I've been thinking a lot lately about the environment and how, despite even the best effort, it always finds a way into machinery.
Most recently, this involved water getting into a junction box on a tugboat, even though the crew worked hard to prevent it. The J-box and the cable fittings entering it were wrapped in Densyl tape. The lid had a thick red silicone gasket. There was RTV silicone smeared around, as well. The crew really wanted that box to stay dry. And yet, when I opened it, the terminals inside were corroded, and I could feel moisture.
A couple of months ago, I opened a machine electrical enclosure to discover it filled with water up to the lower lip of the door flange. The box was Type 13, and all entries (conduits, wireways, cord grips, etc.) maintained that rating. This wasn't even a washdown environment.
At my previous employer (which did have many machines in washdown environments), the engineers and the workers in the service department expected to open an enclosure to discover standing water. The water may not have originated in that particular enclosure. Liquid-tight conduit can keep water out, or act as a pipeline keeping water in, and transfer it from one enclosure to another.
All of the components in the overall electrical assembly can be selected for proper ingress protection and still not be enough to keep the enclosure protected. In one memorable incident, the third-shift cleaning crew performed their job exactly as instructed, hosing the machine down completely with high-pressure water and caustics. They washed the machine completely, including the open electrical enclosure. That little incident cost the customer more than $20,000 in replacement components. That's an expensive but valuable lesson, if the lesson were learned. The same thing happened again a couple of months after the machine was repaired.
The fight against water ingress constantly escalates. It used to be that sensors were terrific if they had an IP65 rating (4psi water jet). As food plants increased their sanitation methods, this was no longer enough. IP66 became the standard (14psi water jet), then IP67 (3ft complete submersion). And then even that wasn't enough, and IP69K (1,200psi) is now the level necessary to protect sensors from water ingress from high-pressure hot water hoses.
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?
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.
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.
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?
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.
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.
Currently I am working on the characterization and performance evaluation of solar collectors, we are using thermocouples for temperatures measurment in a closed loop of solar collectors in order to read temperature elevation of a recirculation fluid. One of our headaches is water in the terminals of the sensors, it produces oxidation in the ferrite metal of the thermocouple and therefore lecture errors, if anyone can suggest an efective way to isolate thermocouple terminals from water it will be really appreciated.
Capillary action can draw water between the strands inside a cable from the connector outside the watertight box. a gastight gland is no protection against this.
It's worth remembering that a 1$ plastic bottle of soda has a perfect gastight screw-top seal with no o-rings or RTV.
In my experience the scheduled cleaning of the service equipment was the biggest issue with corresion.I noticed that seals were not holdig with time and the quarterly cleaning let some moisture inside. This was the main cause for corrosion on power terminals and transformers, especially where the thermal cycling was evident.
I've had radar lines on a 90' sailboat mast dump water into a junction box that was sealed. Every fall when the mast was removed, the terminals in the box would be severely corroded and the wire needed to be stripped-back to find bare wire. The wire was sealed in the radar dome and sealed in the junciton box, however in winter the mast was stored horizontally with the dome removed and the wire exposed. Somehow water got into the cable and until it was replaced, caused corosion in the sealed box. Once you see this, you can find the same symptom in other areas. It makes for interesting trouble-shooting.
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