Inconel 625 approved for superheater but not boiler?

[Stephen Max]

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

 

[MJCronin]

Anybody else getting a bad feeling ?????

MJCronin
Sr. Process Engineer

 

[racookpe1978]

Yes.

 

[EdStainless]

I have seen these heater blocks, but find it hard to believe that they are rated for these temps and pressures.

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P.E. Metallurgy, Plymouth Tube

 

[Stephen Max]

Thank you for your feedback. Apparently this sounds like a sketchy idea to you guys.

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.

 

[racookpe1978]

Look at each "little" part of this high-pressure, high-temperature system in detail. No superheater design I've seen uses embedded electric heater cores to "stage up" steam from saturated to final superheat. Almost all are radiant heaters for the superheat in banks suspended before the boiler tube walls.

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.

 

[EdStainless]

Be careful, these are not fired boilers so they fall under a different set of rules.
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

 

[SnTMan]

Sec IV, Heating Boilers may apply.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

This is how many home heating boiler units and industrial demand water heaters are built, though no to these temps and pressures.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

The concern with Inconel 625 in elevated temperature boiler water service is stress corrosion cracking, which is why Section I will not approve it's use in boiler water applications.

 

[racookpe1978]

If he has a limited exposure, limited use, limited cycle test assembly in limited access lab, will the high cycle fatigue limits apply? It could still break, could still crack out under loads, but it would not be a commercial threat to outsiders.

 

[GenB]

The amount of steam you want to produce makes a difference in designing a boiler of any type, fuel or electric powered.

General Blr. CA,USA

 

[Stephen Max]

First of all thanks to all of you for your replies, I appreciate every one.

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

 

[davefitz]

as per Meteng, the section I ( power boilers) provision is to prevent chloride stress corrosion cracking in the evaporator. many power boilers are fed from a condensate system that originates in the surface condenser hotwell, and a single leaking condenser tube can cause chloride contaminated condensate to enter the boiler, requiring replacement of all evaporator tubes as be the end result , implying a 1 yr outage and megabucks to fix. Generally a bad design decision . Another source of chloride contamination is the acid regeneration stage of the water treatment system.

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

 

[chicopee]

Per racookpe1978 "... but it would not be a commercial threat to outsiders." true but have you considered the threat to employees at which time OSHA, if within the USA, may invoke stiff fines if the electric boiler is not registered with National Board.

 

[Stephen Max]

The lab will be unmanned during operation, so no safety hazard. But I think I will stick to the approved list for the proof-of-concept prototype and deal with the increased iron content problem later.