TITLE
A method of maintaining ratio of constituent ceramic suspensions for improving
manufacturing yield of high voltage porcelain insulators.
FIELD OF INVENTION
The present invention relates to a method for manufacturing high voltage
porcelain insulators.
The manufacturing of electro-porcelain insulators is a very complicated process due
to the number of parameters involved: three major raw materials with host of
additives, lengthy manufacturing process, long drying time, reaction during heat
treatment process and stringent electrical and mechanical strength requirement of
the final products. One of the most important parameters is to maintain the
composition of porcelain to achieve the final properties of the product after
sintering. This is because, during manufacturing, the generated wastes prior to
firing are also used in processing with the virgin materials. This includes the wet
scrap and the dry scrap and both have to be maintained at a proper ratio with the
fresh material, otherwise, the overall composition is changed. This affects the
overall quality of the product, because during sintering there are phase changes in

the material and the composition is highly sensitive to impurities. Therefore, the slip
house where the raw materials are mixed and milled at different proportions with
water is very crucial in deciding the properties of the final product. The present
method of manual measurement by dip stick method is very inaccurate in precisely
measuring different ceramic suspensions during final mixing. Therefore, a new
system is developed to precisely measure such parameter to maintain strict control
in composition.
BACKGROUND OF INVENTION
The level measurement of many liquids forms an integral part to many process
controlled industries. Both the point level measurement sensors and continuous
level sensors are being used to monitor the level of mainly clear liquids like
water, oil etc. The measurement techniques vary from simple inaccurate dip
stick method to float switches, capacitance level sensors, radar level sensors
to highly accurate non-contact ultrasonic sensors. A low-energy ultrasonic device
within the contact ultrasonic sensor measures liquid level at a certain point. The
sensor assembly consists of a field-mounted sensor and integral solid state
amplifier with having no moving parts and require no calibration. These sensors
are used in vessels or pipes to automatically operate pumps, solenoid valves,
and high/low alarms. The highly aerated liquids and liquids viscous enough clogs
the sensor gap causing problems in measurement.

The non-contact ultrasonic sensor on the other hand incorporates an analog
signal processor, a microprocessor, binary coded decimal range switches, and
an output driver circuit. The sensor transmits pulses and a gate signal from the
microprocessor route through the analog signal processor to the sensor, which
sends an ultrasonic beam to the liquid surface. The sensor detects the echo from
the surface and routes it back to the microprocessor for a digital representation of
the distance between the sensor and the surface level. Through constant
updating of received signals, the microprocessor calculates averaged values to
measure liquid level. Like ultrasonic sensors, capacitance sensors use a probe to
monitor liquid level changes in the tank, electronically conditioning the output to
capacitive and resistive values, which are converted to analog signals. The probe
and the vessel wall equate to two plates of a capacitor, the liquid to the dielectric
medium. Because the signal emanates from level changes alone, material build-
up on the probe has no effect. Non-conductive fluid vessels may dictate dual
probes or an external conducting strip. Rigid probes offer higher stability,
especially in turbulent systems, where swaying of the probe can cause signal
fluctuations.
Capacitive level sensors known in the prior art utilizes frequency sensitive
circuitry wherein a change in sensor capacitance caused by a change in the
dielectric constant of the surrounding medium produces a change in the

frequency of a high frequency oscillator. US patent 5 042299 has described a
capacitive fluid level sensor containing three capacitors: a measurement
capacitor, a compensation capacitor and an offset capacitor. The charge applied
to each of their capacitors discharged to an associated differential amplifier when
the capacitors are disconnected from a charging voltage supply. The US patent
5 097703 cites an invention related to a similar capacitive probe for use in a
system for remotely measuring the level of fluids. The probe is continuously
charged and discharged. The discharge current from the probe is measured and
converted to corresponding voltage representing the level of fluid. US patent
5073 720 highlights an invention pertaining to the development of an electro
optical device which uses a light source and an optical detector to measure the
level of a liquid in a container. The light beam is passed through the liquid and
received by the optical detector; the analysis of the same determines the liquid
level or liquid volume. In another invention US patent 8429965, a level sensing
apparatus and method is described based on capacitive sensing technology to
sense the level of beverage retained in the server.

In another invention 6 199428, a fluid level measuring device is
described for measuring fluid filling level in a container. A magnetic field sensor
is disposed in a zone of influence of a magnetic field generated by the magnet
and outputs an electric signal which is representative of a height of the fluid level.
This device is suitable for measuring fluid level of inflammable or explosive fluids.
Radar level gauges are often used to measure process fluids or process solid
levels in tanks, where the process materials range from benign materials to
severely corrosive or abrasive compounds. In US patent 7255002, a microwave
level gauge is utilized for measuring a level of a process material in a tank. The
system includes a ceramic seal and a microwave conductor. The ceramic seal is
disposed adjacent to an opening in the tank and adapted to isolate circuitry from
the process material. The microwave conductor is electrically coupled to the
circuitry and extends through the hermitic seal and to the process material in the
tank. Similarly, the US patent 7492 859 discloses a system and method for
measuring the density , level or interface position of a fluid in a vessel using the
intensity of the gamma ray backscatter. The multiple gamma ray emitters and/or
detectors may be positioned at opposite sides of a vessel, where the presence or
absence of a signal may indicate the presence or absence of a fluid in place
between the source and the detector. In one of the ceramic process in US
patent 4302 171, an apparatus was built for controlling the transfer of the

ceramic slurry material or similar materials from an initial preparation tank to a
working tank by providing a pivoting support assembly which will shift when the
working tank has reached a predetermined weight and provide an indication of
the slurry level in the tank without using sensors or other members inside such
tank.
Non-contact level sensing instruments using ultrasonic or radiation
detectors are among the most sophisticated and accurate measuring devices
used today. Because of their complexity and cost, they are restricted mostly in
industrial and commercial applications. US patent 775106 describes a system
arid method for using ^ probe based guided wave radar sensor to measwe^fluid
level in a container in a non-contact mode. The level probe is mounted external
to the container, adjacent to the radar transparent panel. By avoiding the contact
between the sensor probe and the fluid, whether the fluid be cement slurry or
•another fluid prone to probe degradation , it is possible to eliminate the
possibility of contamination , build-up, caking and/or damage to the probe
alongwith the associated degradation in the sensor performance. In US patent
2005/0241391, the inventors described an improved guided wave level
measurement device for measurement of fluid level in a storage tank. The

system includes a waveguide, a signal generator and a signal receiver. The
target presents a reflective surface and the position of the target is detectable
through time of flight measurements of a signal generated by the signal
generator.
OBJECTS OF THE INVENTION:
An object of the present invention is to propose a method for maintaining
accurate ratio of ceramic suspension of three types in a single storage tank with
continuous stirring during manufacturing of high voltage porcelain insulators.
Another object of the present invention is to propose a method to develop a
•control mechanism for accommodating the variation in composition during
manufacturing.
Further object of the present invention is to ascertain improvement in green
recovery of the electroporcelain product.

BRIEF DESCRIPTION OF THE INVENTION:
The present invention relates to a method for manufacturing high voltage
porcelain insulators comprising of maintaining accurate ratio of ceramic
suspension of three types of ceramic slips in a single storage tank with
continuous stirring.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig 1: shows the schematic of ceramic suspension movement for a typical
ceramic manufacturing line
Fig 2: shows the schematic of sensor for level measurement. The different slip
levels in the figure are only representations. In real term, as soon as the slurry is
fallen to the tank, it is mixed uniformly with the remaining slurry in the tank.
Fig 3: shows the schematic of the control system for non-contact measurement
of ceramic slip ratio
DETAILED DESCRIPTION OF THE PRESENT INVENTION:
The manufacturing of Porcelain material is a complex process involving
many steps such as: composition development, slurry preparation, storage, filter
pressing, forming, finishing, drying, glaze making, glazing and sanding, firing,

cutting and grinding, cement mixing, assembly, testing by different methods, final
inspection, packing and dispatch. The initial steps of composition development
and slurry preparation play, the most important role which determine the
properties of the final product. It also influences the productivity of the
manufacturing process by avoiding rejection and improving recovery. The
rejection can originate both at green stage and at fired stage. The green stage
rejection can be handled by reusing in the slurry preparation stage but fired
rejects cannot be reused. Therefore, the maximum utilization of rejects before
firing need to be taken care to improve the manufacturing productivity. However,
the green rejects are of two types: wet and dry. The scraps generated during
forming stage is known as wet scrap and is contaminated with oil and other
impurities during forming. The other major green rejection occurs after drying of
the article. The rejected piece at this stage can be reusable, but the material
has no moisture and hence it is termed as dry scrap. The slurry must be stored
for ageing prior to use in the main line. Other sources of green scraps during
manufacturing originated during finishing stage. These are semi solids with
moisture content between the dried mass and the wet scrap.
The manufacturing productivity depends on the zero rejection at the green
level. This implies that all the wet and dry scraps generated during the
manufacturing process must be reused up to the maximum extent along with the

fresh composition. This scheme however poses challenge since the dry scraps
contain no moisture and wet scraps contain impurities. The addition of both
these materials to the fresh slip will deregulate the properties of slurry such as
viscosity and composition. Therefore, the ratio of such addition needs to be
controlled. The ratio of fresh slurry mixed with separately made slurry from dry
scrap and wet scrap is maintained at different proportion in order to maintain
the overall composition and viscosity of the blended slurry which will finally be
sent for filter pressing from a storage tank. Adequate mixing of all the three
types of slurry in the final tank plays an important role and this is achieved by a
continuous moving paddle in the tank as the three different slips are filled in.
The present method of measuring the ceramic suspension slip ratio is mainly
focused on conventional manual type dip stick method due to easy operation of
such systems and cost effectiveness. However, the precise measurement of slip
ratio is not maintained by this method and the error in measurement can be as
high as 5 %. This has led to the recovery problem because if the dry scrap ratio
or wet scrap ratio is increased then the plasticity of clay gets altered which
eventually leads to defect formation during drying resulting in poor green
recovery. In BHEL, especially for 420 kN HVDC line, this is very crucial since the
final product quality requires stringent properties to be maintained. One of the
major factor of high green rejection of 10-15 % is the non-maintenance of the
proper slip ratio.

In this development, the source tanks of fresh slip, dry scrap slip and wet scrap
slip are interconnected with the final mixing tanks. In the present case, tank
no. 6 represents fresh slip, 12 and 13 represent wet scrap and 15 and 16
represent dry scrap slip. Provision has been already made for filling the slip from
each tank to the final mixing tanks 17 and 18. A paddle is continuously
moving inside the final tanks 17 and 18 to homogenize the slip mix., In the
present invention, the slip composition of fresh: wet: dry is decided by the
technology group and the same is fed into the computer. The final tank has
been calibrated both in empty condition and in fully filled condition. The piezo
electric crystal in the sensor sends an ultrasound to the level of liquid in the tank
and receive the echo developed by the fluid level. The echo is converted into
electrical energy for onward processing by the control circuit. Figure 3
summarizes schematically the principle of non-contact measurement system
for ceramic suspension in a tank.
The ratio of each slip is fed into the system percent wise by taking care
the volume of the calibrated tank. Based on the input data, first the tank contain
the fresh slip is energized and the filling from tank no. 6 starts to tank 17 or 18.
The ultrasonic sensor is attached to both the final tanks. Once the programmed
value is attained, the pump stops filling in from that tank. Then the next pump is

energised with the required percentage value. This process continues till the
required value is attained. Similarly, the third slip is also operated. Finally, 100
% slip ratio is maintained in the final tank. The movement of the paddle helps in
uniform mixing of the three slips in the final tanks. Provision has been made to
decide the tank numbers for filling at any particular time depending on the
.availability of that particular slip. Similarly, the final tank also can be chosen
based on the body composition and availability of slip.
After the slip ratio is maintained and the slip is aged for the required
duration, again the system is energized to pump out the mixed slip from the
final tank to the filter pressing. The level of the tank can be monitored
continuously during the entire operation of draining out of the slip. Once the
drainage is complete, the tank is again ready for filling with the programmed slip
ratio. This cycle continues every day and the manual intervention on
measurement has been totally stopped using this method. The accuracy in
measurement is achieved at 0.1 %. This has resulted in improving the overall
green recovery of the product in the range of 5-7 %.
One of the difficulty in measurement faced was the disturbance due to
continuous paddle movement for homogenization in the final tank. This was
taken care in the non contact measurement system with suitable programming

which filters the disturbance in the input data so that the movement does not
interfere in the final measurement of suspension level in the tank.
In this invention, a new non-contact type level control measurement system was
developed to accurately measure the slip ratio for electro porcelain composition.
Figure 1 summarizes the schematic of three different ceramic suspensions
from a typical manufacturing line of BHEL from Tank Nos. 6,12, 13,14 and 15
are uniformly mixed in final tanks 17 and 18. The Non-contact
measurement systems are placed in tank nos. 17 and 18 to precisely control the
suspension ratio of all the three constituents up to 0.1 % accuracy.
The ultrasonic signal is used to accurately measure the depths of the ceramic
slip filled in from different tanks which are pre-calibrated and computer controlled.
The ratio of three slips can be altered depending on production requirement and
depending on composition requirement for fabricating different types of
insulators. The calibration of the system also takes care the continuous
movement of the paddle which is used in the final tank to continuously mix the
different slips. The introduction of the system has also helped in minimizing the
error committed from the operator in different shifts.

The entire system can be connected with intranet for monitoring the slip filling
and draining progress from any personal computer inside the factory. This will
help in taking immediate decision on the manufacturing process.
The present invention can best be understood with suitable examples
Example 1
In a composition of the ratio Fresh: wet: dry :: 60:30:10 , the slip from
tank no. 6 will be drawn till 60 % of tank 17. Then the wet scrap slip will be
drawn from tank 12 for 30 % of the tank. This will be put off once 90 %
reached in tank 17 volume. Finally, the filling from tank 14 with dry scrap slip will
commence and will automatically shut off after the tank no. 17 reached 100 %
volume. This ensures the maintenance of proper ratio upto one decimal place
accuracy for each of the three types of slips.
Example 2
In a composition of the ratio Fresh: wet: dry:: 60:30:10, the slip from either
of the tank no. 6 or 7 will be drawn as outlined in example 1. The variation in
specific gravity from 1.4 to say 1.42 or 1.38 of incoming slips due to process
variation can be adjusted keeping the ratio fixed by calculating appropriate

volume and maintaining in the final tank. Therefore, the level of different slips
can also be different with the same final volume. This is achievable by the
accurate method of each of the suspension using this technique.
Example 3
In a composition of the ratio Fresh: wet: dry :: 70:20:10 , the slip from
tank no. 6 will be drawn till 70 % of tank 17. Then the wet scrap slip will be
drawn from tank 13 for 20 % of the tank. This will be put off once 90 %
reached in tank 17 volume. Finally, the filling from tank 15 with dry scrap slip will
commence and will automatically shut off after the tank no. 17 reached 100 %
volume. This ensures the variation of ratio upto one decimal place accuracy for
each of the three types of slips as per the requirement of the manufacturing
process.
The present method as described above can accommodate any ratio formulation
for any number of incoming ceramic slips . Further, the system is capable of
use in any type of viscous ceramic slip condition including corrosive or wear
prone material based slips as the probe does not contact the liquid.

WE CLAIM:
1. A method for manufacturing high voltage porcelain insulators comprising
maintaining accurate ratio of ceramic suspension of three types of ceramic slips
in a single storage tank with continuous stirring.
2. The method as claimed in claim 1, wherein the three types of ceramic slips
used are fresh, dry scrap and wet scrap.
3. The method as claimed in claim 1, wherein the fresh raw material in water, wet
scrap material in water and dry scrap material in water are subjected to the step
of mixing to form a ceramic suspension.
4. The method as claimed in claim 1, wherein the ratio of fresh: wet: dry ceramic
scrap slips 60 to 70: 20 to 30:10
5. The method as claimed in claim 1, wherein the three ceramic slips are
taken upto 0.1% accuracy.

6. The method as described in claim 1 wherein the system can filter the
disturbance in level measurement of ceramic slip due to any disturbance
during mixing in the tank.