pickling solution severely damaged end caps

[Shala]

gday fleet
wondering if anyone else has had the experience of recommisioning a water maker only to find the end caps leaking the pickling solution out and the end caps severly corroded and compromised. the watermaker is only 2 years old and the pickling solution had been inside the pressure vessel for six months.
all input welcome

[Tellie]

What make and model and what solution?

[thinwater]

Originally Posted by Tellie

What make and model and what solution?

And what are the end caps made of, and any other metals in contact.



Some people use just propylene glycol, which is rather corrosive by itself.

[Cheechako]

I had a 3 membrane water maker and the end caps were all severely damaged from salt water. Are you sure it's the pickling solution? I guess it doesnt matter. Mine were black anodized aluminum, but still corroded readily. In the end I decided on using one new long membrane and new housing. It still put out 24 GPH.

[a64pilot]

Originally Posted by thinwater

Some people use just propylene glycol, which is rather corrosive by itself.

You sure on that? I’m under the impression that it is essentially old style car antifreeze, which isn’t usually corrosive.
However the Sodium Metabisulphate if that is spelled correctly sure is, just get some on your skin.

I plan on pickling with propylene glycol myself from now on, unless someone can tell me why I shouldn’t

[Shala]

the watermaker is an aquamax. the end caps are black anodized aluminum and the fittings are stainless steel. im not sure what the pickling solution was. it came with the watermaker in a sealed cartridge with the instructions for pickling. the pressure vessel leaked right out of the box but once the pressure came up it stopped leaking. not sure if the seawater or the pickling solution was the culprit but i am thinking of having the end caps turned out of stainless as replacements as the original are now useless. what is the opinion of that move?

[Shala]

ironically the stainless fittings that screw into the end caps are actually the only place where no corrosion is found

[a64pilot]

Stainless on Stainless is preferable to Stainless and aluminum. That set up dissimilar metals corrosion, add an electrolyte which seawater is a great one, and your going to have trouble pickling solution or not.

[billknny]

Originally Posted by Shala

ironically the stainless fittings that screw into the end caps are actually the only place where no corrosion is found

That’s your problem! Stainless fittings into aluminum? Recipie for disaster. What is the pressure vessel made of? If it is Al then you’ll destroy it with stainless end caps.

This is classic bad engineering.

[Fuss]

I see no problem with stainless fittings onto aluminium end caps, the stainless fittings are on the outside and not immersed.
Maybe the watermaker had been leaking saltwater over an extended period of time?

[Shala]

the pressure vessel itself appears to be made of fibre glass therefore i figure the stainless end caps will be okay. problem might be finding someone who can turn the stainless to the right tolerances. has anyone else ever had this problem and what was the solution other than replacing the pressure vessel. i guess the question is : should i just replace the pressure vessel and membrane or should i try and repair the old one. if i do replace the pressure vessel and membrane, what is the best "engineered" one out there.

[Shala]

also would be interested in knowing more about the propylene glycol pickling method pros and cons

[charris]

I went through this about 5 years ago - exactly the same, and replaced the end cap. A hole was eaten out from product side to salt side. I suspect in my case the fault was that I did not carefully measure the proportion of the chemical water. Since then I have pickled using the same solution (SMS) and have had no further trouble. One needs to account for the water in the mixing bucket and the water remaining in the system. Here is a general guide that should help calculate the correct amount of powder (2% by weight)

Powder Form Preservative
After thoroughly flushing your watermaker with fresh water, disconnect the brine discharge hose and run it to a clean bucket filled with fresh, dechlorinated water. You can use RO product water or tap water that has been run through a carbon filter.

Disconnect your boost pump inlet hose (make sure you close the seacock) and run it to the same bucket. Make sure you don't allow the boost pump to lose its prime. Add the preserving chemical to the bucket so that you end up with a 2% solution overall. The preservative needs to be 2% by weight of the total water in the system, not just the bucket! Here is a quick way to estimate the total water in your system:

Start with the water in your pressure vessels:
2.5 x 21" Membranes - About 3 Lbs each
2.5 x 40" or 4.0 x 21" Membranes - About 6 Lbs each
4.0 x 40" Membranes - About 15 Lbs each

Then add the water in your prefilters:
2.5 x 10" Standard Housings - About 2 Lbs each
2.5 x 20" Standard Housings - About 4 Lbs each
4.5 x 10" Big Blue Housings - About 5 Lbs each
4.5 x 20" Big Blue Housings - About 10 Lbs each

Then add the water in the plumbing system and your bucket:
Plumbing - About 1-2 Lbs
Bucket - 8 Lbs per gallon

For example, a system with two 2.5 x 40" membranes and two 2.5 x 10" prefilter housings will have about 17 pounds of water, not including the bucket. If you add two gallons for the bucket, the total water in your system will be about 33 pounds. Calculate the amount of preservative powder to use: Total Water (Lbs) x 0.02 = Preservative (Lbs) In our example, 33 Lbs x 0.02 = 0.66 Lbs, or 2/3 of a 1 Lb Jar

Carefully mix the preservative chemical in the bucket of fresh water. Wear safety goggles and a mask, and work in a well ventilated area. This stuff is pretty harsh. Set the system pressure to ZERO, and make sure the product water will not go into your tanks. Run your watermaker boost pump and feed pump to recirculate the preservative for 30 minutes.

[Shala]

thanks for that charris

[Tellie]

Any SS fitting on any watermaker that see's salt water is going to corroded eventually, add a dissimilar metal to it such as aluminum and the less noble metal will suffer first.