What is the best material for boiler tubes - Stainless or Titanium? 4

[Harold007]

I have a small diesel boiler, with steel coils which only last 12 months. If I change the material, which is best Stainless 316 or Titanium GR2? Or do any of you have any other recommendations? The tube size is 20mm, working pressure is 350Bar at 90 Deg Cel. Thea pplication is hot water for a high pressure water cleaner.
Thanks

 

[macmet]

I do not have a lot of experience with diesel boilers, but that seems like an extremely short service life for such a product. Maybe the problem isn't so much the material selection as it something else in your process.

Is your burner working properly? What is the failure method you're seeing?

I worked on a firetube boiler that had excessive corrosion b/c the hot gases were being cooled too much in the tubes and the HCl was condensing out of the gas. We recirculated some of the hot water back into the return to raise the temperature of the return water and it reduced the corrosion rate.

I don't think your boiler is firetube, I'm just trying to say that there are other, possibly better or more economical, approaches to solve these problems other than simply paying for upgrading the material.

 

[StewPad]

Yep, it seems like a water problem more then material issue.
But if you can't do anything with your water source,
you might want to go with duplex 2205 SS or 904SS

 

[vpl]

Harold007

Does your hot water contain chlorides? If so, stainless is not a good solution. You might consider copper or bronze tubes. Titanium just seems a bit much for this application.

Patricia Lougheed

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[Harold007]

Thanks guys for the above feedback. My application is rental fleet, so I have no control over how and what water they are using. 2knagar's idea of using 2205 duplex looks like a very good idea. Can any of you comment on wall thickness for 350 Bar?

 

[davefitz]

The failures need to be metallurgically analyzed to determine the root cause. If it is corrosion and the flow velocity is low, then it could be exacerbated by subcooled DNB .

You might try recirculating hot outlet fluid back to the inlet so as to ensure a minimum mass flow rate in excess of 800,000 lb/hr/ft2.

The low temps do not imply an upgrade beyond carbon steel. However, there is a canadian boiler vendor that uses incolnel for their once-thru steam generators , to allow "dry firing" operation. If your unit must also be suitable for "dry firing" or you cannot adequately control the flow + firing rate, then a similar upgrade to ilcolne may be considered.

 

[rmw]

This looks to me like a coil out of a 'store bought' commercially made steam generator or high pressure hot water heater.

If it blows up and kills someone, it is the responsibility of the designer and builder of the unit (and you as the rental agent to some minor extent - I hope you are insured).

But if you redesign it, choose a different tube metallurgy, change the design in any way, put it back together and then it harms someone, you are on the hook all the way.

The original Mfg'r will disclaim any responsibility and you (or your insurance company) will be exposed to the max. You just bought the farm.

I'd run from any idea of modifying this in any way and just figure the cost of maintenance or replacement in your rental rates.

Check with your insurer before proceeding.

Good luck.

rmw

 

[MJCronin]

All,

Be careful when using 2205 duplex stainless. There is an upper temperature limit of 600F

This limit imposed by the ASME code is to prevent long term embrittlement in usage.

http://www.alleghenytechnologies.com/pages/Markets/ATIOilgas/docs/oilgas-datasheets/ATI 2205.pdf

Look at 904 and possibly 317 LMN stainless.

Get rid of the chlorides !!!

-MJC

 

[Shmulik]

Harold,
The worst idea was of welding the coil tube loops between each other not allowing any flexibility for thermal expansion.
Stainless is not a good solution.
I would consider 254SMO. It is not sensitive to chlorides, and has a quite high allowable stress upper temp. limited to 750°F

 

[hacksaw]

if you are burning high sulfur diesel you'll have corrosion like crazy, get the tubes basted and then aluminized, be cautious in getting away from C.S. tubes as there are serious design issues that could create a bigger problem than you already have

 

[metengr]

There is not enough information to make any informed material recommendation. I could envision service conditions where oxygenated water is left in the coils and under exposure to air you have local internal pitting and a sprinkling can. Or you have inadequate flow (cooling) and failure from short term overheat or you have external corrosion from the fuel being burned.

Figure out what is happening.

 

[Harold007]

Thanks guys for all this input.
I cut a faulty section of a coil out to see there / how it failed. It failed on the bottom coil where the baseplate was welded onto the coil. The inside of the tube (at the bottom )was corroded quite badly, and it failed at the weakest spot, which was the weld. There are always water left in the coil, so i assume it will always be an issue with CS? What are your opinion on Shmulik's surgestion of 254SMO?

 

[Harold007]

Here is a photo of the faulty section

 

[metengr]

Switching to a more corrosion resistant material is not necessarily the answer. The coil needs to be flash drained to avoid stagnant water conditions or drained. So, either improve drainage or modify operating to flash drain after use. No water, no corrosion.

 

[moltenmetal]

Was the baseplate the same grade of stainless steel, or was it carbon steel? If the latter, there's your problem.

 

[Harold007]

Curretly all are made from CS

 

[moltenmetal]

OK, missed that. Sorry! Agree with meteng- if this thing is left part full of water but open to the atmosphere, you're going to get a nice oxygen concentration cell set up and the thing will be toast. The pocketed non-drainable design isn't helping matters.

 

[kalens99]

Thanks macmet. So you don't always have to give up and upgrade, troubleshooting usually works?

Kalen Smith
Engineer-a-Business

http://www.engineer-a-business.com

 

[chicopee]

Looking at the damage section, it appears that there was a weldment at the longitudinal crack in the top picture. Is that so?

 

[Harold007]

Yes, that is correct.