Copper/Nickel Tube Corrosion in Cooling Water

[charlesflau]

We just have a heat exchanger tube failure. The equipment is used to cool product hydrogen gas at the outlet of a compressor. The H2 is in the tube side and the cooling water in the shell side.

The tubes are made of Cu/Ni (90/10) and the tube sheet & shell are made of carbon steel. We are aware of active corrosion in the carbon sheet components because of galvanic action and accepted it. We were surprised today when we discovered 2 small pinholes on 1 tube. Other than these 2 pinholes on the same tube, the other tubes look good.

Has anyone experienced similar failures? Possible cause?

Your help is appreciated!

 

[metengr]

charlesflau;
Have you established visually if the pin-holes were caused by external or internal corrosion pits or local wastage in the tube? This is most important.

Is the shell side cooling water treated? How long have the tubes been in service? When the cooler is out of service, is it drained?

 

[charlesflau]

We are in the progress of cutting the failed tube out and have it analyzed. As far as I can see the corrosion was external (on the water side). The area right next to the holes are pitted and clean. The reason it is clean, I think, is once the hole is made the higher pressure H2 from the tube would start jetting to the water side.

The cooling water is treated under a GE/Betz program without any problems. The cooler was fairly new installed in year 2004. When we shut the compressor down, we always keep cooling water circulating through the cooler.

 

[metengr]

charlesflau;
I would say you are headed in the right direction with having the failed tube analyzed. The cause of local pitting could be several factors including contamination from original start-up or during fabrication of the cooler. Once you review the results of the analysis, you should be able to decide on further nondestructive testing of the tubes to decide if this is an isolated case or the start of something new. Sometimes, even during start-up or during fabrication things can happen resulting in damage that creates problems later in service.

 

[EdStainless]

You should also consider Eddy Current testing the rest of the tubes. After all you want to find corrosion before you get a failure. Those tubes would be valuble for your analysis.

Considering the situation I would suspect some surface damage or contamination.

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Rust never sleeps
Neither should your protection

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[charlesflau]

Update - we found the cause of the problem. It is mechanical.

Upon closer inspection, we found the tube above the holed tube was loose. The vibration actually caused the baffle plate to cut through the tube. When that happened, the high pressure H2 from the top tube would impinge on the tube below creating the hole (similar to steam cut).

We are happy to find the true cause as we know the Cu/Ni tube should stand up well in cooling water.

Thanks for all your comments!

 

[EdStainless]

Remeber that Cu alloys have low modulus and are prone to vibrate. You need to find a way to validate the baffel hole sizes.
In a unit this new, if the builder can't satisfy you that holes are the correct colse fit sizes, and that the unit meets reasonable vibration design limits, then you need to be looking at new bundle. You may end up seeing a lot of these failures if you don't.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[charlesflau]

Excellent comments, EdStainless! We will definitely follow up on it.

Thanks!

 

[rmw]

In the interim, in order to prevent failures until you can evaluate a new bundle, you may want to consider staking this bundle to prevent vibration damage.

rmw

 

[charlesflau]

We have installed bands and secured all tubes (as best as we could). It should help reduce vibration of the tubes.

Thanks!