Published on 21-Aug-2023

Procedure for Liquid Penetrant examination

Procedure for Liquid Penetrant examination

Liquid Penetrant Examination (LPE), commonly known as dye penetrant inspection or liquid penetrant testing, is a widely used non-destructive testing method to detect surface defects and discontinuities in various materials.

Penetrant Testing is a surface defect detection method, that can be used on any material (i.e.: or non-ferrous, conductive or non-conductive, metals/non-metals). The procedure for LPE involves several steps. 

Liquid Penetrant Examination(PT)

Penetrant Testing is surface defects detection method, the method can be used on any material ( i.e: Ferrous/ non-ferrous, conductive/non-conductive, metals/non-metals)

But PT is limited to materials having a porous surface or very rough surface.

PT can be carried out on castings, raw materials, machined faces, and weld joints. It is more often used in Austenitic Stainless Steels, where MPI inspection is not practically possible.

 Most standards insist that the penetrant materials used, should not contain contaminants that spoil the quality of weld and metals. ( contaminants such as Sulphur, Chlorine & halogens )

 Penetrant materials are mostly suitable for materials with surface temperatures less than 52°C. Therefore the weld has to be allowed to cool before carrying out PT test on it.

Principle Of Liquid Penetrant Examination

Liquid Penetrant Examination is a non-destructive testing technique used to identify surface defects on materials such as metals, plastics, ceramics, and composites.

The procedure involves the application of a liquid penetrant to the surface of the test object. Any PT inspection at an elevated temperature (other than 4°C to 52°C) should have a special qualification for the Consumables.

The penetrant seeps into surface defects by capillary action, making them visible and detectable. It is widely employed in industries like aerospace, automotive, manufacturing, and construction to ensure the quality and safety of components and structures.

Also Read, What is Pipeline Isometric Drawings?

Types of Penetrants in Liquid Penetrant Examination (LPE)

Liquid Penetrant Examination (LPE) is a powerful Non-destructive Testing method that utilizes various types of penetrants to identify surface defects and discontinuities in materials. Two common types of penetrants used in LPE are visible dye penetrants and fluorescent penetrants. 

Visible Dye Penetrants

Visible Dye Penetration test procedures are widely used in LPE and are the traditional choice for detecting surface defects. These penetrants contain colored dyes that are easily visible to the naked eye, making indications easily discernible during the examination.

Visible inspection uses a color contrast die, and inspection is carried out under white light. Visible Light should have a light intensity of 1000 Lux.

Advantages of Visible Dye Penetrants

1. Simplicity and Ease of Use

One of the main advantages of visible dye penetrants is their simplicity. The application and removal processes are straightforward, in NDT  making it easy for inspectors to use them effectively.

2. Cost-Effectiveness

Visible dye penetrants are generally more cost-effective than other types of penetrants, making them a preferred choice for many applications, especially where budget constraints exist.

3. Suitable for Various Materials

Visible dye penetrants are compatible with a wide range of materials, including metals, ceramics, plastics, and composites, allowing for versatile usage.

4. Ideal for Field Inspections

In remote or field inspection scenarios where sophisticated equipment might not be available, visible dye penetrants are advantageous due to their minimal equipment requirements.

Fluorescent Penetrants

Fluorescent penetrants are a more advanced option in LPE and offer improved sensitivity compared to visible dye penetrants. These penetrants contain fluorescent dyes that emit visible light when exposed to ultraviolet (UV) radiation. Fluorescent inspection using Glowing fluorescent dies is done under Ultraviolet Lamps, and the UV Lamp should have a light intensity of 1000 W/cm2 at the surface inspection. A dark room setup is required for Fluorescent inspection with a maximum white light intensity of 20 lux.

Advantages of Fluorescent Penetrants

1. Enhanced Sensitivity

Fluorescent penetrants are highly sensitive to small surface defects, making them a preferred choice for critical inspections where even minute flaws need detection.

2. Greater detection capabilities

Due to their enhanced sensitivity, fluorescent penetrants can reveal finer details of surface defects, aiding in accurate defect characterization.

3. Reduced Ambient Light Interference

Fluorescent penetrants are particularly useful in well-lit environments or under natural light conditions, as they can be used with UV lamps to minimize ambient light interference.

4. Improved inspection accuracy

With the ability to identify smaller defects, fluorescent penetrants contribute to higher inspection accuracy, ensuring that no critical flaw goes undetected.

Step-by-Step Procedure for Liquid Penetrant Examination (LPE)

The Liquid Penetrant Examination procedure is a widely used Non-destructive Testing method for detecting surface defects and discontinuities in various materials. This step-by-step procedure outlines the key stages involved in conducting an LPE.

1. Pre-cleaning

Before commencing the examination, it is essential to pre-clean the test surface thoroughly. This step involves removing any dirt, grease, oil, paint, or other contaminants that might obstruct the penetration of the liquid dye. A clean surface ensures that the penetrant can access surface defects effectively, preventing false indications during the examination.

2. Application of Penetrant

Once the surface is clean and dry, the liquid penetrant is evenly applied to the test area. The penetrant is left to dwell for a specified period, usually determined by the material and the type of penetrant being used. During the dwell time, the penetrant seeps into any surface defects present on the material.

3. Penetrant Dwell Time

The dwell time is a critical stage in the LPE process. The time the penetrant is allowed to remain on the surface directly affects the sensitivity of defect detection. Longer dwell times generally lead to increased penetrant absorption into defects, resulting in higher sensitivity. However, excessively long dwell times might cause the penetrant to dry out on the surface, reducing its ability to penetrate smaller defects. Striking the right balance in dwell time is essential for obtaining accurate and reliable results.

4. Penetrant Removal

After the dwell time has elapsed, the excess penetrant is carefully removed from the surface. The removal process should be carried out meticulously to avoid unintentionally wiping away penetrants from actual defects, which could lead to false-negative results. Some methods of penetrant removal include solvent wiping, water rinsing, or using special emulsifiers.

5. Application of the Developer

Following the removal of the excess penetrant, a developer is applied to the surface. The developer acts as an absorbent, drawing the trapped penetrant out of the defects and making them visible. There are various types of developers, including dry powders and wet suspensions, each offering different benefits depending on the specific inspection requirements.

6. Inspection

The developer brings out the indications of defects on the surface, which can now be inspected under appropriate lighting conditions. Adequate lighting, often achieved through ultraviolet lamps, enhances the visibility of indications. Trained inspectors carefully examine and interpret the indications, considering their size, shape, and location, to assess the significance of the detected defects.

7. Post-cleaning

After completing the inspection and recording the results, the test surface is thoroughly post-cleaned. This step is essential to remove any residual penetrant and developer from the surface, ensuring that subsequent inspections or other processes are not affected. 

liquid penetrant testing principle

Applications of Liquid Penetrant Examination

Liquid Penetrant Examination (LPE) is a versatile Non-destructive Testing method that finds extensive applications across various industries. Let's explore some of the diverse applications of LPE in different sectors:

1. Manufacturing Industry

Application: LPE is widely used in the manufacturing industry to inspect finished products, machined components, and fabricated structures for surface defects and discontinuities.

Real-world Example: In the manufacturing of turbine blades for gas turbines, LPE is employed to detect surface cracks, porosity, and other defects that could compromise the integrity and performance of the blades. Early detection of such defects ensures that only flawless components are used in critical applications.

2. Aerospace Industry

Application: LPE plays a crucial role in the aerospace sector by inspecting critical components of aircraft, such as engine parts, landing gears, and structural components, for surface flaws.

Real-world Example: During regular maintenance inspections of aircraft engines, LPE is used to detect cracks or defects on the surface of turbine blades and compressor discs. Identifying flaws early on helps prevent potential engine failures and enhances flight safety.

3. Automotive Industry

Application: LPE is commonly utilized in the automotive sector to inspect various components for surface defects, ensuring the safety and quality of vehicles.

Real-world Example: In the production of automotive engine blocks, LPE is applied to identify surface cracks and defects. By using LPE as part of the quality control process, manufacturers can ensure that the engine blocks meet the required standards and do not contain any defects that could compromise performance.

4. Weld Inspections

Application: LPE is extensively used in weld inspections to detect surface cracks, incomplete fusion, and other imperfections in welded joints.

Real-world Example: In the construction of pressure vessels, LPE is employed to inspect welds for any defects that might weaken the structure. The ability of LPE to identify surface flaws aids in maintaining the structural integrity and safety of pressure vessels used in various industrial processes.

5. Power Generation Industry

Application: LPE is widely applied in the power generation industry to inspect components such as turbine blades, steam pipes, and heat exchangers for surface defects and cracks.

Real-world Example: In nuclear power plants, LPE is used to examine critical components for surface cracks and discontinuities. Early detection of defects in these components ensures the safe and efficient operation of nuclear power plants.

6. Petrochemical Industry

Application: LPE is employed in the petrochemical industry to inspect components such as pipelines, storage tanks, and pressure vessels for surface defects and corrosion.

Real-world Example: In the inspection of storage tanks for chemical substances, LPE is utilised to identify surface flaws or leaks that could lead to hazardous spills or environmental contamination.

Also Read, Importance of Non-Destructive Testing (NDT)

What Are The Requirements of Liquid Penetrant Testing?

The ASME Section V, Article describes the requirements of a Liquid Penetrant Test (PT)

 API 650, ASME B31.3, and ASME Section VIII, Division 1, all require the PT inspection to be done as per ASME BPVC- Section V- Article 6.

For carrying out the inspection and signing off the reports the personnel should be trained and qualified by ASNT norms SNT-TC-1A.

 The person should have cleared eye tests – Near Vision test Jagger’s J1, Color blindness test – Ishihara test, and the employer should carry out the test on personals annually.


Liquid Penetrant Examination is a powerful Non-destructive Testing procedure that allows the detection of surface defects and discontinuities in various materials. Its sensitivity, cost-effectiveness, and versatility make it a widely utilized technique in various industries. By identifying defects early on, LPE contributes to ensuring the safety and reliability of critical components and structures. 

LPE is a versatile non-destructive testing technique with diverse applications across industries. Its effectiveness in detecting surface defects and discontinuities has proven crucial in ensuring the safety, reliability, and quality of various components and structures. LPE's ability to identify flaws at an early stage allows for preventive measures, reducing the risk of failures and accidents in critical applications.

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Application Notes