Fatigue and creep resistant material for steam generator 1

[Peregrino7]

I am working in the design of a steam generator (like a Kettle or similar heat exchanger type) for a thermal solar power plant. Working temperatures will be around 390ºC @ 120 bar.
The problem we are trying to solve regarding material selection is the fact that this equipment will have start-up and shut-down every day. In the morning, the equipment is cold, and it will receive a hot fluid almost instantly. And this will occur every day for 30 years. And also the operating conditions will vary every day according to weather conditions, etc.
If we only consider Pressure, Temperatures and fluid types, Carbon Steel would be OK... but we have the concern regarding creep and fatigue resistance.
Aside the mechanical design of the equipment in order to be able to handle the thermal expansion, we need to select a good material (but not expensive) for a life of 30 years and avoiding creep and fatigue failure (which may happen in the mid/long term)

Any suggestions regarding what would be the optimum material specification for heat transfer tubes, tubesheets (forgings) and for shells (plates) are much appreciated.

Kind regards,



PGh.

 

[rustbuster]

Sounds like an un-realistic operating strategy for any material and or process. Could you not maintain the equipment in hot standby over night? This would give you economic options for materials.

Good luck, keep us posted....

 

[davefitz]

390 C is low enough for carbon steel and low alloys, and creep is not a major issue for ferritic alloys at this temperature. Carbon steel has a high thermal diffusivity and high yield stress, these parameters are key for minimizing thermal stresses during thermal shock events. In other words, avoid austentitic stainless steel at this temperature range and for thermal cycling service. Thin thermal sleeves, if used , should be made of light alloy or of incolnel.

The design, monitoring, inspection of the unit should be focused on minimizing thermal fatigue damage. Some key items may be :
a)become educated in modern fatigue theory. It's not your grandfather's theory anymore- some major advances have been accpeted and incorporated in the EU design codes in the last decade.
b) become familiar with the pressure part design Code paragraphs on fatigue damage , espescially EN 12952-3:2001 or the old german boiler code TRD 301 annex 1.
c) The process may need to be dynamically simulated , and the detail design analyzed by 3D finite element methods, to accurately estimae the response to thermal transients in the design phase.
d) Design should be reviewed by an experienced designer of similar components in similar service, such as a large boiler mfr. There is something to be said for experience, and some of it is positive.
e) employ design details aimed at mimimizing thermal stress, such as : use thin wall pressure parts and avoid thick walled vessels at all costs, welds to be full pen welds with radiused edges and avoid partial pen welds, thickness transitions to be gradual and not sudden, use thermal sleeves at junctions of pressure parts that may operate at disparate temperatures, headers that collect or ditribute fluid to / from lutiple tubes to be symetrically configured such that a sudden temperature change will not cause bowing or "bannana" deflection of the header, provide generous tube flexibility to minimize reactions at nozzles during upsets, and the pipiong supports to allow for sudden overheats well beyond max expected without bottoming out to solid supports or solid bumpers.

 

[davefitz]

By the way, the operating mode you descirbed is no longer considered unusual in any way , shape, or form. Large heat recovery steam generators ( HRSG's) in combined cycle mode have been expeosed to the same daily thermal shocks for over a decade now. It hasn't all been successful vis a vis fatigue, but there is enough experience that you can find a few desingeers out ther that are no longer frightened or surprised by sudden daily startups. ( a large gas turbine can startup to full load in about 20 minutes, and the downstream tubes can headers can be thermally shocked, in the worst case, from ambient to 564 C in about 30 minutes).

 

[EdStainless]

Some of the alloys steels have shown to be good options for this type of application. In cycling I see T-11 and T-22 used commonly. I have seen ferritic stainless used also (430). Proper design (to minimize thermal stresses) and fabrication (heat treatment included) is needed to get good results.

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