T91 Water wall

[owenmaterials]

Does anyone have experience with T91 material used in a water wetted application, i.e, waterwall in a boiler? If so , how was it?

Thanks in advance.

 

[metengr]

Why would you even consider this material for boiler water wall service???? Given the fact that this material requires extremely tight controls for welding (repairs in the field), having this in a boiler water wall is asking for trouble. Also, any boiler chem clean would result in hazardous waste because of the elevated chromium content.

 

[owenmaterials]

Metengr,
Special circumstances make us to consider the use of T91 in waterwall application. We may be able to find a way to heat treat it after welding, but don't know if there are unexpected problems down the road.

 

[metengr]

I would recommend T23 tubing, not T91.

 

[davefitz]

You specified water wetted application. There are some cases of this , but not as a 'waterwall". Superheaters downstream of spray water attemporators may experience wet steam at some loads , or if the mfr did not properly design spray manifold.

The superheater tubes normally must be loose tubes, not in a membrane wall configuration. If it is a very old boiler with radiant superheater upper furnace enclosure walls, the superheter walls are not fabricated with a continuous membrane fin weld but are stitch welded on approx 2 - 4 ft O/C.

The transition from P91 to T22 or F22 downstream of the spray station is subject to extreme variations in temperature as the spray water alternately wets then drys the surface, so this transition is subject to severe thermal stresses. This means that transition must be a smooth thickness transition- although the allowable stress variation between P91 and F22 is on the order of 2:1, you cannot allow a sudden 2:1 variation in wall thickness in this case unless you want the part to crack within 1 year of service- must be s smooth variati9on of thickness with a slope not greater than 0.33 : 1. We have seen cracking downsteram of attemporators on piping transitons from P91 to F22.