Thursday, 13 September 2018

NON – DESTRUCTIVE TESTING


Adequate NDT and inspection by suitably qualified personnel is very much essential during all stages of pipe manufacture, construction and operation. The newly used inspection methods and equipment assist in obtaining the maximum life expectancy from a pipeline reducing the overall operating costs. Fluoroscopy, computed radiography, digital radiography and automated ultrasonic testing helps in improving the probability of detection (POD) of discontinuities. Modern NDT methods have become more quantitative and less obtrusive, which many a time, results in saving over time. Thus it can be said these advanced NDT methods have the potential that could lead to significantly lower repair rates while maintaining existing safety standards .

The Radiation detectors that used are image intensifiers in fluoroscopic and real time imaging systems The equipment that is used to perform Radiographic inspection can be either an X-ray machine or a radioactive isotope that produces gamma radiation. The isotopes help in increased portability as no electrical power supply is required. Electronic imaging panels and phosphorescent imaging screens are used to create digital images for computed and digital radiography. Real time imaging can be used close to the welding station and can detect all the defects at an early stage thus reducing the number of faulty welds produced. Phosphorescent imaging plates in digital radiography replaces X-ray film and processing chemicals. They can be used again and the X-ray images are stored electronically on optical disc. These images can be electronically enhanced to increase or reduce density leading to discontinuities which may have been previously . The phosphorescent imaging screens are flexible and are used in conventional x-ray film. The phosphorescent screens store a latent image which is scanned with an infrared laser scanner and are then viewed on a monitor. Magnification and measuring tools are then used for further evaluation of images. The use of phosphorescent screens require shorter exposure time which can amount to considerable savings.
The primary benefit of UT is that it is a truly volumetric test which means it is capable of determining not only the approximate dimensions and location of a defect, it also provides information to the testing technician regarding the type of defect. Another major advantage of UT is that it requires access to one side of the material to be tested and it will best detect crack’s and incomplete fusion which may not be possible with Radiographic testing. Since a variety of beam angles can be used, UT can detect defect which may not be detectable by radiography .UT requires highly skilled technicians because interpretation of indications are difficult. Reference standards are used for calibration and setting up of the equipment. Test scans can be recorded by equipments providing automated scanning. This test method is generally limited to the inspection of butt welds of materials that are thicker than 6 mm. Automated UT is used in pipe mills where the welds are inspected by a multiple array of probes, scanning the entire weld and detecting any discontinuities at an early stage. AUT is an in-field test method. An array of probes mounted in a scanner are placed on the pipe and the weld area is scanned. An encoder records the probe position with respect to the distance traveled, which allows the weld to be tested in a shorter period of time, resulting in a complete volumetric test of the weld and reducing the number of errors.

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