by Jim Worman, Senior Staff Engineer with The National Board of Boiler and Pressure Vessel Inspectors
If you have contractors or on-site engineers doing pressure vessel inspections, this is one method they may choose. This article is VERY WELL written and is a MUST READ for those not formally trained in this method of inspection.
The liquid penetrant examination is one of the most popular Nondestructive Examination (NDE) methods in the industry. It is economical, versatile, and requires minimal training when compared to other NDE methods. Liquid penetrant exams check for material flaws open to the surface by flowing very thin liquid into the flaw and then drawing the liquid out with a chalk-like developer. Welds are the most common item inspected, but plates, bars, pipes, castings, and forgings are also inspected using liquid penetrant examination. Over the years, liquid penetrant examination has been called many names: penetrant testing (PT), liquid penetrant testing (LP), and dye penetrant testing (DP). The American Society for Nondestructive Testing (ASNT) uses the name liquid penetrant testing (PT). The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME B & PVC) and the National Board Inspection Code (NBIC) use the name liquid penetrant examination (PT). The first documented use of PT was in the railroad industry. Cast railroad wheels were dipped in used oil, dried off, and then coated with powder chalk or suspension of chalk in alcohol. Once the wheels were dry, any oil stored in the flaw would bleed out into the chalk and be detected. This was called the oil and whiting method. The ASME Boiler & Pressure Vessel Code recognizes six different techniques of PT. They vary by type of penetrant and method of cleaning before applying a developer. The two penetrant types are either fluorescent or color contrast (dye) penetrant. They can then be used with any of the three methods of cleaning: water washable, post-emulsifying, and solvent removable. The most popular is dye penetrant that is solvent removable. This method is referenced throughout this article. The dye penetrant solvent removable method is most popular because it is low cost and very versatile. It typically comes in three aerosol cans – cleaner, penetrant, and developer. The cans can be purchased from welding supply distributors for typically $5 to $15 a can. For less than $50 you can have all the equipment you need to conduct liquid penetrant examinations. The aerosol cans are very versatile, which allows them to be taken up ladders, inside boilers, down into pits, and into very tight places. Most nonporous materials (steel, stainless steel, cast iron, aluminum, brass, bronze, titanium, rubber, plastics, and glass) can be examined using PT. Porous materials (concrete, wood, paper, cloth, and some types of fiberglass if the fibers are exposed to the surface) should not be examined using PT. It is important to remember penetrant is a very thin liquid designed to seep into the smallest crack. Consequently, if an assembly has stitch welds or material not sealed by a weld, the penetrant will travel behind the welds and between layers of unfused material. Penetrant can be nearly impossible to remove from these areas. Trapped penetrant will cause defects in welds if further welding is done, or will bleed out over time and contaminate paint and process fluids. For PT to be used on ASME Code construction or NBIC repairs or alterations, a written procedure must be followed. This must comply with ASME Boiler and Pressure Vessel Code, Section V, Article 6, and address all essential and nonessential variables. Many liquid penetrant examinations are done for informational purposes only, and do not follow a written procedure. For instance, a written procedure does not need to be followed if a welder grinding out a weld crack for repair is using PT to ensure removal of the entire crack. However, if the PT is being done to comply with Code, the written procedure needs to be followed by qualified NDE personnel. Click Here (pdf) to read more of this article from the National Board Bulletin (great publication for FREE!)