1. About SRG Insulators

!SRG Insulators is one of the prominent Manufacturers, Suppliers and Exporters of wide array of

Insulators, Industrial Cables, Transformer Brushings and Lighting Arresters. We develop the range

of porcelain insulator in compliance with IEC/AS/BS/IS and as per customer’s unique

requirements. These products are manufactured from various raw materials, semi finished and

finished products. Our entire product is available as per CPRI Test Reports.

!SRG Insulators was incepted with the sole aim of catering to the requirements of clients for

Insulators, Industrial Cables, Transformer Brushings and Lighting Arresters. Our company is

engaged in manufacturing, supplying and exporting of electrical porcelain insulators from Bikaner.

!1.1 Mission

Our Mission is to provide our clients with Products and Services that fully satisfied their unique

requirements and to provide superior Products and Services that are competitively priced with the

highest quality and delivery in a timely manner.

!1.2 Quality Assurance

Our products are tested electrically and mechanically as per Indian standard procedures through

different types of testing equipments to ensure quality. We also have a unique kiln design that is

designed by highly qualified engineers, which vitrifies uniformly all the material fired in it.

!1.3 Infrastructure & Geographical Advantage

The state-of the-art infrastructure is supported by a well-maintained manufacturing unit having

latest machinery and modern techniques. We frequently upgrade our infrastructural facilities that

enable us to produce quality products with utmost precision. The unit is ideally located in respect to

the availability of good quality raw materials in abundance within the state.

!1.4 Human Resource

The team comprises of both young and experienced members. Their dedication and commitment

towards work not only help providing excellent quality in bulk orders but also assure supply within

the stipulated time frame.

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1.5 Products & Marketing

They are manufacturing various types of insulators for transmission and distribution lines.They are

supplying products to almost all the state electrical utilities including National Thermal Power

Corporation Ltd., National Hydro Power Corporation Ltd. and other government-undertaking units

to their utmost satisfaction.They are also one of the leading manufacturers of Transformer Bushings

and regularly supplying our products to more than 200 transformer manufacturing units spread all

over India.

!Some of the Valued Customers for Porcelain Insulators

• Central Power Distribution Co. Ltd. of A.P.

• Southern Power Distribution Co. Ltd. of A.P.

• Northern Power Distribution Co. Ltd. of A.P.

• Eastern Power Distribution Co. Ltd. of A.P.

• Jaipur Vidyut Vitran Nigam Ltd.

• Jodhpur Vidyut Vitran Nigam Ltd.

• Ajmer Vidyut Vitran Nigam Ltd.

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1.6 Machinery

Table:-1 Various Machinery Installed in the Plant

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2. Insulators

!A true insulator is a material that does not respond to an electric field and completely resists the

flow of electric charge. In practice, however, perfect insulators do not exist. Therefore,

dielectric materials with high dielectric constants are considered insulators. In insulating

materials valence electrons are tightly bonded to their atoms. These materials are used in

electrical equipment as insulators or insulation. Their function is to support or separate

electrical conductors without allowing current through themselves. The term also refers to

insulating supports that attach electric power transmission wires to utility poles or pylons.

Some materials such as glass, paper or Teflon are very good electrical insulators. Even though

they may have lower bulk resistivity, a much larger class of materials are still "good enough" to

insulate electrical wiring and cables. Examples include rubber-like polymers and most plastics.

Such materials can serve as practical and safe insulators for low to moderate voltages (hundreds,

or even thousands, of volts).

The advantages of porcelain insulators include superior electrical properties, good mechanical

properties (especially tensile strength), good creep resistance at room temperature and high

corrosion resistance.1

!An insulator might reach the end of its working life for many reasons. Cracks in the body might

be caused by a mechanical failure or a thermal mismatch between the ceramic part and the metal

part. In addition, flashover might result from contamination on the glaze or weathering that

leads to a small crack on the surface.2 Thermal cycling causes material failure since it promotes

the growth of micro-cracks on the surface. Thermal stress is generated by temperature

differences between day and night, which may result in 80ºC differences on the glaze surface.

The heat generated may also come from the passage of fault-current arcs in the insulator.3

!Generally, the mechanical strength of the porcelain body can be improved by reducing the

internal thermal mismatch between crystalline particles and glassy phases.4 An increase in

mullite crystals can result in a change of the thermal expansion property of the body. The

coefficient of thermal expansion is a thermal property related to chemical composition, firing

temperature, particle size distribution of raw materials, particle packing of green and fired

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bodies, crystal structure, and the glassy phase of the body. Crazing of the glaze can be caused by

a thermal mismatch between the body and glaze. Usually, the thermal expansion of the body

should be higher than that of the glaze in order to generate compressive stress (instead of

tension) in the glaze layer.

!Many methods are available to determine the coefficient of thermal expansion, including

calculation from the proportion and types of oxides in the body composition. However, this

method is not accurate because many factors affect the calculated values. The most precise

method is through measurement by a dilatometer.

!Insulators are produced in many shapes, which are formed by different forming processes. For

complicated shapes that cannot be formed by jiggering, slip casting is used. Other products, like

a fused support with a simple shape, can be formed by pressing.

!It is important to understand the physical, mechanical and thermal properties of porcelain bodies

that are formed by slip casting, extruding and pressing. The effects of forming processes on

fired properties can be explained by the phases and microstructures of the fired products.

Understanding all of these relationships will result in increased accuracy when predicting and

determining the thermal mismatch and life cycles of porcelain insulators.

! The insulators for overhead lines provide insulation to the power conductors from ground. The

insulators are connected to cross arms of the supporting structures of the power conductors and

the power conductors pass through a clamp of the insulators. The insulators are mainly made of

either glazed porcelain or toughened glass. The materials used for porcelain are silica 20%,

feldspar 30% and clay 50%. The porcelain should be ivory white, sound and free from defects.

It should be vitrified because the presence of air lowers the dielectric strength of porcelain. It is

therefore desirable that the porcelain to be used for insulators should be air free and impervious

to entrance of liquids and gases .The dielectric strength of porcelain should be 15 to 17 KV for

every 1/10th inch of thickness. Normally it is difficult to manufacture homogeneous porcelain,

and therefore for a particular operating voltage, two, three or more piece-construction is adopted

in which each piece is glazed separately and then they are cemented together. Porcelain is

mechanically strong, less affected by temperature and has minimum leakage problem.

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Toughened glass is also sometimes used for insulators because it has higher dielectric strength

(35 KV for1/10th inch thicknesses) which makes it possible for single piece construction,

whatever be the operating voltage. As glass is transparent, the flaws like trapping of air can be

detected. It has lowered co-efficient of thermal expansion. It has a disadvantage that moisture

condenses very easily on its surface, limiting its use to about 33KV.

! It is not desirable to allow porcelain and metal pieces to come together. So cement is used

between them. It is seen that cement does not cause fracture by expansion or contraction.

!The principal dielectric used on overhead power lines is air at atmospheric pressure. The air,

surrounding the bare high voltage aluminium or steel- cored aluminium (ACSR) conductors, is

a good insulating material, provided that the electric stress is kept below the ionisation

threshold. It is, however, necessary to attach the conductors at certain points onto the cross arms

of the pylons. The problem of reliably suspending the conductors of high voltage transmission

lines has therefore been with us since the turn of the century. The task is particularly complex,

bearing in mind the multiple extreme stresses present: mechanical, electrical and environmental.

!!

!

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2.1 Types of Insulators

There are three types of insulators used for overhead lines.

Pin type

Pin type insulators are normally used up to 33kv. In any case it is not desirable to use them

beyond 50 V as the cost of insulation becomes too high.

Fig 2.1 Pin Type Insulator ! Suspension type

! Suspension type insulators, in addition to being economical as compared to pin type for

voltage more than 33KV have the following further advantages:

!1) Each insulator is designed for 11KV and hence for any operating voltage a string of

insulators can be used.

!2) In case of failure of one of the units of string, only that particular unit needs replacement

rather the whole string.

!3) The operating voltage of existing transmission can be increased by adding suitable number of

disks.

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Fig 2.2 Suspension Type Insulator

Strain Type

Strain insulators are placed in horizontal plane rather the vertical plane

as is done in case of suspension. These are used to take the tension of conductors at line terminals,

angle towers, road crossings and junction of overhead lines with cables. For low voltages of order

11KV, the shackle insulators are used.

Fig 2.3 Strain Type Insulator

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2.2 The Advantages of Porcelain Insulators

!1. Environmental friendly. At its disposal, the porcelain insulator is not dangerous waste.It is

manufactured form natural materials by simple blending and curing; it may be stored in dumps with

other waste. It may serve as a recycled material for the production of ceramic and similar products.

!2. In comparison to the polymer, electrical strength of porcelain is higher: 25+ kV/mm v. 20 kV/mm

at the polymer. The porcelain insulator in the dry state as electric insulation material has better

electrical properties than the polymer, type electrical tests show better results, giving longer useful

life in terms of loads generated by electric charges and other temporary electrical phenomena.

!3. The porcelain insulator has demonstrably higher resistance to degradation of the surface, does not

degrade or carbonate during charges; the conductive path is created very slowly in comparison of

the surface of a composite-material insulator.High thermal resistance and strength, ceramics is

resistant to temperatures as high as 1000°C: the surface is resistant to any type of degradation

within the temperature range. The surface is stable against the effects of UV radiation.

!4. The ceramic material is resistant to rodents, termites, birds and other animals capable of

compromising the integrity of polymers.The surface of the insulator is highly glazed and hard,

making the product unfavourable to the tastes of the fauna.

!5. The ceramic insulator has a wide scope of application: Contractors, disconnectors, equipment

transformers, condensers, grommets also with extreme surface, atypical insulators (filters). The

features of high plasticity during production, the possibilities of precision grinding and quite easy

cementation and bonding with excellent mechanical properties permit that a multitude of shapes be

created and used in any type of application.

!6. The ceramic insulator is suitable for extreme hot/cold changes in the environment. It is suitable

for environments with dust, salt and high moisture, or for combination of all of the above. The

highly glazed surface gives the product better self-cleaning properties in high-pollution areas. The

product shows stable results in charges and short-circuit in this type of environment; it is highly

resistant to corrosion in acidic as well as caustic environments.

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7. The ceramic insulator does not suffer from defects in the ceramics-to-metal interface. The

combination of the ceramic insulator with cast-iron or aluminium structures using traditional

cementing agents is resistant to transition phenomena during the discharge or brush discharge.

!8. The ceramic material offers very high mechanical strength under pressure and hardness. The

ceramic insulator does not deform unless external force is deployed. Long useful life can be

guaranteed of lengths up to 40 years. Therefore, many users have provided long-term operational

references in a number of applications.

!9. The design is modified to suit the environment.The product offers many shapes during

production; glazing uses a wide scale of colours based on the needs of the customer, for example

grey or sky blue.

!10. The ceramic insulator is nicer to the eye

It has a timeless design.

!2.3 Terminology

When applying insulators, it is necessary to describe the insulator dimensions, using the

following terms:

• Creepage distance: the shortest distance between the metalware at the two ends of the

insulator, when following the contours of the insulator, excluding intermediate metal

fittings. This distance is easily measured by sticking masking tape to the insulator

surface.

• Specific creepage distance: The quotient of the creepage distance in mm and the line-

to-line rms. voltage of the three phase system in kV

• Connecting length: the axial length of the insulator between the end terminals

• Arcing distance: the distance between the metalware, measured as the length of a

tightly pulled piece of string

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• Intershed spacing: the distance between corresponding points on adjacent sheds.

2.4 Failure Modes of Insulators

Flashovers, caused by air breakdown or pollution, generally do not cause physical damage to the

insulators and the system can often be restored by means of auto closing. Some other events,

however, cause irreparable damage to the insulators.

Puncture

As previously mentioned, porcelain pin-type and cap and pin insulators may suffer punctures

between the pin and the either the pin or the high voltage conductor. These occurrences are usually

caused by very steep impulse voltages, where the time delay for air flashover exceeds that of

puncture of porcelain.

Punctures caused by severe stress over dry bands also occur on composite insulators on sheds and

through the sheath. A puncture of the sheath is particularly serious as this exposes the glass fibre rod

to the environment (see brittle fracture below).

Shattering

Glass insulators shatter when exposed to severe arcing or puncturing due to vandalism. One

advantage is that they retain their mechanical integrity.

Erosion

Prolonged arcing of glass insulators leads to erosion of the surface layer of the glass. This may lead

to shattering of the glass discs - a result of the tempering process used during manufacture. Arcing

and corona over long periods may cause removal of shed or sheath material in the case of polymeric

insulators. Severe erosion may lead to the exposure of the glass fibre core (see brittle fracture)

Tracking

Tracking occurs when carbonised tracks form because of arcing. These tracks are conductive. This

phenomenon only occurs in carbon-based polymers.

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Brittle fracture

Water entry into the glass fibre core of composite insulators, coupled with the influence of weak

acids, has been shown to lead to brittle fracture of the rod. The by-products of partial discharges in

the presence of water can lead to the formation of weak acids. The integrity of the metal/polymer

and glass/polymer interfaces is therefore extremely important - especially if acid-resistant glass is

not used.

Corrosion

The corrosion of metal fittings clearly affects the mechanical performance and lifetime of

insulators. The corrosion products, running onto the insulator sheds can also initiate deterioration.

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3. Production Processes 3.1 Overview

!

Fig 3.1 Flow Chart of the Production Process in Porcelain Insulators

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3.2 Raw material select

!Clay a universal component used in preparing ceramics owing to its extraordinary ease of

transformation through moulding and hardening processes. Although there are numerous

classifications, the most common classification in the manufacture of ceramic coverings is based on

the colour of the body-red or white-. Red-body clays are characterised by their high iron content,

low melting temperature, and moderate plasticity, whereas white-body clays are more plastic and

have no colouring oxides.

The Principle raw materials are ball clay, china clay, feldspar and quartz, roughly as 50% clays,

25% feldspar and 25% quartz

Both ball clays and china clay are decomposition products of granitic rocks and both comprise

mainly kaolinite.Kaolinite is a layered silicate: even at molecular level its structure comprises plans

of atoms, in a multi-deck sandwich assembly of oxygen, silicon, oxygen with hydroxyl, aluminium

and finally hydroxyl.The remarkable rheology of clays arises both from this layered form and from

the manner in which water is embodied into the material.

Fig 3.2 Structure of a clay

!!Feldspars are alkali-aluminium silicates.The alkali feldspars are as follows:

• orthoclase (monoclinic),[7] — KAlSi3O8

• sanidine (monoclinic)[8] —(K,Na)AlSi3O8

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• microcline (triclinic)[9] — KAlSi3O8

• anorthoclase (triclinic) — (Na,K)AlSi3O8

3.3 Ball mill

!A ball mill is a type of grinder used to grind materials into extremely fine powder.A ball mill works

on the principle of impact: size reduction is done by impact as the balls drop from near the top of

the shell.

A ball mill consists of a hollow cylindrical shell rotating about its axis. The axis of the shell may be

either horizontal or at a small angle to the horizontal. it is partially filled with balls. The grinding

media is the balls, which may be made of steel (chrome steel), stainless steel or rubber. The inner

surface of the cylindrical shell is usually lined with an abrasion-resistant material such as

manganese steel or rubber. Less wear takes place in rubber lined mills. The length of the mill is

approximately equal to its diameter.

In case of continuously operated ball mill, the material to be ground is fed from the left through 60°

cone and the product is discharged through a 30° cone to the right. As the shell rotates, the balls are

lifted up on the rising side of the shell and then they cascade down (or drop down on to the feed),

from near the top of the shell. in doing so, the solid particles in between the balls are ground and

reduced in size by impact.

Fig 3.3 Ball Mill

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Grinding in ball mills is an important technological process applied to reduce the size of particles

which may have different nature and a wide diversity of physical, mechanical and chemical

characteristics. Typical examples are the various ores, minerals, limestone, etc. The applications of

ball mills are ubiquitous in mineral processing and mining industry, metallurgy, cement production,

chemical industry, pharmaceutics and cosmetics, ceramics, different kinds of laboratory studies and

tests. Besides particle size reduction, ball mills are also widely used for mixing, blending and

dispersing, amorphisation of materials and mechanical alloying

!3.4 Electro magnetic filtering

!Electro magnetic filtering is a key step, the iron in the raw materials will influence parts mechanical

and electric property, Exotic iron also will influence raw material,so we need the work environment

tidy and without contamination.

3.5 Filter pressing

!Much of the water is removed by filter pressing, sometimes with applied heat to reduce the

viscosity of the water.

!Filter press is a separation process, specially employed by solid/liquid separation using the principle

of pressure drive, provided by a slurry pump. Filter press is a fixed volume and batch operation,

which simply means that the operation must be stopped to discharge the filter cake before the next

batch can be started. The major components of filter press are skeleton and filter pack. The skeleton

holds the filter pack together while pressure is being developed inside filter chamber. It however

can only hold a specific volume of solids.

!Generally, the slurry needed to be dewatered is injected into the center of the press and fill up each

chamber.The filling time should be as quick as possible in order to avoid cake formation in the first

chamber before filling up the last chamber. While the chambers are being filled up, the pressure

inside the system will increase due to the formation of thick sludge. Then, the liquid is filtered out

through the filter cloths by adding streams of compressed air or water. The use of pressurised water

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require more time to pass into the chamber compared to pressurised air, however this method is

much more cost efficient.

Fig 3.4 Filter Pressing

!3.6 Vacuum pug mill

!The process of vacuum pug mill is for ejecting the air in the pug ,it can let the pug more compacted

and improve its plasticity. The word "pug" means "to mix with water to make more elastic." A pug

mill, then, is a machine used to grind and mix wet and dry materials, usually for clay, asphalt and

similar compounds. Generally, a pug mill consists of a container and one or more rotating arms with

blades or paddles on them. These act like a blender, producing a smooth, moist mixture that is easy

to work with.

A pug mill may be a fast continuous mixer. A continuous pugmill can achieve a thoroughly mixed,

homogeneous mixture in a few seconds. Mixing materials at optimum moisture content requires the

forced mixing action of the pug mill paddles, while soupy materials might be mixed in a drum

mixer. A typical consists of a horizontal boxlike chamber with a top inlet and a bottom discharge at

the other end, 2 shafts with opposing paddles, and a drive assembly. Some of the factors affecting

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mixing and residence time are the number and the size of the paddles, paddle swing arc, overlap of

left and right swing arc, size of mixing chamber, length of pug mill floor, and material being mixed.

!

Fig 3.5 Extruded Cylinders

Fig 3.6 Various Sections in Vacuum Pug Mill

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3.7 Turning

!Blanks are put in on turning machines to be turned in hollow cores (machining inside and outside).

Fig 3.7 Turning Process on Vertical Lathe Machine

!!3.8 Drying

Remove the mechanical combined moisture in the semi-product. Insulators stay in drying rooms 3

weeks with temperature of 373K to reach approximately 0.3 to 0.5 percent of moisture. During this

step , each element losses 12% of its volume.

Fig 3.8 Porcelain Insulators in Drying

!!

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3.9 Visual check and quality control

Visual and dimensional test according to drawing.

3.10 Glazing & Sanding

!This process consists of applying a thin glaze layer that adheres to the surface of the piece by a

fusion process, generating a protective, impervious coating that adds new technical and ornamental

properties, such as slip resistance, chemical resistance, and mechanical strength, as well as gloss or

colour.

The raw materials used may be wet milled, yielding a slip resembling the suspension used in

preparing the ceramic body, though it contains particles with a smaller size and a larger quantity of

water. They may also be dry milled and dispersed in water, or applied dry for special effects.

There is a great variety of glazes, which are adapted to different bodies and firing temperatures. At

present, glazes may also be found with self-cleaning or biocidal properties, these being the result of

the ongoing technological innovation in the ceramic industrial sector.

Glaze compositions are mostly made up of frits, unfritted raw materials, additives and ceramic

colorants. These ingredients are variously combined,depending on the targeted product.

Components

Frits. Inorganic chemical substances obtained by fast cooling (quenching) a complex combination

of materials that look like broken glass. The use of frits reduces the toxicity of the process, widens

the firing range, and provides a more uniform finish.

Unfritted raw materials. Substances that contain the different types of oxides responsible for

providing the glaze composition with the desired characteristics.

Additives. Substances that, added to the raw materials, improve certain technical qualities.

Particularly to be noted among the various types are the suspending agents, deflocculants, and

binders.

Colorants. Substances that, added to the raw materials, modify the colour qualities and provide the

glazed surfaces with their definitive appearance.

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Process

To perform the vitrification process on the surface of the ceramic piece,an engobe layer needs to be

applied beforehand, which acts as a bond between the body and the glaze, assuring proper

anchoring between both. The engobe eliminates external irregularities and hides the colour of the

substrate, providing a uniform surface and creating an impervious layer to prevent problems of

porosity.

The glaze is applied on the engobe layer in the form of an aqueous suspension, all its properties

being acquired once the firing has ended, yielding a consistent, compact coating with a uniform

thickness known as a consolidated layer. There are different application methods: manual, dry

application, spraying, and continuous curtain application, the last two being the most widespread in

the ceramic industry.

Manual. This is done using manual techniques such as pouring or dipping, or using different

utensils, and it is performed by a craftsman or an artist.

Dry application. This is performed with devices that proportion dry-ground frit particles of different

sizes, or products obtained by granulation or thermal softening of the glazes.

Spraying. This is performed using a spinning disc that deposits small drops on the surface of the

piece, forming a continuous thin layer with a rough finish.

Continuous curtain. A glazing bell is used. The slip flows down over the outer surface of the bell as

a continuous waterfall,coating the ceramic body with a glaze layer of greater thickness and smooth

texture

3.11 Kiln

The firing process is performed in different types of kilns and, depending on the

characteristics of the product, the ceramic pieces are subjected to high temperatures in a

range of 900 to 1300 ºC. The kilns have three different stages: loading, firing cycle, and

unloading of the elements. Depending on how these stages are carried out, the kilns are

divided into kilns with an intermittent cycle or a continuous cycle.

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Intermittent

Intermittent or periodic kilns are characterised by a break in the energy input after each

firing cycle. The heating system stops and loses the heat accumulated during the process,

which involves a necessary waiting time and lower productivity.

Fig 3.9 Loaded Wagon undergoing Firing in Kiln

This is customarily carried out in bell kilns, which consist of a working base on which the ceramic

pieces are set and a moving structure that surrounds the elements during firing. Heating is

performed by radiant tubes installed inside the walls or by direct fire in the base, in both cases

achieving a high degree of temperature uniformity as a controlled atmosphere is involved. It is

customarily used in manufacturing moderate production volumes, and it is suitable for firing large

pieces.

Continuous

Continuous kilns are characterised by uninterrupted development of the firing cycle. The heat

released by the pieces during the cooling stageis reused in the initial heating stage, leading to

greater energy recovery and higher productivity. This customarily takes place in single-deck roller

kilns, the deck consisting of a line of constantly moving rollers that allow the pieces to be

continuously conveyed through the kiln, providing multiple advantages, such as greater heat

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recovery, shorter firing cycle, low energy consumption, and less air pollution. This completely

automated process is suitable for very high production volumes, as a result of which its use has

prevailed at present in the manufacture of flat ceramic coverings.

!3.12 Firing

!Firing is the most important manufacturing process stage because, when the semi-hardened

elements travel through the kiln, they undergo fundamental transformations in their

physicochemical properties. The processed pieces are subjected to a high-temperature thermal cycle

that varies as a function of the product to be obtained, giving rise to a hard, resistant material, with

technical characteristics that determine its ceramic condition.

!Complex processes, both physical and chemical, occur during firing, which is performed in either

type of kilns. The ambient atmosphere is controlled and is usually oxidising for firings up to 1473 K

Durations may be several days: the temperature/time characteristics, both heating and cooling, is

monitored and controlled.

!The sequence of events, as the temperature rises, is in essence loss of remaining water,

decomposition and recombination of the clays and their associated impurities, formation of viscous

liquids, as the feldspars react with parts of the clay residues and silica, sintering of the solids under

the influence of the glassy components.

!The regimes are approximately as follows:

Up to 373K the pore water and surface-bound water is lost.

Between about 673K and 913K organic impurities volatilise

Residual carbon sulphurs and carbonates burns out at about 1173K

!!At about 850K an important physical transformation occurs which produces a significant kink in the

thermal expansion characteristic. This is the transformation of quartz from one crystalline form to

another.

!�23

The clay lose chemically bound water from 715K to about 950K. This reaction is irreversible and

endothermic and is accompanied by significant contraction, which may cause shrinkage-cracks in

large pieces. Above 1173K the clay residues react chemically to form mullet, an aluminium silicate

and silica, both of which take part in the fluxing reactions with the feldspar.

!Rigidity during firing depends on the viscosity of the liquid glass, governed by the proportions of

Na and K in the feldspar and also by the presence or absence of alkaline earth like Ca. High

viscosity gives wide firing, i.e tolerance of temperature variations without loss of properties. It is

this viscosity which allows long and heavy pieces to be hung from one end without bottom support

and to survive firing without failure in tension

!As the piece is allowed to cool, the body, now a compact mass of crystals and grains bound by

glassy matrix, hardens at about 1373K. The principal hazards during firing are seen to arise from

departure of volatiles, which must not be allowed to generate bubbles or pores, from volume

changes associated with chemical reactions and tending to cause cracks, but above all from

differences in thermal expansibility. The clays, quartz, alumina and feldspars all have widely

different expansion curves, and the composite characteristic, for the porcelain body, is dominated by

different components in different regimes.

!Even when the body is cooled to room temperature the internal stresses, arising from the differences

in expansibility, remain. In particular, the quartz grains are left in tension and constitue a potential

source of failure micro cracks.

!

3.13 Cycle

The firing cycle consists of two different stages: heating and cooling. Heating is performed in two

steps,known as initial heating and firing, with a view to drying the pieces and hardening them.

Cooling, in which it is sought to accelerate the cooling of the fired product, takes place in three

steps: forced cooling at high temperature, natural cooling, and forced cooling at low temperature.

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Heating

Initial heating. This consists of gradually raising the temperature to completely remove the moisture

from the pieces and avoid the risk of breakage caused by fast expulsion of water, preventing the

appearance of surface defects in the finished product.

Firing. This consists of a high-temperature thermal cycle in which the raw materials are

transformed into a single complex structure that provides the pieces with strength and hardness,

giving them the technical characteristics that determine their ceramic condition.

Cooling

Forced cooling at high temperature. This takes place by forced convection and consists of impelling

air at ambient temperature into the kiln, close to the pieces, shortening the firing cycle.

Natural cooling. This takes place by natural convection and consists of reducing the temperature

without any artificial means, with a view to obtaining gradual cooling that minimises any possible

stresses caused by volume changes.

Forced cooling at low temperature. . This takes place by forced convection and consists of

impelling air so that the pieces can be handled once they have acquired thermal shock resistance.

Number of firings

Single firing. The element is fired once. This is the most widely used system as it involves

important savings in time and energy. When glazes and decorations are applied, this must be done

on the unfired body.

Double firing. The element is fired twice: the first firing serves to obtain the ceramic biscuit, while

the second firing is performed after the glaze application.

Third fire. Three firings of the element are performed, the third firing being for special decorations

on the already fired glazed body. A fourth firing may be performed on pieces that require very

complex ornamentations.

!!

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3.14 Summary

Table 2 Summary of the Production Process

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4. Calibration Lab The calibration lab is used to calibrate different instruments used in the plant. Firstly the slave

instruments are calibrated from the standard instruments and then they are used for the calibration

of instruments used in plants time to time.

!The instruments used in calibration lab are:- !4.1 Name of Instruments !1. Vernier Calliper

2. Depth Calliper

3. STD Pressure gauge(40 KG/cm2)

4. STD Pressure gauge(250 KG/cm2)

5. STD Pressure gauge(700 KG/cm2)

6. STD Vacuum Gauge(760 MMHG)

7. Digital multimeter(750 Volt)

8. Stop watch

9. Thermocouple(R type)

10. Thermocouple(K type)

11. Thermocouple(B type)

12. Mer. Thermometer

13. Slip gauge

There are other instruments also like IC TESTER which are used to test the functioning of IC.

!!!!!!!!!

�27

5. Ceramic Lab In Ceramic lab different tests are done on slip and the bar prepared from the plant to check the

different contents of slip and physical properties of the insulator.

Generally insulators are made from feldspar,quartz,clay etc. So the iron is injurious to the

insulators.In this lab the physical & chemical testing are done to determine:-

5.1 Physical Test !

1. Particle size

2. Thermal Expansion

3. Residue

4. Sp. Gravity

5. Viscosity

6. pH !5.2 Chemical Test

!1. Determination of SiO2

2. Determination of Al2O3

3. Determination of Fe2O3

4. Determination of CaO

5. Determination of MgO

6. Determination of K2O

7. Determination of Na2O

!Different processes like Spectrophotometry, Flame photometry, EDTA testing, Viscometer,

thermocouple etc are used in this lab.

Every day slip & different clay are taken from the slip house to determine different contamination

in them and the moisture content in them.Then the report is prepared and send to the slip house so

that they follow the instruction of the report. !!

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6. Manufacturing Units: The various manufacturing units are as follows:

i. Storage

ii. Slip House

iii. Jiggering Unit

iv. Finishing Unit

v. Glazing Unit

vi. Firing Unit

vii. Assembly Unit

viii.Testing Unit

ix. Dispatch Unit

!!6.1 Storage

The different raw materials for manufacture of insulators are stored in the identified bins.

The raw materials are:- 1) Calcined Chaibasa clay

2) Dolomite

3) Manganese dioxide

4) Chrome ore

5) Calcite

6) Talc

7) Ellur Clay

8) Than Clay

9) Quartz for brown glaze

10)Quartz

11)Feldspar

12)Bikaner clay

13)Pyrophyllite

14)Felcite

15)Sericite

16)Japan ball Clay

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!6.2 Slip House !Slip house performs the following operations:

!1. Weighing

Different raw materials from the storage bins are collected and weighed according to the

percentage composition given by the E&D and according to the ball mill capacity to be charged.

Weighing should be rounded to nearest multiple of least count. Then the weighing card is

attached to the lot.

A sample data for preparation of suspension body slip in a 5 ton ball mill for the first

charge are as follows:

2. River-pebble charging:

!For initial charging of river pebble in the ball mill after reling, large, medium and small pebbles

are collected as per standard samples and weighed as per specification No. PPSL: 0100 R-2 and

pebbles are charged as per the quantity specified

!!3. Preparation of first charge Body slip:

!�30

Weighed quantity of first charged raw materials are first charged into ball-mill by attaching a

cone to its hole from the charging hole on the platform. The pebbles are cleaned with water

before charging. Then specified quantity of water is charged into the ball mill. The ball-mill is

run for the specified time and initial and final meter reading , time , ball-mill number are also

noted. Then a little quantity of first slip is collected for checking slip parameters. Generally

charging takes 1-2 hrs and ball-mill runs 14-16 hrs.

!!

4. Preparation of second charge body slip

!Weighed quantity of second charge is charged into the ball-mill with required quantity of water

after preparation of first charge body-slip and the ball-mill is run for specified time. After

complete grinding the ball-mill is stopped and sample is collected for checking slip parameters.

Then the slip is discharged into mixing tanks and is agitated continuously.

!5. Blunging return slip:

!Scrap and water returned by the process is charged into the blunger in specified proportions to

get required specific gravity of the slip. Then it is blunged to achieve homogeneous slip. Then

the slip is transferred to return storage tank by passing through Vibroscreen. The timely removal

of foreign materials from the Vibroscreen should be ensured. !6. Preparation of cake:

The filter press cloth should be free from folding and clay sticking before pressing operation

and this must be ensured. The adequate power pag pressure is applied and filter press is closed.

The slip from the final tank is pumped into the press using plunger pump at 10-12 kg /sq. cm

pressure to get cake of specified hardness as per process specification No. PS: 0100 R-Z. Then

power pag pressure is released and cake is removed. Cake hardness is checked and cake is

stored in specified area in stacks. Here the slip house work is completed.

!6.3 Jiggering Unit

!�31

The cake from the slip-house is fed into the pug mill. Pug mill makes dough of the cake and

releases it in continuous cylindrical moulds. This continuous cylinder is cut into the required

length by thread cutters. In some pug mills the cutting process is automatic. Then these moulds

are shaped into required shapes by automatic jiggering machines. In case of pin insulators it is

done manually by cutting machines.

!6.4 Finishing Unit

!First the tools and the smoothing plates are checked. Before finishing, the article should be

checked and and defective ones should be discarded. Then the article is placed on the rotating

wheel and adequate layer of mass is removed without affecting die formation. Finally

smoothening plate is used over entire finished portion to achieve smooth surface finish. The

rotating wheel should be at rest while placing and lifting the article. Ambient humidity is

maintained in the finishing area by special water sprayers in order to control specified finishing

hardness of the article. Then the articles are placed on tunnel drier cars and the cars are led into

temperature drying chambers. The stream pressure / waste heat is regulated and temperature of

each zone of dryer is maintained as per process specification No. PPS: 0202 R-1. The racks are

pushed into the drier at one end and released at the other end. Necessary corrective action is

taken in case the moisture exceeds specified limits.

!!! 6.5 Glazing Unit ! Operations carried out:

! 1) CMC Solution preparation

Required quantity of water is taken in a bucket and then weighed quantity of CMC powder is

added slowly to water with continuous stirring. The concentration of CMC solution should be as

per specification no. PPS: 0203 R-0. The solution should be free from modules.

! 2) Sanding Glaze Preparations

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Measured quantity of sanding glaze is mixed with CMC solution and the mix is agitated for 30

min.

3) Brown Glaze Preparation

Brown Glaze from slip house is mixed with CMC solution as per process specification no. PPS:

0203 R-0 for specified concentration. The glaze is agitated for 30 min before se.

!4) Glazing operation

The visually inspected articles are loaded on the conveyor belt and article is again inspected for

chipping, sharp edges etc. Such defects are removed by rubbing with sharp tools and steel wool.

Sufficient compressed air jet is applied to the articles for removal of dust from the surface.

Glaze is applied on the ‘g’ portion of the article with the help of brush of specified thickness.

Then the articles are loaded on glazing machine’s spindle. The water spray nozzle is adjusted for

proper spraying. The glaze nozzles are also adjusted in such a way that the glaze is poor on the

entire surface of the article for a specified glaze thickness. The glaze article is removed from the

machine spindle and is placed on auxiliary turn table. Glaze from ‘a’ portion of the article is

removed with the help of sponging machine. !5) Sanding Operation

Sanding glaze is applied on the ‘f’ portion of the article with brush of suitable width and then

sanding grog is poured. The article is then turned upside down and then placed on the wooden

planks with sponge bedding. Then the article is placed on another hand wheel. Sanding glaze is

now applied on ‘b’ portion of the article along with sanding grog. Then the articles are placed on

the conveyor hangers.

!6) Stamping Operation

Stamping solution is prepared by mixing stamping powder with glycerin as per ratio given in

process specification no. PPS: 0203 R-0. The solution is stirred till no nodes are left. A clean

stamp as per rating is used to stamp the glaze articles. The articles are placed on the conveyor

racks after stamping. At this point the job of glazing is completed.

! 6.6 Firing Unit

The firing work is done in the kiln. It is a continuous process. When one car is entering the kiln

from the entrance door, one exits out through the exit door. The kiln temperature and the cycle

�33

time is maintained as per guide lines of E&D for controlling standard and routine rod thermal

expansion at 250 and 650 deg C by making suitable adjustments of kiln operation parameters.

When the wagon is discharged, the articles are unloaded for visual inspection. !!6.7 Assembly Unit

The various operations performed during assembly are

!1. APPLICATION OF BITUMINOUS PAINT

Bituminous paint and thinner are mixed in specific ratio as per specification no. PPS: 0400 R-0

for applying coating on cap, pin and shell surface which come in contact with cement mortar.

This paint is applied on the pin by dip method and is allowed to dry before assembly. Paint is

also applied on inside and outside grog portion of the cap by using a spray gun. The coated

shells and caps are allowed to dry before assembly.

!!2. PREPARATION OF CEMENT MORTAR

Cement and quartz are mixed in cement mixer for 5 min. then the required quantity of water is

added. Then the flow value of cement mortar is checked. If it is slightly less as specified in PPS:

0400 R-0 , then water is again added and mixed for 3-5 min. to bring it to required value.

!3. ASSEMBLY OF INSULATORS

Cement mortar is poured inside the cap portion as well as the shell portion. A suitable cork is

placed to insert the ball-pin. The pin is placed manually. The shell is assembled in the cap and is

pressed manually. The shell is twisted while pressing to eliminate entrapped air. The assembly

quartz is sprinkled around the pin. The insulator is lifted and placed on the pre-curing chamber

without disturbing the alignment of the components.

!!4. ASSEMBLY OF INSULATORS

Specified temperature and humidity of curing chamber is ensured during initial curing. This is

called temperature curing. The temperature is attained by blowing steam into chamber from the

�34

boiler house. Then the insulators are water cured for 24-36 hrs. in underground water tunnels at

specified temperature. Finally the insulators are air cured for 24 hrs.

6.8 Testing Unit

The testing section is described in details in a later section

!6.9 Dispatch Unit

The tested insulators are packed in seasoned mango wood crates with weather proof nature.

They are then dispatched to customers according to work orders. !!

!!!!!!!!!!!!!!!!!! !!

! !!!!

!!!!!!�35

7. Testing of Insulators Testing of insulators is of three types namely:

1. Routine test

2. Acceptance test

3. Type test.

!7.1 Routine Test

It consists of three sub tests.

!a) Physical verification test

After manufacture, the insulators are inspected visually for cracks and any visual physical

defects. The defected ones are discarded.

!b) Routine Mechanical test

In this test 40 to 50% of rated mechanical load is applied on the insulators. The insulators

incapable of bearing this load breakdown and are immediately discarded. !c) Routine electrical test

The insulators are subjected to high voltage discharge (up to 80 KV) for 10 seconds followed by

proper frequency voltage discharge (up to 80 KV) for 5 min. Then the insulators which are

punctured re labeled with red stickers and others with green stickers. In case of faulty insulators

the normal blue discharge arc is changed to yellow flame and insulator is immediately removed

by stopping the machine.

!7.2 Acceptance Test

Acceptance tests are conducted as per IS-731 and IEC-575. Acceptance tests are also called

performance tests. These consist of 7 tests. They are: !!a) Verification of standard dimensions

The dimension of insulators are verified as per IS-731. They should confirm G.T.P. of drawing.

Diameter, spacing, creepage distance also measured. Then eccentricity of insulators is checked

by eccentricity testing machine. It needs venire height gauge and dial gauge for its

�36

measurements. It ensures pin to cap spacing with venire gauge which should be according to

specification.

!b) Temperature cycle test(TCT)

This test is conducted to check the thermal stability of insulators. The entire trolley of insulators

is dipped in hot bath for 15 minutes followed by dipping in cold bath for 15 minutes. This cycle

is repeated three times. The difference in temperature between hot bath and cold bath should be

higher than 70 deg C . Usually the temperature of hot bath is maintained at 85-90 Deg C with

the help of water heaters. The temperature of cold bath is maintained at 15-20 Deg C with the

help of water chillers.

!!c) Mechanical Performance Test (MPT)

This test is conducted as per IEC-575 in thermo-mechanical chambers. In this test 60% of rated

mechanical load which is tensile is applied to string of insulators. The load is 4-6 tons. The load

is applied for 24 hrs. Hence it is called 24 hr mechanical performance test. This test is done by

thermo mechanical performance testing machine.

!d) Electro –Mechanical test:

For electrical test 50-60% of rated voltage at power frequency is applied to the insulators. For

mechanical test Universal Testing Machine (UTM) is used where 60 tonne tensile mechanical

load is applied to the insulators to measure mechanical strength and breaking load of metal

parts. Breaking load must be less than rated value.

e) Puncture test

This test is conducted to verify or note the voltage at which puncture of insulators occur. This is

done in particular tank which contains transformer oil. First the insulator is dipped in the oil and

then high voltage (AC) is applied to the insulators till puncture occurs and this voltage is noted.

Puncture voltage should not be less than the specified voltage.

!f) Porosity test

The non-porous property of insulators is checked by porosity test. For this purpose a solution of

methyl spirit and a dye (1%) is prepared. Broken pieces of insulators are dropped in the solution

and then pressure is applied by the compressors to the insulator loaded chamber for some hrs.

�37

Then broken samples are taken out and again broken to check porosity. Pressure in the chamber

should not be less than 153 kg/sq cm. !g) Galvanising test

This test ensures proper coating of zinc on metal parts of insulators to avoid corrosion. It

consists of 2 tests: one for mass of zinc on metal parts and other for uniformity of coating. Mass

of zinc coating on the cap should not be less than 610 gm/sq-m. For this test we need

hydrochloric acid. First pin or cap is weighed and then it is dipped in HCL for sometime. After

reaction it is again weighed and the difference in weights give weight of zinc. Then taking

standard area of zinc coating mass in gm/sq.-m is calculated. Similarly pin or cap is dipped in

copper sulphate solution (of specific gravity 1.186 gm/cc at 18±2 Deg. C) for 1 min. to check

uniformity of coating. This procedure is repeated 5 times. Uniform blue color indicates

uniformity of coating.

! 7.3 Type Test

a) Power frequency with standard voltage test

b) Dry flash over voltage test

c) Wet flash over voltage test

d) Pollution flash over voltage test

e) Impulse flash over voltage test

f) Sweeping surge test

g) Steep wave front test !Impulse Voltage withstanding test

It is a type test conducted on insulators to check impulse withstands or flash over voltage. Impulse

generator gives impulse for some time and voltage is noted. Impulse may be lightening or

switching. For lightening impulse rise time is equal to 1.2 micro seconds 30%. Fall time is equal to

50 microseconds 20%. For switching impulse rise time is equal to 250 microseconds. Fall time is

equal to 1250 microseconds.

!!!!

�38

References !1. http://www.ceramicindustry.com/articles/forming-porcelain-insulators

2. http://www.ceramicarchitectures.com/fundamentals/

3. http://www.meisterintl.com/transformer-bushing.html

4. http://www.kosic.si/attachments/sl/101/LAYOUT_engl_klein.pdf

5. http://www.elektroporcelan.co.rs/eng/production_process.html#oblikovanje

6. http://www.ceralep.fr/en/PRODUCTION-PROCESS-_pageid15.html

7. http://www.powerinsulator.com/list1.asp?id=275

8. www.wikipedia.org

9. http://www.srginsulators.com

!!!!!!!!!!!!!!!!!!!!!

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Conclusion

From the experience gained during my training period, it became evident that why are insulators

today’s inevitable needs.The manufacturing plant need to be kept clean and a healthy environment

must be maintained for efficient and safe working. A manufacturing plant needs a rigorous house-

keeping due to various source of dust, viz. coal dust, ash, etc. Due to the heavy machineries and

large components, it becomes quite risky to work in a production plant. Proper nose masks, gloves,

ear plugs and other safety equipments must be used by the workers to minimise the health hazards.

During my training period, I got to learn about the maintenance and operations of all the

components in the plant. I experienced the work environment and how the hierarchy helps all the

workers and staff members of the plant to work in perfect serenity. It is not without actually

working in the company that one gets to feel the importance and support of human resources

department. I was highly obliged to work and learn in such a company where people were ready to

give and accept.

As I paddled my wheel through the different sections of the plant, the graph of my knowledge kept

on reaching its new peaks. Although there is a regret for not getting to learn even more about the

processes due to the lack of time, it was an immense pleasure to watch and learn how porcelain

insulators is actually produced. My experience in the industry helped me get a glimpse of what a

working life could be and how even a small worker in the company plays such an important role

that if he/she is taken out of the chain, the whole chain breaks and its direct effect is on the output of

the plant.

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