Equivalent to ASME Section 1 for austenitic stainless steel in wetted service

[deejanb]

Hi everyone,

We're working with a Client who's designing a linear Fresnel concentrating solar power plant - a boost plant for a coal fired power plant in Asia/Pacific.

We are looking into replacing ASME A213 T22 carbon steel boiler tubes with A213 T304H austenitic stainless steel; PG-5.5 of the ASME Boiler and Pressure Vessel Code states that the use of austenitic alloys for boiler pressure parts that are water wetted in normal service is prohibited. As the saturated section of the economiser tube will most likely have the water contacting the bottom of the tube, an alternative standard is needed to allow the use of T304H austenitic stainless steel tube. The entire steam line is unfired as the steam is being generated by the use of solar energy, and later plugged into the coal plant.

Would anyone have any insight on alternative standards (DIN in particular) for ASME / ASTM T304H boiler tubes that would allow such application?

Kind regards,

DB

 

[davefitz]

stainless tubed economizers are usually supplied as asme sect VIII div I components. The problem that asme sect I is trying to avoid is chloride stress corrosion cracking of austenitic stainless steels.

3 ways chlorides get into the wetted tubes:
a) surface condenser tube leak
b) demin system mal-operation during flush + rinse cycle lts hydrochloric acid into feedwater
c) routine acid clean of boiler tubes to remove internal deposits

So the problem is theoretically solved by using an ACC air-cooled condenser( no possible inleakage of salts) , use a nicorfilter + RO + EDI for demineralization, and use correct modern all volatile water chemistry, plus use a duplex ( non austenitic) stainless steel. And supply the water wetted parts as section VIII div I.

"Nobody expects the Spanish Inquisition!"

 

[deejanb]

Hi davefitz,

Thank you for your most comprehensive and helpful reply. We were weary of using stainless steel economizer tubes, but due to Client's request, we had to check all possibilities.

Kind regards,

Dejan

 

[moltenmetal]

If you're using the tubes to generate steam for power generation, I have no idea how you're going to get around the requirements of Section 1. The device walks and talks like a power boiler. The prohibitions against austenitic stainless steels are there for good reasons as noted above.

Why is the T22 being replaced, and how is it hoped that 304H will solve that problem?

Could you get away with a duplex? Maybe, but NOT for the hot parts. Duplexes are not high temperature materials, unlike T22 Cr-Mo or 304H stainless steel. Duplexes have upper service limit temperatures in the 600-700 F range.

Section VIII might apply if your purpose wasn't power generation, i.e. if you were using the steam as a feedstock to a chemical process, or boiling something other than water.

 

[metengr]

There is an ASME Code case to allow installation of Section VIII, Div 1 components in Section I. The typical application is an HRSG. However, as a metallurgist I would not recommend it unless treatment is such that chlorides can be avoided AND material has been thermally treated to remove cold working from fabrication.

 

[moltenmetal]

A heat recovery steam generator is definitely arguable, as its intention is primarily to be a process cooler rather than a boiler per se.

Unless the water is essentially closed loop, I would have little confidence in the practicality of eliminating chlorides entirely. We've used austenitic tubes in a HRSG-like service in past with success, though it was done under conditions where the safety risk of a brittle tube failure was mitigated by design, and the water was essentially closed loop so that neither chloride nor oxygen were being continuously fed in any appreciable quantity in the form of boiler feed water. As to eliminating the stress arising from cold work, that's a good idea, as is pickling/passivating etc. But the OP talked about 304H, which is particularly poorly resistant to chloride SCC. Best to keep that entirely out of contact with liquid water by design if at all possible.

 

[EdStainless]

Even if SCC weren't an issue there are other things that make using an austenitic stainless an issues:
huge thermal expansion
low thermal conductivity

If the issue is the surface corrosion and condition I would suggest that you look into laser clad T22. They can coat it with anything from 304 to C22, though a high alloy stainless (317LMN) might be the best balance.

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P.E. Metallurgy, Plymouth Tube