304H oxide formation

[JEB66]

Recently we had several Superheater overheat failurs on a new bank of tubes. The platens are made from SA213 TP304H. They are less than 2 years old.

The failure is the result of severe exfoliation of the magnetite scale that plugged the tight radius bends and ID transitions. This stuff is very light and fragil.

I understand that this is not a new problem for the rest of the world. And with time a spinel layer will form and we will not have this problem any more.

My question is: Is there some way to prevent the magnetite formation? would passiviation have helped?

 

[metengr]

Yes. Are you force cooling the boiler?

 

[JEB66]

Normal routine for an outage where entry into the boiler is needed is to run the fans for cool down.

This particular incident we had an upset after a very long run. The upset spiked the main steam temp down which we feel is the event which dislodged all of this at once. The thermal expansion difference.

What I would like to know is what processes we could explore to prevent this r=from reoccuring on the next set of elements we make out of 304H.

 

[metengr]

JEB66
We have replaced several RH and SH tube assemblies in our Power Boilers with 304H and never had an incident with serious oxide exfoliation that resulted in stress rupture failure downstream of tube bends. We are going on 10 years of operation with no problems. So, I don't see an issue related to how the tube material was supplied as long as it is supplied in accordance with ASME SA 213.

 

[JEB66]

The problem goes back many many years. It is only going to happen on new tubes. When stainless is new it forms magnetite and hemitite oxides until the spinel layer takes over.

If you have a new section and the boiler runs for a long period after the installation and as a trip. The early layer of oxide comes off all at once. But if you have several small shedding events, you will not have any problem. This stuff is very fine and light. It will not get caught on screens.

But if it all comes off at once, oh boy.

ASME has nothing to do with this.

 

[metengr]

I understand it, and it has nothing to do with new stainless tubes. Our recent installation was 2 years ago on a 540 Mw unit. The formation of Cr₂0₃ occurs immediately during tube fabrication. Conditioning has nothing to do with this on new versus "old" tubes. My point is going above and beyond by pickling and passivating will do nothing to guard against this phenomenon. Having sharp bends is a design issue, having rapid cool downs or abnormal events is an operational issue.

 

[metengr]

One other item to check operationally is how fast the operators are firing the boiler during start-up. In the past, our operating personnel had a habit of over-firing to compensate for condensate entrapped in SH tube circuits. As you can imagine, the SH tube circuit can become exposed to excessive metal temperatures (gas side temperatures + radiant heat affects) until the trapped condensate turns to steam to allow for flow.