FIELD OF INVENTION
The present invention relates to a method of monitoring of
screw conveyor rotation in ball tube mills to avoid damage of the conveyor and
mill proper. More particularly, the invention relates to a method of producing an
advance warning during monitoring to stop the ball mill before any damage takes
place.
BACKGROUND OF THE INVENTION
The Coal pulverizes (typical shown in Fig.1) are used for
pulverizing coal by feeding pre-crushed raw coal into long rotating. Tube having
large quantity of small balls (normal quantity of balls is 60 to 80 tons having
diameters as 30, 40 and 50 mm). Due to impact of balls on coal and due to
attrition of coal and balls on the shell liners the coal get pulverized. The
pulverized coal is carried up by hot air to classifier zone. The classifier is fitted in
the pulverized coal air path for segregating the coarser coal particles from fine
coal particles. The coarser coal particles are sent back for further grinding while
the fine coal particles are further transported to its end use i.e. burning in
suspended condition.
The raw coal is pushed into the grinding zone with the help of
flexible ribbons fitted on hollow shaft and rotating along with the shell.
The screw conveyor (shown in Fig.2) is fabricated Tube with thick
plate ribbons attached to the tube. The conveyor is supported inside the shell by
connecting rods on one side while on the other side bearing support is provided.
In Ball tube mills, used for pulverization of coal to specific
classification for easy and complete burning of coal. The raw coal is fed in to the

shell having hard, thick material lining (having lifting surface contour) and
containing large quantity of hard balls. The coal feeder installed after the coal
bunker allows the coal into the Ball Mill. The screw conveyor pushes the coal into
the grinding zone (shell). The Shell is rotated slowly with the help of electric
motor and step down gearboxes. The coal is pulverized due to impact of loose
hard balls on the coal and also due to attrition. Hot air is sent through the inner
pipe of the screw conveyor for picking the pulverized coal.
The screw conveyors are provided with special steel ribbons for
pushing the coal. The rotation required for the conveyors is taken by connecting
them to shell body by connecting rods.
Due to extraneous foreign material like pyrites, stones, rods, bolts
and other non-magnetic materials there are chances of breakage of screw ribbon
and connecting rods. With this the coal gets piled up; resulting in breakage of
other components and explosion in case the mill continue to operate.
This invention gives advance warning on stoppage of screw
rotation; thus facilitating stoppage of the feeder and avoid catastrophe.
At present, there is no provision of monitoring the screw conveyor
rotation and only by observing the high current of drive motor, it is concluded
(based on the experience); that there is something wrong inside mill. Many
times, it becomes too late to take corrective action and explosion takes place.
The existing Ball mill consist of two equipment, Noise level
transmittal and Delta Panel level measurement. Both these equipment assist in
coal feeding and ascertaining the quantity of pulverized fuel available. There is
no equipment to monitor the rotation of the screw conveyor.
Both the Instruments namely Noise level transmittal and Delta
Panel Level Measurement cannot detect failure of screw conveyor rotation and

respond to the control room of the Power Plant. By the time the operator detects
something wrong the vertical pipe gets chocked up and mill explosion takes
place. It takes less than 20 seconds to fill the pipe while any one of this
equipment or the operator takes 5 to 7 minutes to detect the fault.
At present, there is no method or equipment in Ball Mill to detect
failure of rotation of screw conveyors. Many accidents/mishaps have taken place
in Power plants.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of
monitoring of screw conveyor rotation in ball tube mills to avoid damage of the
conveyor and the mill parts, which improves the performance of the ball mill and
makes it safe to operate.
Another object of the invention is to propose a method of
monitoring of screw conveyor rotation in ball tube mills to avoid damage of the
conveyor and the mill parts, which is capable of transferring the rotational signal
to electrical signal.
A further object of the invention is to propose a method of
monitoring of screw conveyor rotation in ball tube mills to avoid damage of the
conveyor and the mill parts, which is able to pick up the electrical signal without
physical contact.
A still further object of the invention is to propose a method of
monitoring of screw conveyor rotation in ball tube mills to avoid damage of the
conveyor and the mill parts, which is capable of controlling the Coal feeder
operation and stoppage of the same when the screw rotation is stopped to avoid
piling of coal inside the shell resulting in explosion.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - Shows a typical Ball tube mill
Fig.2 - Relates to mill air and coal circuit and shows inside working of coal flow,
Hot air flow and pulverized coal air mixture flow in the Ball tube mill.
Fig.3 - Shows screw conveyor assembly with hot air, box showing typical raw
coal feed pipe, hot air entry duct and PF outlet duct.
Fig.4 - Shows screw conveyor with helix used for pushing the raw coal into the
shell.
Fig.5 - Shows screw conveyor rotation monitoring system according to the
invention.
Fig.6 - Shows the mounting details of screw rotation monitoring system with
housing and support according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Extensive laboratory tests were carried out to study the screw
conveyor stoppage phenomena and pick up signal mechanism to optimize the
total system.
Extensive Site tests were carried out on three varieties of Proto-
types to study the screw conveyor stoppage phenomena and operation of pick
up signal mechanism under dusty, vibrating environment.
As per the invention, the movement of an object can be detected
by using proximity sensor without physically contacting the rotating part.
The simulated study of rotation of screw conveyor and pick up of
signal was carried out.

The invention relates to Screw conveyor, critical component of Ball
Tube Mill, which is exposed to damage due to extraneous foreign material in raw
coal. With damage to screw conveyor the coal mill is exposed to sudden
stoppage resulting in breakage of many parts of coal mills and many times
results in mill fire. With application of this invention, there will be advance
warning and the Ball mill can be stopped before any catastrophe takes place. It
is an advance warning safety device to avoid major accident in coal fired thermal
power plants with Ball Tube mills as pulverizer of coal.
Fig. 1 Shows the Ball tube mill. This consist of drum (Shell) (6)
supported on two trunions with electric motor and gear box drive system. The
shell (6) is covered from inside by shell liner (7) to protect the shell plate and to
lift the balls (5). The raw coal is fed through the feeder (1) onto screw conveyor
(3) which pushes the raw coal into the shell (6). The balls (5) fall on raw coal
resulting in breaking of the same. The coal and balls rotate on the shell liners (7)
inside the shell (6). Over a period the coal is ground and is taken out to furnace
by hot air which is sent through the inner pipe of screw conveyors.
Fig. 3 shows a typical raw coal feed pipe (10), hot air entry duct
(11) and pulverized Fuel outlet duct (9), hot air entry pipe inside the shell (12)
and screw conveyor ribbon (4) for pushing the raw coal.
Fig. 4 shows a screw conveyor with helix for pushing the raw coal
into the shell for pulverization, having connecting rods (13) for transmission of
rotation from shell to screw conveyor, a pipe (14) for hot air entry, screw ribbon
(4) for pushing the coal inside, and the conveyor shaft (15) for second support.
Fig. 5 shows the screw conveyor rotation monitoring system having
a rotation pick up system (16), a coupling (17) and a rotation transmission disc
(18).

Fig. 6 shows the screw conveyor or rotation monitoring system with
rotation pick up system, (16) with housing (19) a support, rotation transmission
disc (18), connection (20) from pick up system to process panel and signal
process panel (21) to give alarm for stoppage of the raw coal feeder (1).
The screw conveyor system (M) uses the rotation of the shell (6) to
get required rotational force by connecting through connecting rods (14). Once
the connecting rods are broken due to foreign material the screw conveyor shaft
stops rotating.
The monitoring system consists of wheel with sprockets on the
outer periphery. The wheel is fitted on the extension of screw conveyor shaft
(15) so that the wheel rotates at the same speed as the conveyor shaft. Since
the sprockets are provided there is air gap between the sprockets. The inductive
proximity sensor (proximate switch) is provided with pick up point (16) which
detects the sprocket movement. If a smooth wheel is provided then there is
continuous signal and stoppage of wheel cannot be detected. Due to sprocket
and air gap the Inductive proximity sensor gives fluctuating signal (upward due
to sprocket and down ward due to air gap). With this signal it is concluded that
the shaft is rotating. In case there is breakage of connecting rods resulting in
stoppage of the shaft, then, the signal is continuous (if the signal is picked from
the sprocket surface then it will continue to give sprocket level output) thus it is
concluded that the shaft is stopped and the mill is getting filled up with coal
which will result in explosion. Once we get straight line output signal, immediate
instructions are given for stoppage of coal feeding and necessary action for
evacuation of pulverized coal to avoid raw coal piling up in screw conveyor area
and mill.
The inductive proximity sensor works on the principle of continuous
smooth straight line signal output if there are no undulations on the surface. The

Wheel is provided with sprockets to get undulation resulting in fluctuating signal
showing shaft rotation in vogue. In case the connecting rods are broken then the
pickup is either from sprocket or from air gap, which is a smooth output.
To organize pick up system without mechanical contact was the
main aim in laboratory trials. A make shift proto-type was made giving the
required rotation to the wheel with sprockets to arrange the desired signal to the
optical proximate switch. The wheel was rotated at different speed and number
of slots / sprockets was determined to give clear signal. The shape, size and
depth of sprockets were designed and laboratory tested to give clear signal. The
weight and width of the wheel was another critical area for getting proper
continuous readings. The sudden stoppage of wheel and its effect on inductive
proximity sensor (proximate switch) was analysed. The switch electronic
equipment and the control panels were modified to ensure proper signal in case
of stoppage of wheel due to connecting rod breakage. The Fig 05 shows the
arrangement wherein Sl.no. 18 consists wheel with sprockets for signal pick up.
On the other side a lovejoy coupling is fitted to take care of any misalignment
and proper pick of the signal.
As given above while designing and manufacturing of the wheel
different materials like aluminum, steel, plastic and Bakelite were tested with
different number of sprockets, width and shape of sprockets . Tight fit fixing of
wheel on the shaft arrangement was made by interference fit and providing key.
The inductive proximity sensor works on the principle of continuous smooth
straight line signal output if there are no undulations on the surface. The wheel
is provided with sprockets to get undulations, resulting in fluctuating signal
showing shaft rotation in vogue. In case the connecting rods are broken then the
pickup is either from sprocket or from air gap, which is a smooth output. This
phenomenon is the heart of the invention and gives signal for stoppage of the
feeder in case the rotation stops.

WE CLAIM
1. A method of monitoring of screw conveyor rotation in ball tube mills to
avoid damage of the conveyor comprising;
fitting a wheel on the extension of screw conveyor shaft (15);
providing sprockets on the outer periphery of the wheels creating air gaps
between the sprockets;
providing inductive proximity sensor with pick up point (16) to detect the
sprocket movement;
characterised in that,
the inductive proximate sensor gives fluctuating signal which is upward
due to sprocket and downward due to air gap to indicate that the shaft is
rotating wherein the proximate sensor gives continuous signal when there
is a stoppage of the shaft due to breakage of connecting rods (13) with
the continuation of the mill getting filled up with coal so that once this
straight line output signal is received, immediate instruction is given for
stoppage of coal feeding.
2. The method as claimed in claim 1, wherein the inductive proximity sensor
produces continuous smooth straight line signal output when there are no
undulations on the surface.
3. The method as claimed in claim 1, wherein the inductive proximity sensor
produces fluctuating signal where there are undulations on the surface.


ABSTRACT

A method of monitoring of screw conveyor rotation in ball tube
mills to avoid damage of the mill comprising fitting a wheel on the extension of
screw conveyor shaft (15), providing sprockets on the outer periphery of the
wheels creating air gaps between the sprockets and providing inductive
proximate sensor with the pick up point to detect the sprocket movement when
the inductive proximate sensor gives fluctuating signal which is upward due to
sprocket and downwards due to air gap to indicate that the shaft is rotating
wherein the proximate sensor gives continuous signal when there is a stoppage
of the shaft due to breakage of connecting rods (13) with the continuation of the
mill getting filled up with coal so that once this straight line output signal is
received immediate instruction is given for stoppage of coal feeding. The
advance warning system and stopping of raw coal feeding results an operation
free of explosion.