Hot Water Stainless Steel Storage Tank Failure 1

[madeincanada]

I have been trying to get ansewrs to discover why I have had trouble with Indirect fired hot water heaters failing.

The tanks and heat exchangers are made from SA 240-316 Ti stainless steel.

I have read that pitting may be a possibility, galvanic corrosion, stress corrosion cracking.

Could someone please offer some suggestions as to what I should be looking for..

 

[TBP]

Are they being installed with relief valves on the cold water side? If they aren't (and if you're relying on the field installation guys to select and install the correct relief valve, the vast majority are not), then that, combined with the increased use of backflow preventors could be the cause of at least some failures. After the last use of hot water for several hours (typically overnight), the last shot of cold water enters the tank, and it does what it's supposed to - heat the water. All of the taps in the house are shut. The heated water used to be able to expand back into the municipal water system, but now, with backflow devices, it can't. You can develop some pretty amazing hydraulic pressures in this scenario. I've seen a number of industrial heat exchangers fail for this very combination of reasons.

 

[chicopee]

you have not indicated the type of failure encountered w/ the hot water heaters. How often are these tanks failing and what is their application, fuel, size and type? You may have to strip the insulation off and determine whereabout failure takes place.

 

[arto]

Insulation on Stainless Tanks should be low in "leachable chlorides" [i.e., per ASTM C795]. Otherwise you can get corrosion problems [from the outside], even though the vessel contents are non-corrosive.

 

[gvanbeek]

In addition to chlorides in the insulation, you must also look at the environment that the vessel is installed in. I have a case history where low Cl insulation was installed on a vessel and it experienced SCC due to its close proximity (<10 miles) from the gulf coast.

Is there a source of chlorides near the vessel (e.g. water softner)?

 

[mcguire]

Stress corrosion cracking will occur at the temperature you cite. 316Ti will pit at these temps if chlorides are present. Pitting will be followed by SCC if stresses are sufficient, say greater than 25% of yield strength. This is one reson such tanks are better made from grade 444 or 2205.

 

[Heula]

Pl. tell us where exactly the failure occured and at what temperature the tank is being used.
Then we can suggest you some solution.

 

[dooron]

We use stainless 316 hot water tanks with copper tubing with a 10 year warranty. There is a relief valve set at approx. 760 KPa (110psi). The water used is very soft, almost pure rain water. I believe that with hard water (containing sediments and chlorides) there is SCC, intergranular corrosion and pitting attack on the Chrome part of the Stainless. This should be visually apparent to the naked eye and occur in areas of water/stainless contact. There may also be sulfur/acid moisture/condensation but I would have thought that 316 is resistant to these. Galvanic corrosion or earth leakage corrosion, I don't know.

 

[unclesyd]

Restating Heula's post.
What is the physical location of the failures, top, bottom, sides, or penetrations of the tank?
This type information is very important in determining what to look for that may be causing your failures.

I seen S/S tanks fail from every reason in the book and sometimes a combination of several.

 

[GenB]

What type of indirect hot wate htr. is that,can you please explain?
Is the fire side also ss or just the heat exckanger?
what's the temp. of the heated water?
what's the temp of the incoming heated water?
or is it a steam source?
ER

 

[smckennz]

Stress corrosion cracking due to the presence of chlorides in the ppm range. Bet you are at 80 deg C or more. We had this problem in NZ big time about 25 years ago; swimming pool heaters and meat works hot water tanks ($$$$). Simple answer is to change to duplex stainless. There have been millions of words written on the subject; not aware of any way of getting 316(L) to be immune to the problem.

Cheers

Steve