3.2
Porcelain
Porcelain insulators and bushings have been used
in the electrical industry since its inception.
They have excellent dielectric properties, long life, are resistant to
atmospheric corrosion and U/V radiation.
The infrastructure cost to manufacture porcelain component is very
heavy. The process is extremely labour
intensive and requires very skilled engineering and operating staff.
The dielectric properties of a porcelain insulator, in most cases, can be fully restored, after a flashover across its surface, by washing with solvent and water and cleaning the same. Porcelain Insulators have excellent resistance to water absorption.
The porcelain manufacturing process, in brief,
comprises:
3.2.1 Raw material
The raw material comprises silica, feldspar, and
clay that is mixed in the ratio of 3:2:5
Alumina (Al2O3
3.2.2 Slip House
The raw material mixed with water is charged
into a ball mill and ground. The mixture
is called slip. The slip is
continuously agitated in a tank. The
slurry is passed through magnetic separators and vibrating screens to remove
Iron and coarse particles in a process called slip filtration.
The slip is then passed through a set of filter
presses to remove excess water and gets converted to filter cakes. The hardness of the cake is checked, stacked,
and is sent either for Pugging or Jiggering.
3.2.3 Pugging
In the Pugging Process,
the cake is fed to the vacuum / de-airing pug mill which discharges a
cylindrical blank. For porcelain
manufacturing industries, the diameter of the pug determines its capacity. The pug size is approximately 20% larger than
the diameter of the finished product to cater to shrinkage during the
manufacturing process. The pug is taken
for machining by vertical or horizontal lathe depending upon the size and shape
of the finished product.
3.2.4 Jiggering
In this process, the final shape is given to the
cake. It is normally carried out in two
stages wherein the cake is pressed in a machine and then finished on a
turntable.
3.2.5 Finishing
The product is fine finished as per the
dimensions and sent for drying.
Drying is a very
important process where the moisture content is brought down from 15% to
2-3%. The method of drying is extremely
important and depends upon the shape and size of the component. It should be ensured that the moisture
gradient from the core centre to the exposed surface is kept to a minimum. It is far easier to dry a thin shell
porcelain housing than a solid core insulator for obvious reasons.
The natural drying process is the best. The accelerated process of drying may leave a
higher moisture gradient resulting in distortion & cracks. Some
manufacturers go to the extreme limits to ensure uniform minimal moisture
gradient by rotating the finished component by 90 degrees at an interval of 1
day before undertaking sandering and glazing.
3.2.7 Sandering & Glazing
Most of the porcelain products have metal
inserts. To get a grip on the metal
insert, a rough surface must be made available on the finished housing. Sand grog mono mesh of a particular size is
embedded on the surface with an adhesive and the process is called sandering.
Glazing is carried out to ensure a smooth
surface that is impervious to moisture.
A glazing paste of the desired colour is applied to the entire surface
of the product before firing.
3.2.8 Firing
The product is loaded onto trolleys for firing
in kilns. This can be a continuous or
batch type of kiln depending upon the product, shape, size, and quantity. Standard components such as disc insulators,
pin insulators, stack insulators are fired in continuous kilns. Big shells and special insulators are fired
in a batch kiln.
Firing of components in the kiln is a very tricky process that requires experience and a thorough understanding of temperature distribution inside the furnace and the geometry of the product and other parameters. The success or failure of all preceding processes will be evident only after the firing and no component can be recycled after the firing has taken place.
3.2.9 Metallics
The metal component is normally malleable cast
iron (MCI) for most of the applications in transmission & distribution. Metal inserts and flanges made of Brass,
Bronze, and Aluminium are also used in the porcelain insulator manufactured for
use in Busbar Systems.
The design of the metal insert to mate with the porcelain housing is very critical and depends upon the bonding material.
3.2.10 Assembly
The fired components are
machined (if required) before taking up for assembly with the metal parts. Most of the metal parts are attached to the
housing with Portland cement. The curing
process time is protracted and cannot be shortened. Many manufacturers have used quick-setting
cement or resin for bonding the metallics to the shell, to cut down on
the processing time. A rubberized
cushion gasket is interposed between the metal and the porcelain for ensuring
even distribution of the load.
Assembly is a very important process in the
manufacture of the insulator and bushings for a Busbar System to
establish the guaranteed technical parameters.
3.2.11 Testing
The in-process and finished product testing
procedure for the porcelain component is very well defined. The component has to be inspected for
conformance to parameters at every stage of production. ANSI / IEEE and IEC have very clearly defined
the testing procedures and acceptance norms, for porcelain insulators and
bushings.
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