Main steam pipe material in high-temp coal power plants 1

[TMBeau]

I have been doing some research on coal-fired super-critical and ultra-super-critical power plants (steam temperature and pressure >565C and 250Bar). It seems that for the non USA market, the materials of choice for main steam pipes/headers are A182 Gr P91 and P92 but in the USA the materials of choice seem to be A182 Gr 304 and 347.

1. Is this true?
2. If yes, why?
3. Are there other pipe materials commonly used for >565C, <620C steam temperatures?

Thanks!

 

[stanweld]

Our super critical plants under construction or in bid phase in the USA use P91 mainsteam piping. Ultra-super critical boilers have not been made in the USA in decades.

 

[steelmtllrgst]

P92 is a common grade overseas for the steam pipes but never really caught on in the US, this is true. The reason is not 100% clear to me but likely has to do with the higher cost of the material.

 

[stanweld]

I also know of at least one recent super critical unit in the USA where P92 was used. I think steelmtllrgst hit the nail on the head re P92; higher costs with only marginally higher strength at operating temp.

 

[davefitz]

I am aware of US supercriticacl plants built in the 1960's with stainless steel HP main steam lines ( eddystone, Huscon), but none built with SS main steam lines past that era.

But it could happen again - the US design code ASME sect I has zero references to fatigue, and B31.1 code could be read by an inexperienced person to allow approving the fatigue issues with thick walled stainless steel piping.

The relatively lower yield stress and lower thermal diffusivity of SS compared to ferritics ( P91, P92, P112)would immediately frighten the bejiggers out of a designer that uses the european codes that explicitly demand such a fatigue analysis of transient events, such as startup and shutdown.

If you know of any modern applciations of SS to HP main steam piping in new ultrasupercritcal units, kindly advise- it would then be informative if the designer is assuming only a handful of startups per year.

 

[TMBeau]

Thanks everyone!

For davefitz - I am not aware of any modern applications of main steam piping using austenitic (304, 347) stainless steel by site or plant name but I have been reading countless white papars that claim the US prefers austenitic over ferritic stainless steels (P91, P92) because the austenitics are easier to weld. I can understand why the US market wants an easy-to-weld material for main steam piping but why would the international market choose ferritic steels if they are harder to weld??? I saw one vague reference that on the fire side of boilers, austenitic steel has better corrosion resistance than ferritics. This would make austenitics a preference for boilers burning the high sulphur coal found in the US. I do not think the international market has to worry about high sulphur coal so much. Maybe the main steam pipes are made from the same material as the boiler water walls to prevent welding dis-similar metals in the field??? Since US boilers are presumably austenitic steel, so are their steam pipes? Just a guess.

 

[stanweld]

I was in error. There is/was one ultra-supercritical boiler being (or not) erected in the USA. It has just been halted by the Arkansas Supreme Court. I am unaware of what material was specified for the main steam pipe.

 

[davefitz]

In terms of fire side corrosion, the ultra supercritical steam temperatures cause the outside tube metal temperature to exceed the melting temperature for some eutectic salts formed by coal ash, and these molten salts will flux away the protective oxide layer of normal ferritic tube materials. As a result, special austenitic alloys, such as HR3C ( modified 310 SS), are used for the final superheater and final reheater tubes in the boiler.

For thick walled vessels and pipeline components, the designer needs to adjust his standard analyisis of pipline axial growth ( + earthquake + other loads) with special consideration of the transient Id vs OD temperature gradient that occurs during a fast startup or shutdown of the plant. The transient temperature gradient thru the wall of the thick walled pipe causes thermal stresses to be generated, and these concentrate to high values at drilled bore holes ( for vents, drains, instrumentation), at tees and wyes, and at weld interfaces between thick valves and thinner pipes. Validation of the design at these particular locations now should involve a 3D FEM model of these highest stressed components and a ftaigue analysis conducted using modern fatigue methods ,which now include fracture mechanics due to the recognition that a pre-existing crack always exists at every welded interface.

As indicated earlier, the relatively lower yield stress and lower themal diffusivity of SS would suggest that a SS pipeline would experience much shorter fatigue life than an equivalent ferritic pipeline. But then again, one could make the false assumption that the plant only starts up once and runs continuously for 50 yrs, and blame fatigue cracking on operator error/ feedwater quality/ incorrect zip code.

European mfrs had for years needed to deal with a very difficult ferritic alloy, (X20 ?), and had evolved a very disciplined approach to welding this nasty alloy. When P91 become available in the 1980's, it was relatively simpler to weld than ( X20), and the installed techical capabilities of the qualified EU mfr's was easily able to cope with P91. The US, on the other hand, had evolved from the opposite extreme, being used to facile procedures associated with alloy P22. The QC procedures and technical capabilities installed at most mon & pop foundries was not ( and maybe is not) adequate to reliably deal with the QC requirements of P91.

 

[Welder4956]

We have about a dozen supercriticals in the US and all use P22 for main steam. Design temperature is lower though at 538 C. If we were building a supercritical with 565 C temps, we would likely use P91.

 

[MJCronin]

davefitz rocks !!! Thank You again for your background and insight....

Truly he has not gotten the stars he deserves.

There has been much information placed on the internet on the fascinating subject of high-temperature alloys.

The tip of the sword in this field seems to be held by the Japanese (Hitachi) in thier quest for alloys that can meet the USC 600C+ MS/HRH needs.

Turbine casing issues are being pursued by GE

Fun reading..

http://www.netl.doe.gov/publications/factsheets/project/Proj463.pdf

http://www.ms.ornl.gov/FEM19/Proceedings/papers/Session II/maziasz.pdf

http://www.asiapacificpartnership.o...Gen Strategy Ultra-Supercritical Technlgy.pdf

Remarks ?

 

[MJCronin]

More fun:

http://www.eng-tips.com/viewthread.cfm?qid=172967

Especially, I like rumor of the Chinese ban on the use of any alloy piping in a Chinese power plant.

Can it get any better....?