NDT of Turbines

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The picture shows a huge structure of complicated geometry fabricated in thick steel plates. Water gushing  in from large dams rotates the turbine located at the center of the casing and the generator at the top of the turbine housed in the upper floor of the building to produce electricity in quantity of several mega watts.  The fabrication is inspected utilizing several techniques of NDT (non destructive testing). 

Like the technique of taking the X-ray of human organs industrial radiography is carried out to get the image of flaws and defects in the welding, castings, or components.   X-ray tube or gamma ray isotope source is used to expose the x-ray or radiographic film.   Images of the defects on the films are watched carefully and the defects interpreted on the basis of agreed specifications.

Ultrasonic flaw detection is a technique of NDT that utilizes the property of very high frequency sound of the order of several mega hertz. These ultrasonic waves behave like light when traveling through dense metallic mediums. That is this type of sound can cast a shadow of an object hence this property of the ultrasonic waves helps detecting flaws utilizing appropriate instruments.

There is another technique of Non-destructive testing called dye penetrant test.  This technique is utilized to  check the defect and flaws exposed on the surface in the metal.  Red coloured volatile dye is available for this purpose specially prepared and filled in pressurized aerosol cans.  After cleaning and degreasing the surface of the metal to be tested the dye is sprayed on the area to be inspected.  Certain time is given for the dye to penetrate and fill the openings of the defects like cracks etc.  When the specified time is lapsed extra dye on the surface is carefully cleaned with thinner like special material that too available in the pressurized cans.  As the last stage of the testing a liquid called developer is sprayed.  The developer actually is the suspension of very fine power in a volatile medium that also is filled in the pressurized aerosol cans.  The volatile medium of the developer evaporates immediately leaving a very thin layer of power very evenly spread over the surface to be inspected.  The extra dye, which is in the flaws and defects open on the surface is blotted out by the powder layer, giving very distinct impressions of the cracks etc.

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