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Penetrant Testing: a badly-designed drying oven

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Written by Administrator
Saturday, 19 November 2011 10:54

A short "Oldies but Goodies" story, again about a faulty design. It dates back to the early seventies … yes, an old story! A story for veterans, right.

A well-known French manufacturer of valves for car, truck, even ship engines had bought a complete penetrant testing line. The valves were inspected using a water-washable fluorescent penetrant and a dry developer, listed in the QPL (Qualified Products List) of the American Military Specification MIL-I-25135 C (ASG)(*) which was in force at that time.

Many parameters on this line, for instance the washing parameters, were … surprising, with a dramatic influence on the quality of detection.

The topic of this month is the drying oven.

Keep in mind that this specification did not state any requirement for the thermal stability of the penetrant fluorescence.
Therefore, many QPL-listed penetrants were prone to the "heat fade"(*).

At high temperature, the fluorescent brightness of this penetrant was lower than at room temperature. However, a drying oven was needed to dry the parts after water washing and before the dry developer application. Therefore, the drying oven should have been set at 60-65°C (140-149°F) for some few minutes.

Even these quite "soft" conditions impaired sensitivity and reliability.

The user complained about the drying oven. Parts stayed in the drying oven for an hour, at 100°C (212°F), but went out with some moisture still in some areas!

We easily found the culprit: a wrong design. There was no fan in the drying oven; the air did not circulate, except for the convection effect due to the temperature difference between the top and the bottom. There were many parts in baskets, many more than planned at the design step. The drying oven was the "jamming step" in the line.

We suggested first that the drying oven be fitted with an air circulating fan, and simultaneously to set the temperature down to 60°C (140°F). We also suggested other fundamental changes, mainly dealing with the way the penetrant was used, such as the washing parameters, etc.

This short story is to remind you that drying a part is FAR MORE EFFICIENT when there is a relative movement between the air and the surface. In fact, the higher the air speed, the stronger the flow, the quicker drying will occur for a given temperature. Not only it is far more efficient, but it also keeps costs lower, as fewer calories are used to get the right result: parts dried without detrimental effects on the penetrant.

Reference

(*) Pierre CHEMIN and Patrick DUBOSC, The specifications which changed the penetrant materials, August 2008, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/news/pt-texts/162-les-specifications-qui-ont-change-les-produits-de-ressuage


In the '60s a UK Company had a motto: "For every surface treatment problem, there is a product xxxxxx" (trademark we do not display).

Engineers and Commercial people in this Company had made a "translation": "With every surface treatment product xxxxxx (trademark we do not display), there is a problem".


This anecdote is there only to remind everyone that problems met in workshops may be due to the suppliers/manufacturers as well as to the users.

Our idea in these documents is NOT to target anyone, but on the contrary to bring to your knowledge some interesting cases which may prevent you to duplicate the same mistakes while performing Penetrant Testing (PT) or Magnetic Testing (MT).

All the ministories you will read are TRUE. We think they will be helpful:
• First as examples of specific technical --or non-technical-- requirements or peculiar problems.
• Second to let you see that the problems do not always come where you think they should come from.
• Third so that users feel free to ask for help from people (the experts) who may know more than they do.

Last Updated ( Saturday, 19 November 2011 15:52 )