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Caution: Corrosion When Wet

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Ann R. Thryft
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Re: Can naval tech help washdown?
Ann R. Thryft   3/9/2012 12:37:14 PM
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Thanks for the feedback. I can see where the corrosion protection would be different in the two environments. I guess I was thinking more about the water aspect of the environment, but it looks like the two are inseparable.


TJ McDermott
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Re: Can naval tech help washdown?
TJ McDermott   3/9/2012 12:24:38 PM
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I would guess less than you might think. Take the junction box design, for instance. Any commercially available industrial J-box has a flange all the way around the door. It adds stiffness, and prevents directed jets from impinging directly on the door gasket. Shipboard means of corrosion protection include layer after layer of heavy paint (assuredly not latex house paint, either). I don't see that sort of anticorrosion coating making a comeback in factories.

Ann R. Thryft
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Can naval tech help washdown?
Ann R. Thryft   3/9/2012 12:12:09 PM
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TJ, thanks for a clear explanation of what can go wrong with water ingress.  Your mention of both the tugboat and industrial washdown environments made me wonder how much cross-over there might be from technology developed for naval environments to industrial contexts?


JimT@Future-Product-Innovations
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Gasket material: TPE vs. LSR
JimT@Future-Product-Innovations   3/9/2012 11:34:43 AM
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During a recent contract, I was hired to design an enclosure intended to pass IP64 and temperature cycling extremes. Since I had experience designing both water-resistant, and water-proof (ambiguous terms invented by marketing,,,) enclosures in the past, I saw no real challenge to the task. 

 

While laying out the preliminary product concept, I specified TPE (Thermoplastic Elastomer) as the gasket material between the housings.  I had used that solution previously, and its sealing capability to prevent water -- even pressurized and/or blowing water -- was proven.  TPEs are highly compressible and provide excellent dimensional compliance when gasketing between harder plastics.

 

Luckily, a peer who had been down a similar road looked over my shoulder and quickly pointed out a problem:  remember, the product must pass not only IP64 but also temperature extremes.  His past experience taught that a TPE, when compressed per design intent, would take a permanent compression-set at extreme cold.  Later, as the product cycled back to higher temperature, the TPE would no longer be compliant as a gasket and allowed significant water intrusion after only one cold cycle.    The solution was to use silicone rubber, which has true spring-back to original geometry, even after hundreds of temperature cycles.

 

While injection-molded Liquid Silicone Rubber (LSR) resolved that particular problem, it was not without a long list of other engineering challenges; but I'll save those for a subsequent post.  The points to remember are, (1) TPEs take a "set" at extreme cold, and (2) don't be afraid to listen to your peers.

GlennA
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Pressurized tank
GlennA   3/9/2012 10:31:22 AM
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One of the machines that I worked on had a pressurized hydraulic tank.  About 4 psi from the plant compressed air supply was supposed to keep water from getting in.  However, the design team had upgraded the reservoir to have a vented cap.  When I advised them that a vented cap would not hold pressure, they told me " you don't understand".  When I installed this version of machine I advised the customer not to try to pressurize the tank - to turn that regulator down to zero.  On one of my service calls for a similar machine, the hydraulic oil looked milky.  I found the plant had wet air.  The air drop did not have a drip leg or drain valve.  The machine's water trap was full of water and the automatic drain had been turned off, and the lubricator was also full of water, instead of oil.  And the pressurization regulator was on.  So effectively they had been adding water to the hydraulic tank through the pressurization system that was supposed to keep the water out.

Beth Stackpole
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Keeping the environment at bay
Beth Stackpole   3/9/2012 6:36:19 AM
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I'm sure engineers across disciplines and industries can appreciate your tales of water ingress protection. Lots of perspective advice, too, as to how to prevent or at least trouble shoot this issue. I'm sure water isn't the only environmental component that finds a way to wreck havoc on engineering efforts--what are some of the other major environmental influences that post common challenges for engineers?

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