×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Austenitic alloy steels, stress corrosion cracking

Austenitic alloy steels, stress corrosion cracking

Austenitic alloy steels, stress corrosion cracking

(OP)
I work in a coal fired power plant, we protect the boiler tubes during shut-down (including the superheaters) using the wet method with water and hydrazine. During start-ups we drain the boiler to is normal level, but the secondary and the final superheaters can't be drained, so we evaporate the water through the main steam vent prior to turning turbine with steam. Our plant is on service only during summer and during surges in energy demand, but is normally not operating because of our country energy market is attended by hydroelectric plants.

The final superheater has a section in SA213-TP304H austenitic steel. According to ASME B&PV PG-5.5, the use of this kind of material is prohibited in pressure parts that are water wetted in normal service to prevent the intergranular corrosion and stress corrosion cracking.

My question is:
Is it right to fill this tubes with water during shut down according to PG-5.5, considering that we don't have the water inside the tubes during normal service so the tubes never have water and pressure at the same time?
 

Javier Guevara E.
Projects, Mechanical Engineer
TERMOGUAJIRA - GECELCA S.A. E.S.P.

RE: Austenitic alloy steels, stress corrosion cracking

Javier'
We have the same situation in our Power boilers and there is no issue for SCC under typical lay-up conditions (wet or using nitrogen blanket, as in our case).

First off, the steam collapses into condensate during shut down and is relatively pure. Second, the temperature decays down to ambient temperature and with no tensile stress present other than residual stresses, you will not have SCC. Yes, you will have sensitization in the 304H but if the boiler is tight and with hydrazine as an oxygen scavenger there is little, if any, risk of SCC (IG or TG).

Where you would possibly get into trouble with austenitic stainless steel superheater tubing is at attachments welded from the OD surface of the tube. If saturated oxygen conditions ocurr in the condensate plus residual stresses from welding the external attachment, these combined conditions could promote SCC in the tube from the water-wetted surface.

Paragraph 5.5 in Section I has more to do with water-wetted surfaces in boiler operation where now you have pressure stress + feedwater with oxygen and other contaminants and operating temperature that could result in SCC.

RE: Austenitic alloy steels, stress corrosion cracking

The time of risk is when you dry out the tubes.  If they actually get dry you can concentrate impurities and that can lead to localized corrosion issues.  Normally the drying with steam happens gradually enough that is not a problem.  This is a fairly common practice.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources