Back to Basics
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Even a Monkey Can Do It
by Daniel E. Mace*
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Liquid penetrant testing (PT) seems to have had a great deal of interest lately. Here is a Back to Basics that is presented very succinctly with humor and evidently from down in the trenches experience. It would seem that such a simple test could easily be done wrong! Would you believe? Frank Iddings |
How many times have you heard "even a monkey can do it" when non-NDT personnel are referring to liquid penetrant testing (PT)? However, even trained NDT technicians can and have performed PT incorrectly. There are many variables that can affect a correctly performed test, so let's look at a typical PT process and the many areas where it could go wrong.Temperature
You cannot perform PT in all temperatures; penetrant is a liquid and is subject to the laws of physics. There are some types of penetrant that can be used on high temperature parts, but most are restricted from use in lower temperatures (normally below 10 °C [50 °F]). Performing PT on a part temperature below 10 °C (50 °F), would be inappropriate. In some cases where it is not possible to get the temperature above the 10 °C (50 °F) range you may qualify the inspection at the temperature of intended use and with customer acceptance.Precleaning
This is one of the most critical steps in the PT process. The part's surface must be free of contaminants that could interfere with the flow of the penetrant into any discontinuities that are open to the surface. If the part was coated with oil or grease, wiping it down with a rag is not going to do the job. For example, if the part's surface was examined by ultrasonics using a couplant before the test, you would want to consider steam cleaning the part before applying the penetrant. Some codes require that you do this. The cleaning materials that you use, must also be taken into account as could they damage the material that you are cleaning. Think about whether they contain prohibitive amounts of halogens, sulfur, or mercury, or if they were tested and certified for the above. This is a requirement for PT. The cleaning material may meet all of these requirements and still cause damage if left on a part to go through further processing such as heat treatment. Your boss would be very unhappy if an expensive completed part were rejected by your customer because you used a cleaner with a sulfur content that exceeded their requirements.
"Is the liquid penetrant process as simple to do as most people think?"
Penetrant Selection
The surface condition and number of parts, as well as the accessibility and size/type of discontinuity are some of the factors that will guide your choice in the type of penetrant. You would not want to use a solvent removable penetrant on a part with a very rough surface or on a job that requires inspecting a lot of small parts. There are other types to use that would better suit these needs. In some cases your customer or code will tell you what type of penetrant to use. Is the penetrant you are going to use (if it's not solvent removable) being monitored by a process control system, to ensure system effectiveness? Before applying the penetrant, there are many things that could already have been done wrong if no attention were given to these critical preinspection processes. A monkey would have not made it this far, unless of course, if it was an ASNT NDT Level III monkey.
Application
The entire inspection surface must be wetted with the penetrant and kept wet during the entire dwell time. This means you may have to reapply penetrant during long dwell times. The penetrant can be applied by dipping, spraying, or brushing; however, remember that you have to remove the penetrant. Furthermore, if you are only inspecting a small weld on a large part, it is not necessary to apply penetrant to the entire part. There also may be health considerations if you are applying penetrant while in a confined space.
Penetrant Dwell Time
This part of the process has raised quite a few debates on just how long the penetrant has to remain on the part for an effective inspection. Some codes state that the part "shall be kept completely wetted for a minimum of 15 minutes and a maximum of 20 minutes." ASME Sect V, SE-165 Table 2 lists different materials with dwell times that range from five to 10 minutes. I have seen company procedures that range from 30 to 60 minutes. The point is that there are different dwell times depending on the code/procedure you are following. You cannot assume that a five minute dwell time will always be acceptable; you must review the procedure or code that governs the inspection that you are doing at that time. I have seen situations where the dwell times in a company procedure were changed but the PT personnel were still using the old dwell times because they had not read their own procedures and were unaware of changes.
Penetrant Removal
Depending on the type of penetrant you are using, the removal process can be done in several ways — a water wash for water washable penetrants, an emulsifier for postemulsifiable penetrants, a solvent dampened cloth for solvent removable penetrants. The removal process can easily be done incorrectly. When removing water washable penetrants, the temperature of the water should be maintained in the range of 10-38 °C (50-100 °F). The water spray pressure should be no greater than 276 MPa (40 psi), so you cannot just grab a water hose and remove water washable penetrant from a part. For lipophilic and hydrophilic emulsifiers, quite a few factors come into play. You have to apply the emulsifier correctly, for lipophilics you can immerse the part directly into the emulsifier with penetrant on its surface. However, for hydrophilics the part should be prerinsed with water prior to emulsification. Both have a certain dwell time that the emulsifier can stay on the part, based on the manufacturer or experimentally for each specific application. They also have temperature and psi restrictions for the water spray. The solvent removable penetrant is the one most often removed incorrectly. The surface penetrant should be removed with a clean, lint free cloth, then a cloth should be dampened with solvent to remove the rest of the surface penetrant. That sounds easy, but it is the most often improperly executed removal process. In the past, personnel have started the cleaning process with greasy rags, soaked the cloth with solvent and then applied it to the part, sprayed solvent directly on the part, or used penetrant contaminated cloth for final cleaning.
Developer
When I went through my first training class in PT over 30 years ago, I was told that I should see no trace of the surface penetrant when the developer is applied — it should be on a white background. In fact, during the practical exam a couple of students failed because there were shades of penetrant in the developer. Today, that process is incorrect; you should have a slight shading of penetrant in your developer, because that coloring indicates that you did not over clean the part. Most developers must be either shaken, stirred, or agitated before use. This is an important step; if you pick up a can of developer and spray it on without first shaking the can, the inspection becomes invalid. The amount of developer you apply is also very critical, you do not want a thick, heavy coat of developer because this could mask indications. A thin uniform coat is necessary. The dwell time is important; the developer should stay on the part as required by the code/procedure you are using. ASME, Sect V, Art. SE-165 states "The length of time the developer is to remain on the part prior to examination should not be less than 10 minutes. The maximum permitted times are two hours for aqueous and one hour for nonaqueous developers. Again the solvent removable process is the most misperformed. NDT personnel have sprayed the developer with the nozzle of the can not more than 25 mm (1 in.) away from the part so that the developer runs down the part, sprayed the developer without shaking the can, not applied enough developer, or sprayed on the developer and inspected the part within a couple of minutes.
Inspect the Part
Depending on which type of penetrant you used, there will be light intensity requirements for both visible and fluorescent examination. For the fluorescent part you have black light warm up and visual adaptation times. Some codes require a minimum 550 lx (50 ft candles) for visible dye PT, while others require a minimum of 1100 lux (100 ft candles). The normal everyday flashlight will not work for any of the above requirements. For fluorescent lighting the light wavelength should be in the range of 320 to 380 nm (1.2 x 10-5 to 1.5 x 10-5 in.). The light intensity should be checked and recorded per the timeframe noted in your code/procedure.I have seen very precisely recorded records on the above and the intensity numbers were what they should have been, but, when I checked the filter on the blacklight, it was covered with a thick layer of dirt and grime. If you are going to go through the trouble of documenting the light intensities, do it right. You should also be sure that the inspector has had a vision test within the past year and that it is documented.
Post Cleaning
Some codes tell you that post cleaning is only necessary in those cases where residual penetrant or developer could interfere with subsequent processing or service requirements. Other codes tell you that the penetrant materials shall be removed as soon as possible and how to remove it. If you are inspecting an item that is not covered by a code, your procedure should at least tell you to wipe off all PT materials to avoid part corrosion. If you leave the developer on a part that is going to go through vapor degreasing, the developer could be baked onto the part. Post cleaning materials come under the same restrictions as precleaning materials.Is the liquid penetrant process as simple to do as most people think? It is simple to do it incorrectly, and that is the only thing simple about it. Do you feel that you know everything there is to know about this process and that nobody can tell you something you don't already know? If you do, look real hard in the mirror, you may just see a monkey looking back at you.
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