LIQUID PENETRANT INSPECTION

Liquid Penetrant Inspection (LPI) is a nondestructive testing method used to locate surface breaking discontinuities. A low-viscosity fluid known as a "penetrant" is applied to the part and allowed to 'dwell" on the part for a specified time. The excess penetrant is then carefully removed and a "developer," which is a thin chalk-based powder coating, is applied. The developer, most commonly applied by an aerosol, draws the penetrant out of any discontinuities and provides a contrasting background to improve the visibility of the indications.

Liquid Penetrant Inspections can be performed on ferrous and non-ferrous materials, such as welds and weld overlays, castings and forgings, valves and their components, machined parts, pressure vessels, cladded vessel linings, and structural steel. It is most commonly used on non-ferromagnetic materials where other techniques, such as magnetic particle inspection, cannot be used.

Liquid Penetrant Inspections can only identify surface breaking discontinuities, therefore surface preparation is the most critical part of this inspection technique. Surfaces must be free of oil, grease, scale, paint, rust or other residues, and careful, thorough cleaning is crucial.

The penetrant fluids are either visible or fluorescent and are designated as water washable, post-emulsifiable or solvent removable based on how the excess penetrant is removed. The water washable visible method is commonly used but it is also the least sensitive. Fluorescent penetrants are highly sensitive since they penetrate and detect small discontinuities. This is relevant on critical components such as aircraft parts. Fluorescent penetrants must be viewed under a near ultraviolet light, sometimes called a "black light", and any discontinuities will show as bright green fluorescent indications. Visible penetrants have only one sensitivity level, and generally appear as red indications against the white developer background. They provide a good useful method of detecting surface breaking discontinuities.

Penetrants are also limited by temperature and generally operate best in the range of 40°F to 125°F. Special procedures and penetrants can be used for lower or higher temperatures but these must be qualified by a procedure test.

Test results are interpreted in accordance with the appropriate specifications, or customer requirements. A detailed report is issued clearly identifying the locations of any discontinuities.