Molybdenum pressure vessel 2

[tpt27]

Hi,
I am designing a pressure vessel out of pure moly. It should hold 1 atm of nitrogen and will be heated to 1500C. The max. diameter is 600mm and it will be about 1m long. I am concerned about sealing the vessel (i.e. what metal to use as a seal) and constructing valves ( since welding moly. is very difficult). Does anyone have any experience with these conditions or working with moly.? Thanks

 

[metengr]

How are you designing this vessel with no reference to ASME B&PV Code, Section VIII or other Pressure Vessel Code? I would hope you are aware of this and research proper materials for pressure vessel design and fabrication.

 

[MJCronin]

A pressure vessel from pure Molybdenum ? Why ? I would be surprised if anyone has ever done this before..

What property of this metal appeals to you ?

Explain why you cannot use a high grade of stainless steel in this application

There is an expensive alloy called TZM that is ~99% Molybdenum,... used in high-temperature molten salt reactors

Moly, of course, is an important alloying element for may grades of stainless steel.

MJCronin
Sr. Process Engineer

 

[tpt27]

@metengr I am not trying to comply with the ASME code
@MJCronin I don't believe anyone has done this before. I need to use moly. because of the high temperatures and nitrogen atmosphere, stainless steel will not work for these temperatures.

 

[MJCronin]

At 1500C your choices are certainly limited.... In many metallurgical reference texts, Molly is not tested to 1500C

Regarding Molybdenum, like many metals, the pure form is not always your best choice where strength at temperature is your desired property.

See the following:

http://www.lr.tudelft.nl/organisati...-structural-materials-used-for-rocket-motors/

http://www.fusion.ucla.edu/apex/meeting4/2ghoniem0798.pdf

Consider rocket motor INCONEL 718 as an alternative.

Consider the alloy TZM at 1500C, It seems that it was developed for high temperature service..

Respect us..... tell of your final decision and the reasons for your choice





MJCronin
Sr. Process Engineer

 

[metengr]

tpt27;
I would hope your pressure vessel is exempt from local regulations within the US where pressure vessels are regulated. If it is not exempt, you will be in violation of the law unless you follow ASME B&PV code rules.

Do you have Boiler and Machinery insurance? You may want to consider this when you build this pressure vessel. Do you know and understand pressure vessel theory?

 

[tpt27]

@metengr This is for a research project. I see if I have to follow any regulations, thanks for letting me know. Yes, I understand the theory.
@MJCronin It's between TZM and moly. The other materials just aren't as strong at high temperatures and I'm afraid of this thing blowing up like a balloon. If the design works it won't be built for at least another year but I'll let you know what happens with it. Thanks for you help.

 

[XL83NL]

Is the 1 atm of nitrogen gauge pressure or absolute? Can you consider other high temperature materials? Have you consider ceramics?

 

[tpt27]

@ XL83NL The 1 atm is absolute pressure. I have considered ceramics but they become too brittle and would need some kind of metal lining, which brings me back to moly.

 

[jte]

Well, if the design pressure is truly 1 atm absolute, then at least the shell thickness calculation will be fairly straightforward.

Yes, I understand the theory.

I truly hope you understand mechanical design theory better than I understand Chemistry.

 

[btrueblood]

It can be done. I once helped build rocket engines that operated at that temperature and about 40-80 psi. Much smaller o.d. though. And, we used a Mo/41Re alloy. Electron beam welding in vacuo is ideal for moly.

there are a couple of books you should look at, that will have a wealth of high temperature operating strength data for refractory alloys:

https://books.google.com/books?id=y...ved=0CCoQ6AEwAGoVChMI8_vrhsyQyAIVy6OICh3nEQe8

http://link.springer.com/chapter/10.1007/978-1-4684-9120-3_2

 

[EdStainless]

Build it as a sphere to minimize stresses.
Use TZM or one of the other Mo alloys.
Have the half spheres spun formed, when weld them together in a vacuum chamber.
You may need to use ceramic for the valve, not too many options.
The ceramic could be metallized and them diffusion bonded to the Mo.
A lot of R&D work to build this.

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P.E. Metallurgy, Plymouth Tube

 

[tpt27]

@btrueblood Thanks for the sources, they'll come in handy.
@EdStainless I need to fit another cylindrical chamber inside this pressure vessel so a sphere would not work.I'll look into ceramic valves, thank you!

 

[jte]

EdStainless-

Could you help me to understand how the state of stress in a hemispherical vessel is less than that of a cylindrical shell when designed for a pressure of 0 psig?

 

[moltenmetal]

This is not a pressure vessel- at 0 psig it us basically a furnace liner.

Not too many ceramics are brittle at 1500 C- and after first heat up, your moly will be brittle below 300 C too. So what does using metal really gain you?

 

[tpt27]

@moltenmetal I have been told that ceramics would be even harder to machine than moly. that is why I do not want to use ceramic for the body of the vessel.

 

[moltenmetal]

Ceramics can be machined, but they can also be formed by processes less brutal than machining.

As long as welding is not required, a FeCrAl alloy (Kanthal etc.) might be worth considering.

 

[EdStainless]

Just pure Nitrogen?
Never any positive pressure?
Why not graphite? they make vacuum furnace liners and elements out of it.

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P.E. Metallurgy, Plymouth Tube