Inconel 625 approved for superheater but not boiler?
Inconel 625 approved for superheater but not boiler?
(OP)
Hello all,
I am designing a small boiler/superheater system that will use heaters having inconel 625 tubing cast into an aluminum matrix that has an electric heater embedded in it. This is an off the shelf item. I will use three in series, the first to boil the water, the second to raise the steam temp to 300C, and the third to further raise the steam temp to 400C.
In looking at the list of approved materials in ASME Section 1, PG-9, Inconel 625 is approved for superheater tubing, but is not mentioned in the list approved for boiler tubing. Does that mean I can't use it in the first heater that will boil the water? Why would that be?
Thanks.
SM
I am designing a small boiler/superheater system that will use heaters having inconel 625 tubing cast into an aluminum matrix that has an electric heater embedded in it. This is an off the shelf item. I will use three in series, the first to boil the water, the second to raise the steam temp to 300C, and the third to further raise the steam temp to 400C.
In looking at the list of approved materials in ASME Section 1, PG-9, Inconel 625 is approved for superheater tubing, but is not mentioned in the list approved for boiler tubing. Does that mean I can't use it in the first heater that will boil the water? Why would that be?
Thanks.
SM





RE: Inconel 625 approved for superheater but not boiler?
MJCronin
Sr. Process Engineer
RE: Inconel 625 approved for superheater but not boiler?
RE: Inconel 625 approved for superheater but not boiler?
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P.E. Metallurgy, Plymouth Tube
RE: Inconel 625 approved for superheater but not boiler?
This boiler system will be used to provide superheated steam at 70 bar for a research project I am not at liberty to discuss.
The tubing in the system will have a pressure rating of over fourteen times the system pressure, using the allowable stress values in Section II, Part D for Inconel 625 tubing at 400C in the max allowable working pressure calculation in PG-27.2
We are working with the heater manufacturer to ensure safe operation at 400C. They already have a production unit that operates at 350C.
I have been on several section VIII div 1,2 and 3 pressure vessel projects, but this is my first exposure to boiler design and Section I rules.
I am puzzled why nickel-chromium-molybdenum-columbium tubing is allowed for superheaters in PG-9.2 but is not in the list of materials for boilers in PG-9.1 and PG-9.1.1.
Or am I misreading the rules and PG-9 does not in fact disallow the materials permitted for superheaters to be used in boilers?
Thank you for your attention.
RE: Inconel 625 approved for superheater but not boiler?
You're apparently embedding different metals (with different thermal expansion coefficients! and different heat transfer coefficients) in three series of steam-connected "boilers". What is your seal between tubes and connecting piping, between the cast aluminum blocks and the electric heater alloy and the steam passages? What happens when the Al pulls away from the cooler Inch tubes cast inside the Al matrix? The electric heater inside the Al casting will, by definition, always be much hotter than the water-steam mixture inside the Inc tubing. So you will always have the stress "pulling" the outside of the Inc tube walls away from the cold water inside the tubing. This increases the wall stress -> Either yielding the wall of the tube, or pulling the OD of the tube away from the Al matrix around the tube. If pulled away, you lose you heat transfer at that local spot because of the gap, the matrix gets hotter, has less strength, yields more at that local spot.
Rolled tube ends? Soldered? Brazed? Can't be welded!
How do you attach each steam tubing pipe to its mating steam tube pipe (if that joint is to be welded, and most Inc alloys can indeed be welded if you're careful and have the right WPS), if you are going with a Inconel 325 steam system? What is your stress at the Inc 325-to-aluminum block cast heater joint?
Show a dwg of the proposed system.
RE: Inconel 625 approved for superheater but not boiler?
Offhand I don't remember what this category is called, but there is a name, and rules for it.
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P.E. Metallurgy, Plymouth Tube
RE: Inconel 625 approved for superheater but not boiler?
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand
RE: Inconel 625 approved for superheater but not boiler?
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P.E. Metallurgy, Plymouth Tube
RE: Inconel 625 approved for superheater but not boiler?
RE: Inconel 625 approved for superheater but not boiler?
RE: Inconel 625 approved for superheater but not boiler?
General Blr. CA,USA
RE: Inconel 625 approved for superheater but not boiler?
I was beginning to suspect something like a stress corrosion problem. Thanks for pointing that out, metengr. This will not be a problem with the initial research system, but the final production model will have to last about 100,000 cycles.
The amount of steam generated per cycle is about 60 liters at 70 bar. Operation of the system will be in a lab with no exposure to personnel.
SM
RE: Inconel 625 approved for superheater but not boiler?
Today many HRSG's are designed per asme sect I yet many do not use a surface condenser ( air cooled condensers are used often) and use membrane technology for water treatment and dispense with the acid regeneration, so theoretically have little risk of chloride contamination. Some of those units may use stainless or inconel tubes low temp economizers, and those components are designed per asme section VIII and are not part of the boiler.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick
RE: Inconel 625 approved for superheater but not boiler?
RE: Inconel 625 approved for superheater but not boiler?