Basic Principles of Liquid Penetrant Inspection

Welcome to NDT institute Vijayawada, a premium institute which offers courses in various engineering platforms like welding, piping, quality audit, liquid penetrant testing and international inspection techniques and certification. NDT institute in Vijayawada gives an in-depth training to students from various backgrounds like refineries, petrochemical industries, power plants and fabrication industries. It also gives on the job training o methods like, radioactive testing, ultrasonic testing and liquid penetrant testing. One of these techniques taught here is the Liquid Penetrant Testing which has a great deal of scope for students to find NDT jobs in India and NDT jobs abroad.

 Liquid Penetrant Testing

Principle of LPT:

The basic principle of liquid penetrating testing is capillary action.

Capillary Action: The rise or fall of liquid in a narrow opening is called capillary action. Capillary action is action by which surface of liquid, where it is in contact with solid is elevated or depressed.

The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrant require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Steps of Liquid Penetrant Testing

The exact procedure for liquid penetrant testing can vary from case to case, depending on several factors such as the penetrant system being used, the size and material of the component being inspected, the type of discontinuities being expected in the component and the condition and environment under which the inspection is performed. However, the general steps can be summarized as follows:

1. Surface Preparation: One of the most critical steps of a liquid penetrant testing is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.

2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.

3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible, to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers

or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of discontinuity being inspected for.

Minimum dwell times typically range from five to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.

4. Excess Penetrant Removal: This is the most delicate part of the inspection procedure, because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water.

5. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powders), dipping, or spraying (wet developers).

6. Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks.

7. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Advantages &Capabilities:

·         Penetrant testing inspection is capable of examine the entire exterior surface in one operation even though object may have complex shape.

·         Penetrant inspection can detect very small discontinuities and penetrant bleed out magnifies apparent size of discontinuity making them easier to see.

·         Penetrant testing indications formed provide information on location, orientation and approximate length of discontinuity.

·         The process is relatively simple and rapid and can easily be automated where large volume of parts is to be inspected.

·         Penetrant testing inspection can be used on wide variety of materials including, ferrous and non- ferrous metals, ceramics, cements, glass and many types of plastics.

·         The process sensitivity can be varied by selection of materials and processing procedures. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive.

Disadvantages & Limitations:

·         Penetrant testing inspection will not reveal discontinuities that are not open to the surface. Subsurface discontinuities cannot be detected by penetrant inspection.

·         The part surface and interior of any discontinuities must be clean and free of contaminants, soils and moisture. If it is present organic or inorganic materials of the surface of the part or inside the discontinuities can interfere with penetrant entering and filling the discontinuity void.

·         Porous material or material with seeped-in penetrant that is not removed during post cleaning operations is difficult to inspect.

·         Penetrants usually contain petroleum based liquids that can soften, destruct or attack some non-metallic materials such as certain plastics and rubber.

·         Penetrants, emulsifier and developers contain chemicals that have wetting and detergent properties however they can dissolve and remove natural oil from human tissues if they come in contact with and remain on skin surfaces for a length of time. This causes drying and cracking which is a source for secondary infection.

Liquid penetrant testing taught in NDT institute Vijayawada is a very useful training, which helps lots of candidates to acquire NDT jobs in India  and  abroad.