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Penetrant testing: water pressure in the rinsing/washing booths: a myth?

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Written by Administrator
Tuesday, 01 June 2010 15:46

June 2010

A lot of penetrant inspections require use of water to get rid of the excess of penetrant from the surface, either with or without an emulsifier. This is particular true when PT is performed on PT lines, be there manual or automatic.

In our DPCNewsletter N°023(*), you may read our suggestion for ‘‘water wash’’ and ‘‘water rinse’’ definitions.

A constant requirement is that water pressure shall be lower than a maximum figure … which is not always the same after the different specifications/standards! Not quite easy when a subcontractor processes parts for different primes!

Underneath you may find some maximum water rinsing/washing pressure figures taken from different aerospace specifications which will enhance the previous paragraph wording:

- For water-washable penetrants:
• Water wash with water only: 275 kPa (ca 40 psi), with an air + water gun: 172 kPa (ca 25 psi) each.

- For post-emulsifiable penetrants:
• Water rinse with water only: 400 kPa (ca 58 psi), with an air + water gun: 200 kPa (ca 29 psi) each.
• Water wash with water only: 275 kPa (ca 40 psi), with an air + water gun: 172 kPa (ca 25 psi) each.

Needless to say that this point is an easy target for any auditor!!!

The problem is that this pressure figure may be thought of as a myth; well entrenched in documents, in habits, in minds, since … many years … but a myth whatever you could think !!!

We are to demonstrate that it IS a myth, and that meeting the requirement IN NO WAY is a guarantee against any overwashing of the surface.

What is the purpose of this wash step? It is supposed to wash off all the penetrant (+ emulsifier, if any) in excess from the surface, leaving no background on the part before drying, developer application and inspection. This is applicable both to colour contrast and fluorescent penetrants, though, obviously, in fact installations or inspections using fluorescent penetrants are more prone to the requirement. If insufficient, the water action will leave a background. If too harsh, it may lead to overwashing, i.e. taking some, or a lot of, penetrant out of the discontinuities, hence impairing sensitivity, especially when a dry powder developer is used.

So many are those who believe that coping with the pressure figures stated in documents make them being on the “safe side” of the process.

Just a small story, “Oldies but Goodies’’ style.

Many years ago - must we remind you that, as we are “experienced men”, we have worked for many years “on the spot”, close to users? - one of us, let us call him PCD, was in an aerospace subcontractor plant, dealing with the MT bench, but close to the fluorescent PT line. The young woman who performed the PT inspection suddenly yelled “strong words” and called for the man whom PCD was talking with.

- “Sir, I cannot wash the parts: when I spray water, all the parts are thrown away. What must I do?”

PCD made the few steps to the PT line, and began talking with the woman:

- “Hey, what’s happened?

- ‘‘You see, these parts (ring-shaped) are slipped on a bar, quite useful to bring them all at a time in the penetrant tank. But when I wash the parts, they are all pushed out of the bar!’’

- ‘‘Which water pressure do you use?’’

- ‘‘I do work for an aeroengine manufacturer, and its procedure asks for 1bar (14.50 psi) of water and 1 bar of air’’. (Note that she should have said 100 kPa).

- ‘‘No, the specification states: 1 bar MAX for each; it is not written you MUST have 1 bar each! This condition is only for rough surfaces, difficult to wash; not applicable to your parts with a machined surface!”

As the woman was not convinced, PCD chose an example:

- “Do you come to work driving your own car?’’

- ‘‘Yes.’’

- ‘‘Traffic law allows you to run at 90 km/h (ca 56 miles/h) out of towns. Right ?’’

- ‘’Right’’

- ‘‘When there is black ice on the road, do you drive at 90 km/h?’’

- ‘‘Oh no, I drive very slowly!’’

- ‘‘Nice; so you ADAPT your driving to the external conditions. You have to do exactly the same thing here: 1 bar + 1 bar is only a maximum for specific conditions. With YOUR parts, and given the gun you have, you should set the pressure at 0.2 bar for water and 0.3 bar for air.’’

- ‘‘But it won’t work!’’

- ‘‘It will! Try!’’

Really unconvinced she nevertheless performed as suggested. And, believe it or not, not only parts stay stubbornly on the bar … but there were washed exactly as they had to be!!

Another example, in a large aircraft manufacturer plant. An auditor audits the PT installation - a gigantic one. In the washing booth, a small “cat’s wee-wee”, as we say in French, goes out of the washing gun. But the auditor looks at the manometer, which is some 10 meters (11 yards) upstream. And, horror, when 3 bar (43.5 psi) max are mandatory, the meter displays … 7 bar (101.5 psi)! Immediately the auditor writes a “non-compliance report” (an NCR), though the experienced operator on the line demonstrates that the very gentle water stream going out of the gun could never overwash the part!

With these two examples, you may begin wondering what does the water pressure mean.

Our answer: it is a MYTH!

We agree that some means to check the “efficiency” of washing the parts MUST be used. But a manometer is NOT the right tool!!

What do we have to check then? And how?

The water mechanical effect on the surface

The mechanical action of water on the surface comes from a combination of several parameters:

• Water pressure.

• Water flux.

• Distance between the spray nozzle and the part.

• Aperture angle of the water stream, i.e. is the stream very fine, or very large? This is a MAJOR factor: given the pressure and the flux, the MECHANICAL action of the water stream will be very, very different if the stream is fine or very large. The speed with which water droplets arrive on the surface will be so different that in one case, overwashing is likely while with the large aperture stream, on the opposite, good and reliable washing conditions will be used. In fact, the best way to wash parts is a fine rain, and not a tropical flush, just to make it easier to imagine.

Obviously, a manometer cannot do for the job! So, what to do?

We may imagine using a quite heavy part; heavy enough so that the water flux will not make it move back, modifying the angle between the surface and the water stream, hence the mechanical action. This part would come with a sensor, kind of “strain gauge”, able to measure the mechanical force in plain waterfall!!

Using such a “meter” it would then be easy to make tests by putting the sensor in the vicinity of real parts, in the real conditions, and seeing what would be the right range to get the right background level (no background, a reasonable level, or whatever), without making any reference to water pressure.

Both we understand that other parameters shall be thought about when washing parts: water temperature, washing time, spray nozzle-surface distance, air and water flow-rates and so many others (recycled water containing too much surfactants, water containing too much calcium, etc.), but this paper is focused on the mythic water pressure parameter. Its purpose is to make you, readers, users, think twice when reading specifications...and writing your own!

We would be happy to get some reactions from readers, Level 3s, specifications writers, users, auditors.

Thank you in advance for your involvement.

Reference

(*) Patrick DUBOSC and Pierre CHEMIN, ISO 12706:2009 standard: missed opportunities,
DPCNewsletter N°023, April 2010, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

Last Updated ( Tuesday, 24 May 2011 15:56 )