ABSTRACT
TITLE : ‘A METHOD OF FABRICATING A DEVICE TO RESTRICT ASH
BUILD-UP BEYOND A THRESHOLD LEVEL IN THE HOPPERS’
The invention relates to a method of fabricating a device to restrict ash build-up
beyond a threshold level in the hoppers, comprising fabricating a shear plate (1)
with material having yield strength and Young modulus values lower than that
of the material forming the hopper walls (3); providing a strain-gauge box (2)
having at least two strain gauges disposed on the outer surface of said shear
plate (1); an alarm means located on the hopper to exhibit warning in the form
of displaying light and sound to indicate failure of the hopper walls (3); fixing an
emergency by-pass duct (4) with a bird cage (5) to outlet flange of the hopper
located after the strain-gauge box (2), wherein the shear plate (1) is allowed to
be ruptured prior to failure of the hopper due to weaker material construction of
the shear plate (1), wherein the strain-gauges (2) are enabled to measure the
deflection of the shear plate (1) and causing the alarm means to sound a
warning when the ash level in the hopper exceeds a threshold value, and
wherein the emergency by-pass duct (4) allows transfer of the ash discharged
through the rupture opening from the hopper level to the ground level.

FIELD OF THE INVENTION
The present invention relates to a method of fabricating a device to restrict ash
build-up beyond a threshold level in the hoppers.
BACKGROUND OF THE INVENTION
Dust is collected in industries whenever there is a change in velocity of dust
laden flue gas in chamber (or duct) or dust filtration equipped like ESP, Bag
filter, Cyclone, etc. The settled or collected dust is stored in bins called hoppers
through which periodical or continuous evacuation takes place for providing
space for further dust storage. Whenever the dust evacuation rate is less than
dust collection rate, the hopper gets filled above its storage capacity. As the
settled dust volume crosses the hopper storage capacity, the dust levels
interferes with the equipment parts placed vertically above the hopper and
blocks the path of the flue gas also in some cases.
There have been instances of mechanical failure of equipment and catastrophic
failure in certain cases due to over loading of hopper. This leads to loss to the
plant operation regarding time and money both. The power unit has to be shut
down and electricity generation is stalled. Sometimes, this type of failure of
equipment also leads loss of life. These damages or mechanical failures of
equipment have been attributed mostly due to overloading of hoppers with dust
above prescribed design limit. Similar overloading may happen wherever dust
evacuation rate is not at par with dust collection rate.

The efficiency of equipment also get effected due to ash build-up beyond
hopper, which is re-entrained even after collection. This also results in more
particulate emissions to atmosphere.
As the dust collection crosses the designed dust collection capacity of hopper,
the hopper (plate and stiffeners) and its support structure gets overloaded. This
overloading causes local failure of hopper or damage to the hopper support
structure causing damage to the main equipment also.
Dust filtering systems (like ESP, Bag filter, etc) which are designed for dust
filtering have various signaling systems like Ash Level Indicator, 3D Level sensor
and Non-contact level measurement (based on time of flight principle for
microwave pulses). But these systems are still a challenge for accurate and
reliable monitoring as :
a) Fly Ash density varies widely and its composition changes.
b) Fly Ash has very low density, hard to sense and tends to stick to
everything particularly when moist.
c) Fly Ash has a very low dielectric constant and carries static charge.
d) Fly Ash is at high temperature and is abrasive in nature.
e) There is Suspended (floating) dust in vicinity of the level sensing probe.

Such damages cause loss of operating hours of the equipment. Severe damage
may cause loss of life also.
An Electrostatic Precipitator (ESP) is a particulate collection device that removes
particles from flowing gas (such as air) using the force of an induced
electrostatic charge. Electrostatic precipitators are highly efficient filtration
devices that minimally impede the flow of gases through the device, and can
easily remove fine particulate matter such as dust and smoke from the air
stream.
Bag/Fabric filter is a particulate collection device that removes particulate by
filtering gas streams through large fabric bags.
Ducts are configured with passages through which flue gas passes. Whenever,
there is change in flue gas velocity/ direction of flue gas velocity is below a
certain value, a dust settlement takes place inside the duct depending on dust
particle size and other flue gas parameters.
Hereafter, ESP, Bag filter and Duct will be referred as ‘equipment’ for dust
collection. There have been instances of damage or mechanical failure of such
equipment due to over-loading of the hopper. This leads to loss of the plant
operation cycle. Due to failure of the equipment, the power unit has to be shut
down and electricity generation is stalled. Sometimes, sudden failure of the
equipment also leads loss of life.

The hoppers are used in a boiler system for storing loose bulk material
permanently or temporarily. These hoppers are applicable for fly ash storage in
industrial applications. The ash storage may be due to dust removal (for
equipment like ESP and Bag filter) of dust settlement (in ducts) taking place
along flue gas path exhausted by thermal power plants, cement plants, steel
plants and glass plants etc.
According to the prior art the collected/stored ash is getting built-up inside the
equipment due to improper ash evacuation, which causes damages to the
equipment internals and in some cases catastrophic failure of the equipment.
These damages or mechanical failures of the equipment have been attributed
mostly due to overloading of the hoppers with dust above prescribed limit.
Similar overloading may happen wherever dust evacuation rate is not at par with
dust collection rate.
The efficiency of the equipment also gets affected due to ash build-up beyond
the hopper, which is re-entrained even after collection. This results more
particulate emissions to atmosphere.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of fabricating a
device to restrict ash build-up beyond a threshold level in the hoppers which
eliminates the damage of dust collection equipment in thermal power plants.

Another object of the invention is to propose a method of fabricating a device to
restrict ash build-up beyond a threshold level in the hoppers, which ensures the
protection of the equipment by evacuating ash before the hopper is overloaded.
A still another object of the invention is to propose a method of fabricating a
device to restrict ash build-up beyond a threshold level in the hoppers, which
mechanically protects the Electrostatic Precipitation from shear plate failure in
the event of malfunctioning of sensing device.
A still further object of the invention is to propose a method of fabricating a
device to restrict ash build-up beyond a threshold level in the hoppers, which
eliminates outage of plant due to failure of the dust – collection equipment.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method of fabricating a device to restrict ash
build-up beyond a threshold level in the hoppers.
As per the inventive process, a screen with strain gauges (for strain sensing) is
installed in the hopper wall. When the ash build-up reaches a threshold level, the
additional pressure exerted by the ash on the screen vis-à-vis on hopper walls,
the screen is allowed to get ruptured. The ash would then fall through this
ruptured opening on the ground via an extended portion of the hopper which
delivers the falling ash safely from a height to the ground.

The Screen is called a shear plate. The material of the shear plate is selected
with an yield strength and Youngs modulus lower than those of the hopper wall
material or construction. Due to comparatively low Youngs modulus value, the
deflection in the shear plate for same load application shall be more than that of
hopper wall. This deflection is sensed by a strain gauge box having a plurality of
strain gauges. The strain gauges transmit electrical signals as output, the output
voltage signal triggers a warning alarm in the form of a light and sound,
indicating puncture of the shear plate. The strain gauges are placed on the
cleaner side (outer surface) of the shear plate. Due to lower yield strength value
of the material of the shear plate, the shear plate gets ruptured earlier than the
hopper walls and thus saving the hopper and the equipment. The shear plate is
positioned above the hopper heaters to avoid interference.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 – Isometric view of hopper with shear plate assembly.
Figure 2 – Side view of hopper with shear plate assembly.
Figure 3 – Bottom view of hopper with shear plate assembly and extended
portion.
Figure 4 – Detailed isometric view of shear plate assembly.
Figure 5 – Detailed side view of shear plate assembly.
Figure 6 – Isometric view of extended portion with bird cage.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION.
The screen material in the shear plate assembly plays a vital role to save the
hopper damage by allowing early rupture, and the deflection values before the
rupture are sensed by the strain gauges to provide an alarm indicating the
possible over-loading of the ash in the hopper.
The material of the thin shear plate (1) is of lower yield strength and lower
Youngs modulus to that of the hopper wall material of construction. Due to lower
Youngs modulus value, there will be more deflection in the shear plate (1) for
same load application than that of the hopper wall (3). This deflection is sensed
by strain gauges fitted in strain gauge box (2). The strain gauges transmit
electrical signal as output, which voltage signal is used for triggering light and
sound warning alarm indicating the possible hopper failure. The strain gauges
are placed on the cleaner side (outer surface) of the shear plate. Due to lower
yield strength value, the shear plate (1) gets ruptured earlier than the hopper
walls (3) and thus saving the hopper. The ash falls on ground upon rupture
through an emergency by-pass (4) fixed after the strain Gauge box (2) attached
to the hopper. The emergency by-pass (4) is also fitted with a bird cage (5) at
the same level as that of outlet flange of the hopper. The shear plate (1) is
positioned above to avoid any interference from the existing components (like
hopper heaters).

WE CLAIM :
1. a method of fabricating a device to restrict ash build-up beyond a
threshold level in the hoppers, comprising :
- fabricating a shear plate (1) with material having yield strength and
Young modulus values lower than that of the material forming the
hopper walls (3);
- providing a strain-gauge box (2) having at least two strain gauges
disposed on the outer surface of said shear plate (1);
- an alarm means located on the hopper to exhibit warning in the form
of displaying light and sound to indicate failure of the hopper walls
(3);
- fixing an emergency by-pass duct (4) with a bird cage (5) to outlet
flange of the hopper located after the strain-gauge box (2),
wherein the shear plate (1) is allowed to be ruptured prior to failure of the
hopper due to weaker material construction of the shear plate (1), wherein
the strain-gauges (2) are enabled to measure the deflection of the shear
plate (1) and causing the alarm means to sound a warning when the ash

level in the hopper exceeds a threshold value, and wherein the emergency
by-pass duct (4) allows transfer of the ash discharged through the rupture
opening from the hopper level to the ground level.