Austenitic alloy steels, stress corrosion cracking 1

[wayuu1981]

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.

 

[metengr]

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.

 

[EdStainless]

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.

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