Inspectioneering

Liquid Penetrant Examination (LPE)

Explore this topic

Liquid Penetrant Examination, also referred to as penetrant testing (PT), liquid penetrant testing (LP), and dye penetrant testing (DP), is a nondestructive examination (NDE) method that utilizes fluorescent dye to reveal surface flaws on nonporous materials (metals, plastics, or ceramics) which might not otherwise be visible. 

History and Development

Starting in the 18th century, inspectors in the railroad industry used the “oil and whiting method” to detect defects in railroad wheels. This method involved immersing the pre-cleaned part in hot oil, wiping it dry, and then coating it with a white chalk that was typically mixed with alcohol. Once the alcohol evaporated, a contrasting white surface layer of chalk remained, which also acted as a “blotter” that aided the oil being drawn back to the surface to reveal the presence of surface flaws. This is the earliest documented use of the liquid penetrant examination technique.

Methodology

Liquid penetrant examination works via the principle of “capillary action,” a process where a liquid flows into a narrow space without help from gravity. The liquid is then drawn out with a chalk-like developer.

A critical step of the liquid penetrant examination process is surface preparation. When examining any part or piece of equipment for defects, the surface must first be cleaned thoroughly of all contaminants that may prevent the liquid from flowing into the flaws. Once the surface is properly cleaned, the dye penetrant is then applied and allowed to sit on the surface long enough for the liquid to penetrate and seep into any defects. Once enough time has passed, the excess penetrant is carefully wiped off the surface. The developer is then applied to draw the penetrant back to the surface where it can be easily seen. After inspection, the surface should be cleaned of any remaining developer and penetrant.

Industry Applications

Liquid penetrant examination is a popular technique for in-service inspections on nonporous materials because it is quick, economical, and requires minimal training when compared to other NDE methods. It is primarily used to detect surface defects such as hairline cracks, surface porosity, leaks in new equipment, and fatigue cracks on in-service components.

Liquid penetrant examination can be used to detect surface flaws in pressure vessels, clad vessel linings, valves and their components, machined parts, castings and forgings; however, it’s most commonly used to inspect welds and weld overlays. For welds, extra care should be taken because if the material is not properly sealed, the liquid dye can penetrate through the weld and contaminate fluids or cause defects if further welding is performed in the future.

While this method is effective due to its simplicity and accuracy, it does have its share of disadvantages as well. Liquid penetrant examination can only detect flaws on the surface. For subsurface flaws, techniques like magnetic particle testing (MPT), ultrasonic testing (UT), or eddy current testing (ECT) may be more appropriate. It is also most effective on smooth surfaces, which can make it unsuitable for some parts.

Relevant Codes and Standards

  • ASTM E1417, Standard Practice for Liquid Penetrant Testing, is a standard published by ASTM International that establishes the minimum requirements for conducting liquid penetrant examination of nonporous metal and nonmetal components.

References

  1. Butz, J., 2012, “The Evolution of Nondestructive Testing,” Inspectioneering Journal, 18(3), pp. 20-21.
  2. Worman, J., 2011, “Liquid Penetrant Testing,” The National Board of Boiler and Pressure Vessel Inspectors, https://www.nationalboard.org/index.aspx?pageID=164&ID=374.

Related Topics

Topic Tools

Share this Topic

Contribute to Definition

We welcome updates to this Integripedia definition from the Inspectioneering community. Click the link below to submit any recommended changes for Inspectioneering's team of editors to review.

Contribute to Definition
Articles about Liquid Penetrant Examination (LPE)
Blog

Nondestructive testing (NDT) careers are on the rise in today’s economy. These fields are constantly advancing, and there is an ever-growing need for technicians, quality assurance specialists and inspectors as infrastructure in the United States...

Authors: Berg Engineering
September/October 2016 Inspectioneering Journal

Detecting, quantifying, and sizing indications characterized as a “crack" in critical equipment have long been the global benchmark of asset integrity programs. Therefore, the increased precision that inspection programs obtain using advanced...

Authors: Kimberley Hayes
Blog

Nondestructive Testing (NDT) makes up the majority of testing performed in our industry. There many different types of nondestructive testing techniques. Each method has its own advantages and disadvantages; each can detect different types of flaws....

Authors: Nick Schmoyer
May/June 2012 Inspectioneering Journal

Over the past 100 years, Nondestructive testing has made tremendous advances. New NDT methods have been introduced, while old "tried and true" methods continue to be improved. Although some NDT methods have drastically changed or "evolved" over...

Authors: Jason Butz
Partner Content

TEAM believes that quality inspections are the key to reliability. Our inspection crews are made up of specialty trained, experienced technicians who adhere to the highest quality standards. Our API 653 inspectors deliver thorough, reliable...

    Downloads & Resources related to Liquid Penetrant Examination (LPE)

      Inspectioneering Journal

      Explore over 20 years of articles written by our team of subject matter experts.

      Company Directory

      Find relevant products, services, and technologies.

      Talent Solutions

      Discover job opportunities that match your skillset.

      Case Studies

      Learn from the experience of others in the industry.

      Integripedia

      Inspectioneering's index of mechanical integrity topics – built by you.

      Industry News

      Stay up-to-date with the latest inspection and asset integrity management news.

      Blog

      Read short articles and insights authored by industry experts.

      Expert Interviews

      Inspectioneering's archive of interviews with industry subject matter experts.

      Event Calendar

      Find upcoming conferences, training sessions, online events, and more.

      Downloads

      Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

      Videos

      Watch educational and informative videos directly related to your profession.

      Acronyms

      Commonly used asset integrity management and inspection acronyms.