The NBIC recommends PT for examination of: firetube boiler tube sheets to find leakage around tubes, external inspection of weld joints, evaluating components subjected to fire damage, historical boilers, fiber-reinforced thermosetting plastic pressure equipment, Yankee dryers, and pressure vessels in liquefied petroleum gas LPG service. During inservice inspections, PT should also be used in areas suspected of defects. These include, but are not limited to, nozzles see Figure 1 , tubesheets see Figure 2 , knuckles of heads see Figure 3 , and head spin hole plug welds see Figure 4.
To effectively use liquid penetrant on the tubesheet in Figure 2, an extensive amount of work would need to be done. All rust and scale would need to be removed so the penetrant could be cleaned off.
Rolled, unfused tube ends would also bleed out dye and cause false indications. The head spin hole plug weld looks acceptable to the naked eyes, but shows many defects once it has been liquid penetrant examined see Figures 5 and 6.
In conclusion, PT can be a very valuable tool during new construction and inservice inspections. PT does have limitations and is not the best method for all applications. However, for quick, low cost examinations in any location, PT is often the best choice of NDE methods. Send This Page.
Myers Safety Medal Career Opportunities. The surface to be examined and all adjacent areas within at least 25 mm should be dry, free from any dirt, lint, grease, welding flux, weld spatter, oil, or other extraneous matter that could obscure surface openings or otherwise interfere with the examination. The method of cleaning depends on the nature of the material of the part and contaminants.
Typical cleaning facilities make use of detergents, organic solvents, descaling solutions, alkali solutions, paints removers, vapour degreasing, ultrasonic cleaning, abrasive blasting, etc. Blasting with shots or dull sand is permissible only when it does not peen over or fill the voids, as this drastically reduces the accuracy of the inspection. It is essential that parts be thoroughly dried after cleaning so that FO water or solvent remains in or over the discontinuities, as this will hinder entrance of the penetrant.
Drying may be accomplished by warming the parts with infrared lamps, drying ovens, forced air circulation, etc. In case of small components, they may be dipped in a tank of penetrant.
Where only a local area of a component is to be tested, the penetrant may be applied by a brush or spray. Regardless of how it is applied, it is important that all surfaces are wet by the penetrant. The length of penetration time is critical and depends upon the type of material being inspected, type of penetrant, kind and size of defect anticipated together with the temperature of the penetrant. If water washable penetrants are used, the penetration time shall be about 1.
When a high viscosity fluorescent penetrant is used, the penetration time may be longer than the normal penetration time. In such a case, time shall he subject to agreement between the manufacturer and the purchaser. An extremely, long penetration time does not affect the results except to increase the brilliance of indications slightly and make removal of the excess penetrant more difficult. The rinsing must be through and complete so that the penetrant within the discontinuities of the part alone is intact.
Special attention should be given to drilled holes and threads, which are highly prone to retain penetrant. Using water washable penetrant, rinsing should be done with water spray nozzle. The water droplet from the nozzle should be spray type rather than pointed. Using post emulsifying penetrants, an additional step is required. This is the application of a liquid emulsifier prior to rinsing operation.
Cleaning methods may include solvents, alkaline cleaning steps, media blasting, etc. Sometimes, the sample may even require etching to make the defects open to the surface, dry, and free of contamination. Next, The liquid penetrant is applied on the specimen surface and allowed to soak into any flaws for its dwell time generally 10 to 60 minutes.
The dwell time varies depending on the used penetrant viscosity: longer duration for high viscosity , test material, and the defect sizes smaller flaw sizes require longer penetration time. Dwell time is normally provided by the penetrant manufacturers and depends on the following:.
The excess penetrant needs to be removed from the sample surface. Depending on the dye penetrant type, The removal method is selected from water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable, etc.
Emulsifiers are used for the highest sensitivity level, and it chemically reacts with the oily penetrant, thus making it easier to remove using water spray. The excess penetrant has to be removed thoroughly otherwise, on the application of the developer, it may leave a background in the developed area that can mask indications or defects. Also, while using solvent remover and lint-free cloth, care must be exercised not to spray the solvent on the test surface directly, because this can remove the penetrant from the flaws.
A thin layer of white developer is applied after excess penetrant has been removed. Various types of developers are available like a non-aqueous wet developer, dry powder, water suspendable, and water-soluble.
Based on the compatibility of the penetrant, the developer is selected. The developer basically draws back the dye penetrant from the defects to the surface so that those can be visible.
Advantages and Disadvantages The primary advantages and Disadvantages when compared to other NDT methods are: Advantages High sensitivity small discontinuities can be detected. Few material limitations metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.
Rapid inspection of large areas and volumes. Suitable for parts with complex shapes. Indications are produced directly on the surface of the part and constitute a visual representation of the flaw. Portable materials are available in aerosol spray cans Low cost materials and associated equipment are relatively inexpensive Disadvantages Only surface breaking defects can be detected. Only materials with a relatively nonporous surface can be inspected.
Pre-cleaning is critical since contaminants can mask defects. Metal smearing from machining, grinding, and grit or vapor blasting must be removed. The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is required.
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