Piping Fitting Leak / Corrosion issue...

[quizzical1]

Hi All,

This is piping in a hot water recirc loop. Any ideas on what would be causing the leaking and also the buildup? Been installed for 2 years.

 

[TugboatEng]

Brass is subject to dezincification and that is accelerated by galvanic corrosion. What type of corrosion inhibitors are you using?

Also, as it is leaking on both sides of the 303SS component, are the threads of the correct type? NPT or BSP?

 

[quizzical1]

Hi, it's potable water so no inhibitors used.
Yes all threads are NPT with Loctite 567 on all threads. Doesn't happen on all units - just a few.

Would wrapping the threads with ptfe tape first and then applying the 567 afterwards help prevent the galvanic corrosion?
Should we be using a SS nipple between the CF8M strainer and 303SS body?

Thanks for your help!

 

[EdStainless]

TBE is on point.
You have two options, go all brass, or go all bronze.
From the amount of scale this isn't very pure water.
This means that you will have fairly high electrical conductivity, which will along with the higher temp accelerate galvanic corrosion.
I would also guess that the threads on the SS fittings will show some crevice corrosion.
Especially since neither of these SS parts is very corrosion resistant.
I would also expect to find some Cu deposited on steel or SS components further downstream in the system.
When brass de-zincifies you actually dissolve some of the material besides just the Zn and the Cu then re-deposits locally, but some will be transported.

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P.E. Metallurgy, consulting work welcomed

 

[BenMacier]

If it doesn't happen on all units of the same domestic hot water system, what differ between these units?
Their location in the system? Floor level? Temperature regime? Installation period? Usage?
All these materials are fairly compatible to each other in domestic systems. So what is the external trigger that makes them fail together? is the question to answer.

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

 

[quizzical1]

Thanks guys!

Thanks TBE and Ed - is all-SS not an option?
Ben - what I meant was it doesn't happen in other installations in other buildings they're installed.

 

[EdStainless]

Local temp and water quality.
I used to work with an industrial water heater manufacturer.
Very slight differences had a huge impact.


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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

What kind of velocity and static pressure are you seeing in the pipe? The damage is all down stream of a y-strainer which introduces a lot of turbulence to the flow. In a hot water loop this breeds cavitation erosion. If you find the fittings to be eroded suspect this to be the case.

As Ed said, replace the downstream fittings in the vicinity of the strainer with SS as it resists this kind of erosion.

Various difference in static pressure of different installations can determine whether or not cavitation occurs.

Don't use multiple types of sealants on a thread. The sealant is engineered and tested solo. Any combinations introduces uncertainty. I do like 577 a bit more for SS on SS pipe. It hardens eventually so you don't get covered with it every time you tour your plant. I feel it resists gallon better as well. I don't recall if it's NSF for drinking water.

Standard practice for pipe thread is to apply the sealant a few threads from the end. Those end threads without sealant will form sites for crevice corrosion. With SS pipe start the sealant with the very first thread.

 

[EdStainless]

If I went all SS I would replace the valves also. Make the whole string SS.
I hate the question about applying thread sealant.
You must with pipe threads because the design includes a leak path.
And the risk of crevice corrosion in SS threads is real.
But over the years I have had to rebuild so many needle valves, pressure regulators, and flow meters that had small fragments of sealant or tape in them that I would rather see the first thread left bare. The first thread has a big enough gap that it is rarely where corrosion starts (tight crevices are worse).


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Ed, Loctite 567 and 577 are both non-fouling sealants. As anaerobic sealants they remain liquid in the fluid stream.

However, they do leave barnacles in the threads after disassembly which can be a problem during reassembly without cleaning.

 

[quizzical1]

Thanks guys for all your help - I really appreciate the feedback!

TBE - static pressure is std municipal (~50 - 65 psig) and flow is low from a recirc pump (~1 gpm)

 

So...we have crowdsourced the mechanism, root cause and provided recommendations based on a photo and a bit of description?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

And based on how frequent these types of failures are, yes.


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P.E. Metallurgy, consulting work welcomed

 

I never assume two failures are the same, even if I have seen 10 of them.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

And that's your job as a metallurgist. However, as operators, sometimes just have to make it work. We can use our knowledge and past experience to make the system correct without identifying the exact cause of failure.

 

That approach condemns you to repeat the past. And that approach works fine in some situations; e.g., refineries sticking with carbon steel HX tubes they know will corrode, but at a predictable rate. They just scrap and replace because it makes economic sense. However, it seems to me that an overhead hot water circuit prone to failure presents a significant safety risk.

My job as failure analyst is to recommend solutions to prevent or at least mitigate failures. How owners act on these suggestions is beyond my scope, which is only one input into their decision process.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

We know the correct materials to be used. They were not used here. Replacing with the correct materials will solve the issue. There is no need for detailed analysis.

 

But the owner does not know that, hence the OP.
My ability to make a living hinges on the gap between the client's knowledge and mine.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[quizzical1]

Thanks guys...

The assembly was done this way because some thought that by using brass / bronze DZR valves / fittings that there would be no corrosion issues with the 303SS.

 

[TugboatEng]

The 303 SS is going to have corrosion issues no matter how you assemble it.