Liquid Penetrant Inspection

• Liquid penetrant inspection (LPI) also referred to as Dye penetrant inspection (DPI) is a very economical way of testing non-porous material, such as metals, plastics, ceramics, etc. against surface-breaking defects. Liquid penetrant inspection helps to detect casting and building defects, cracks, and leaks in new products before they are sent for distribution. It also helps to check the weariness cracks on in-service components.

How Liquid Penetrant Inspection is done?

• The working of the Liquid penetrant inspection (LPI) is based upon the capillary method. In this the Penetrant is applied on the object that is to be tested using various techniques, such as dipping, spraying, and brushing. This low surface tension fluid penetrates into the clean and dry surface and helps to break discontinuities. The Penetrant kept for a specific time period before it is removed. After this a developer is applied which helps to take out penetrant from the defect. This entire procedure makes the faults clearly visible to the inspector. The Liquid penetrant inspection (LPI) is either done under the ultraviolet or white light according to the penetrant used.

What are the types of penetrants used?

• Penetrants are categorized on the basis of sensitivity levels. There are basically two types of penetrants that are used in liquid penetrant inspection: visible penetrants and fluorescent penetrants. The visible penetrants are usually of red color and have low sensitivity level. The fluorescent penetrants are a combination of two or more dyes. These dyes shine when passed through ultraviolet radiation. When fluorescent penetrant is used the liquid penetrant inspection is done in a dark environment.
• The factors that need to be considered while selecting the penetrant are the environment under which the inspection is to be done, the finishing of the surface that is to be tested, and the size of the defects that need to be inspected. One thing that you must take care of is that the penetrant that is used is compatible with surface of the object that needs to be examined otherwise it can result in staining or degradation.

What are the steps of Liquid Penetrant Inspection?

• Pre-cleaning: The surface of object should be cleaned properly using methods, such as solvents, alkaline cleaning steps, vapor degreasing, or media blasting. This will help to remove dirt, paint, oil, grease or other stains that cause false indications during the inspection.
• Penetrant application: The penetrant should be applied to the surface of the object that needs to be tested. It should be kept for 5 to 30 minutes depending upon the surface that needs to be tested and the flaws that need to be detected.
• Remove the excess penetrant: Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are some of the methods that you can use to remove the excess penetrant.
• Developer application:Developer application: Apply the developer that is compatible with penetrant. Your options are non-aqueous wet developer, dry powder, water suspendible, and water soluble.
• Final inspection: The inspector will test the surface to examine the defects in appropriate lighting conditions depending upon the penetrant.

Other Related Articles :

• Ground Penetrating Radar - Ground-penetrating radar (GPR)
• Use of Industrial Radiography for Inspection of materials