does polishing direction influence drainage

[wydim]

Hello,

simply put, on 304/316 Stainless steel plates (polished to 32μin or 20μin), does the polishing direction* influence the drainage of water on it?

example : for the same slope, one plate is polished in the slope direction, another one at 90°. which on will have the "better" drainage

context : we are an OEM of S.S. tanks,. We have to do a drainage test in a tank for a pharmaceutical company. the bottom head is a F&D so the slope near the center is almost horizontal, but it has to drain completely. We want to know if the direction of polishing matters.

*I just want to be sure the distinction is clear between the S.S. molecular GRAIN direction in S.S. and POLISHING direction

regards

 

[IRstuff]

"drain completely"; how are you measuring this?

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[EdStainless]

Unless it is a coarse surface I have not seen this have a big effect.
The polish direction can have an influence on how much microscopic debris is retained though.
I have seen these drainage tests done with tracer material in the water so that they could then be flushed with pure water and a quantified measure could be taken.

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[wydim]

IRstuff,

sorry, I could have been more clear on the topic, but it wasn't the main focus of the question.

From our client specs sheet : "The tank shall be filled to the minimum working level (or at least above the bottom weld
seam) with potable water and allowed to drain by gravity using the bottom outlet. There
shall be no water drops left in the vessel greater than 5 mm in diameter, nor stagnant water
in ports or accessories."

EdStainless,
no tracer used, just tap water, so it's not an issue

 

[wydim]

Your opinions or experiences are welcome, but I'm also looking for "scientific" source materials to settle this debate. I spent 1h to look on the web/forums/tank components fabricators website and haven't found a single sentence with the words polishing direction and drainability used together.

regards,
Dimitri

 

[IRstuff]

That's probably because there isn't a connection at the level you're talking about. A 5-mm drop is like a million times larger than the resultant surface roughness after polishing

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[Compositepro]

Polishing direction will have minor effects on what you call drainage. Surface tension of the liquid relative to the surface energy of the vessel surface is a much bigger factor. If the liquid wets the surface a film will adhere to the surface and this film will drain (flow) through the thickness of the film but the film will not detach from the wall. If there is no wetting the liquid will bead and roll off. Particulates in the liquid can increase surface roughness and cause attachment of the drops to the surface, thus impeading rolling off.

 

[MintJulep]

32 micro inch is not polished in my world.

Regardless, it with have trivial affect relative to the requirement you need too meet.

 

[EdStainless]

I would hope that this is sub 24 microinch.
The problem with this test is that it really is a function of the cleaning more than surface texture.
How do you final clean?
Have you ever looked into using plasma for cleaning?

My comment about a tracer is that some people have a quantitative way of measuring the amount of retained material.

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P.E. Metallurgy, Plymouth Tube

 

[3DDave]

It seems odd to ask this as an experiment could be done really easily to see if the liquid will drain from the flat surface.

I would not expect an F&D to be the right answer.

The grain direction won't be 100% radial or 100% tangential so it will be continuously varying.

I agree that surface cleaning is far more influential than grain or polish direction.

 

[wydim]

thank you all for your answers.
it is also my opinion that surface cleaning is a much more important factor than polishing direction. We use a nitric acid solution to cleam the bottom head and that helps in creating a film flow (low surface tension) instead of the water forming beads. Then drainage test, then passivation

F&D is because the tank is pressurized.

thank you all.

Dimitri

 

[Compositepro]

You can apply a mold release coating (something like Rain-Ex on a windshield) to get beading and rapid drainage. Combined with using a squeegee or air knife, you could get almost no residue left in the vessel, which seems to be your customer's goal. The coating would be re-applied after every cleaning. Film drainage gets to be very slow when the film is thin and will always leave a residue, which may build rapidly.

 

[3DDave]

Since there is a drain in it the form is already modified from F&D, so another variation is possible. One could make the drain fitting a cone 20 inches in diameter, or whatever appropriate size, to maintain a usable slope across the entire bottom of the tank.

 

[BUGGAR]

Adding to Compositepro, you can also add a wetting agent to water like firefighters use to reduce surface tension. We used fire trucks to fill our soil percolation test holes to make the soil "perc" better.

 

[Compositepro]

A mold release does the opposite of what a wetting agent does. It more a de-wetting agent but is applied to the solid surface rather than into the liquid. Teflon is an ingredient in some mold releases, but more common are silanes in solvent, which cure into an invisible coating with a low surface energy.

A wetting agent would also improve drainage by preventing the film from breaking into droplets which will stick to the surface and not roll-off. But with a wetting agent, the film would, technically, be considered a droplet of greater than 5 mm diameter

 

[EdStainless]

Your cleaning with nitric acid is the passivation, so why would you do it again?

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[wydim]

EdStainless,
because we wouldn't supply a passivation certificate for "our cleaning" (it's just a spray and wipe type of cleaning). We then pay a passivation company (as per cust. requirement) and they provide an official certificate according to ASTM A380). Then the tank is sealed and delivered.

Compositepro,
no squeegee or air knife is allowed, as it would defeat the purpose of testing the SELF-drainage of the tank head. The client will not re-apply "rain-ex after each cleaning cycle" for reasons related to the plant operation and also because the product is not allowed in a water tank for pharmaceutical use.

Gentlemen, I know you just want to help, but the thread as moved away from my initial question focusing on the polishing, to "the best way to succesfully drain the head" (which we don't have a problem with to begin with).

Regards,