Very High Temperature materials

[MattT1991]

Hi everyone,

I have been trying to find some materials that could be used for very high temperature piping but have been struggling quite a lot. I have been looking at stainless steel alloys mostly.

The specs are really tough:

1200 celcius of helium at 80MPa. My main concern here is finding an allow suitable for sitting under stress at 1200C for a long time.

Any help would be much appreciated,

Thanks,

Matt

 

[Q_Goest]

The problem of course is that as you increase the pipe temperature, the allowable stress drops. There are no pipe materials rated for a temperature above 1650 F in ASME B31.3. Worse yet, at 80 MPa, the pressure is well above typical values for any standard size pipe even at ambient temperature. I can't imagine trying to find a material that can withstand that kind of temperature with any reasonable strength.

From my limited experience in high temperature processes, what is typically done is to line a pipe with refractory material and possibly even cool the outside to maintain a reasonable stress allowable. Monitoring the temperature of the structural portion of the pipe to ensure it doesn't get too hot might also be necessary.

 

[AlephZero]

I can't imagine trying to find a material that can withstand that kind of temperature with any reasonable strength.

You would be pushing you luck at 1200C even with the high temperature superalloys used in jet engines, though it you dropped the temperature to 1000C you would be getting into the plausible region, provided you have a LOT of money to spend.

Note, the cooling system in a jet engine may allow the turbine gas temperature to be higher than the melting point of the metal components, with a film of cooler gas as a barrier between the two - but I don't see how you could play that sort of game with a pipe system.

 

[MattT1991]

Thanks for the reply. I did think aboout this sort of method.

Could you possibly point me towards the sort of materials used for the inner layer and how they are attached to work as a thermal buffer but not to take any stresses caused by the high pressure.

The conditions are extreme however they use similar conditions of around 80MPa and 1000C in Fast Reactors using a helium coolant so there must be some way of doing it. The small nature of Helium adds another problem to the situation too.

Cheers,

Matt

 

[MattT1991]

Also I saw 2111HTR steel rated at 1150 Celcius for continuous use. If anyone has any thoughts about that I would appreciate hearing them.

Matt

 

[Q_Goest]

Also I saw 2111HTR steel rated at 1150 Celcius for continuous use. If anyone has any thoughts about that I would appreciate hearing them.

Matt

But at what stress level? It's probably only good for a few thousand psi (stress allowable) at the most.

I'd suggest doing a search on refractory or ceramic lined pipe and talk directly with a supplier. I'm sure they would be glad to send out an engineer and sales representative to help you through the details if this is a real project.

 

[MattT1991]

True. I will certainly have to look into ceramic lined pipes. Unfortunately the project is theoretical at the moment so I can not arrange to have an engineer specialising in this area come out.

How is the ceramic attached to the inner of the pipe? This is the point where the stresses are highest and as temperature fluctuations occur and with vibrations from the flowing helium (at ~ 100m/s so there are flow issues to watch for) surely the difference in their expansion rates will cause cracking in the ceramic?

Thanks,

Matt

 

[Pkruse]

I’ve seen similar systems for air, not Helium. You will need to insulate the inside. Depending on how you work the design, you may need to cool the outside.

What is the flow rate through this system? Can you afford to let the insulation shed small particles? If not, then you will need to provide a shielding over the insulation, using one of the refractory metals commonly used in jet engines.

What is the application?

 

[MattT1991]

Great. The helium presents more of a problem at joints and valves really.

The mass flow rate will be around 500 kg/s in some areas and as low as 12.2kg/s in others. The density varying between 4 and 7 kg/m^3.

The worst case is 500kg/s, 1200C, 80Mpa, 7kg/m^3 and 100m/s flow.

I can't really afford for the insulation to shed small particles although it would depend on how small. The helium has to go through helium turbines and compressors. The stream to the turbines is relatively very small so filtration may be possible. A pressure drop here is desirable anyway.

The application is for use in a modified, helium cooled, fluidised pebble bed reactor which is being used to provide heat energy mostly for a high temperature chemical reaction on a huge scale. The temperature is increased from 1000C to 1200C. Its not been the easiest of choices for a project.

Thanks for the help,

Matt

 

[Pkruse]

Your design is very similar to many test rigs for gas turbines. I recommend that you contract it out to one of the companies that specializes in designing and building them. It requires a very specialized knowledge base and skill mix that would be much cheaper to contract out rather than to develop yourself. (Disclaimer: I work for one of them, which is why I’m not volunteering details.)

But since you so quickly volunteered the details of the application, I suspect that this is an academic project rather than one that requires real dollars. I wish you well on your learning expedition. But you are right. The sealing of the helium will be one of your bigger challenges for the piping system.

 

[AlephZero]

How is the ceramic attached to the inner of the pipe? This is the point where the stresses are highest and as temperature fluctuations occur and with vibrations from the flowing helium (at ~ 100m/s so there are flow issues to watch for) surely the difference in their expansion rates will cause cracking in the ceramic?

Silicon nitride is pretty widely used and is in the right ballpark for your temperatures. You should be able to get some information on the manufacturing processes for coating metal parts, etc. Compared with a "typical" ceramic (if there is such a thing!), it's fairly tough stuff.