between two portions of the raft foundation on either
side of the Ash pipe trench, wherein all the columns of the Electro static
precipitator are supported on a single raft, and wherein the
interconnection (2) between the two portion of the raft is not supported in
soil.