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Liquid Penetrant Testing for Stainless Steel Flanges

July 14, 2021

What is Liquid Penetrant Testing?

Liquid penetrant testing is a nondestructive testing method for detecting discontinuities that are open to the surface of the stainless steel flange. These discontinuities include cracks, seams, laps, cold shuts, laminations, isolated porosity, through leaks, or lack of fusion. Stainless steel flanges, especially austenitic and duplex, may be subject to cracking if the forging temperature or operation is inappropriately controlled. Such possible defects shall be detected to avoid catastrophic failure in service. The stainless steel flanges may be manufactured in accordance with ASME B16.5, ASME B16.47, API 6A, EN 1092-1, BS 4504, JIS B2220, AS 2129, or GOST 12815, and other international standards.

LPT for 316/316L dual certified flanges

The liquid penetrant testing for stainless steel 316/316L dual-certified flanges, made in accordance with ASME B16.5, supplied to the UK.

There are many liquid penetrant testing methods for stainless steel flanges. The most often used method is the visible penetrant examination utilizing the solvent-removable process. This test method is applicable to in-process, final, and maintenance examination. The visible penetrant testing uses a penetrant that can be seen in visible light. The penetrant is usually red, so that resultant indications produce a definite contrast with the white background of the developer. Visible penetrant indications must be viewed under adequate visible light.

Significance and Use

Liquid penetrant testing indicate the presence, location and, to a limited extent, the nature and magnitude of the detected continuities. The visible penetrant testing with solvent-removable process is intended primarily for portability and for localized areas of examination, utilizing minimal equipment.The test conducted shall depend on the design and service requirements of the stainless steel flange being tested.

Reagents & Materials

The visible & solvent-removable liquid penetrant testing materials consist of a family of reagents such as visible penetrant, solvent remover, and developer. There are many proprietary reagents available in the market. Combined use of reagents from different producers is not permitted. The usage of each reagent shall strictly follow manufacturer’s recommendations.

Penetrant testing reagents

The reagents for liquid penetrant testing: penetrant, remover, and developer.

The visible, solvent-removable liquid penetrant is a red-color reagent, which penetrates into surface discontinuities of the stainless steel flanges. It is formulated so that excess surface penetrant can be removed by wiping with dry, clean, lint-free cloth, and repeating the operation until most of the penetrant has been removed. The remaining traces shall be removed by wiping the surface with clean, lint-free cloth lightly moistened with the solvent remover. The solvent remover function by dissolving the penetrant, making it possible to wipe the surface clean and free of residual penetrant. The developer forms a translucent or white absorptive coating that aids in bringing the penetrant out of surface discontinuities through blotting action, thus increasing the visibility of the indications.

Testing Procedure

LPT for SS304 flanges

Liquid penetrant testing for weld neck flanges made from ASTM A182 F304, ASME B16.5 Class 150 16″ STD, supplied to Bucharest, Romania.

The following general procedure applies to the visible, solvent-removable penetrant examination for stainless steel flanges:

  1. Temperature & Preclean: The temperature of the penetrant materials and the surface of the flange to be tested should be from 40°F to 120°F [4°C to 49°C]. The surface of the flange shall be precleaned to remove processing residuals such as oil, graphite, scale, insulating materials, coatings, and so forth. The precleaning should be done using cleaning solvents, vapor degreasing or chemical removing processes.
  2. Dry after Cleaning: It is essential that the area to be examined be thoroughly dry after cleaning, since any liquid residue will hinder the entrance of the penetrant. Drying may be accomplished by warming the parts in drying ovens, with infrared lamps, forced hot or cold air, or exposure to ambient temperature.
  3. Penetrant Application: After the area to be examined has been cleaned, dried, and is within the specified temperature range, apply the visible & solvent-removable penetrant to the surface to be inspected so that the entire flange or area under examination is completely covered with penetrant. The penetration time or penetrant dwell time shall be between 5 to 60 minutes and shall not be less than the manufacturer’s recommended time for the required sensitivity.
  4. Removal of Excess Penetrant: After required penetration time, remove the excess penetrant by using wipes of a dry, clean, lint-free cloth, repeating the operation until most traces of penetrant have been removed. Then lightly moisten a lint-free cloth with remover solvent and gently wipe the remaining traces to avoid the removing penetrant from discontinuities. Care should be taken to avoid the use of excess solvent. Flushing the surface with solvent to remove excess penetrant is prohibited.
  5. Dry after Removal: similar to step 2.
  6. Developer Application: Apply by spraying the developer onto the flange surface which is dried from step 5. Drain excess developer from the examined area to eliminate tendencies of pooling of the developer, which can mask indications. Then dry the developer using hot air blast, infrared heater, or by exposure to ambient temperature. The dried developer appears as a translucent or white coating on the part. The development time should be between 10 to 30 minutes.
  7. Examination & Evaluation: Perform examination of flanges after the applicable development time to allow for proper bleedout of penetrant from discontinuities onto the developer coating. Visible penetrant indications may be examined in either natural or artificial visible light. Any discontinuity indication shall be evaluated and recorded on the basis of size of the stain created by the developer’s absorption of the penetrant.
  8. Post Cleaning: Any residual penetrant or developer shall be removed carefully.

Standards for Liquid Penetration Testing

ASTM E165, ASTM E1220, EN ISO 3452, EN 571, EN 10228-2, JIS Z 2343, DIN 54152, NF A04 315-2, BS 6443.

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