Material Selection for Molten Salt Service 2

[Smith55]

Hello,

Looking for advice on piping material selection for high temperature molten salt service. Operating temperature is cyclic and up to 460 deg. C. Studying a few options - low alloy carbon steels (P11, P22) and stainless steels (304, 347). If anyone has any experience to share this would be appreciated.

Many thanks.

 

[MJCronin]

Solar Heat concentrating plant ???

MJCronin
Sr. Process Engineer

 

[EdStainless]

First I will presume that these are all nitrate salts with absolutely no Cl or Fl in them.
The steels do well, until there are any impurities (which happens eventually) and then they fail quickly.
The austenitic SS does OK unless there is a lot of temperature cycling where thermal fatigue becomes an issue.
I have seen ferritic SS used successfully in this service. Alloys such as 430, 429, 439, and 444 have been tested in various molten salt services with very good performance.

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P.E. Metallurgy, consulting work welcomed

 

[Smith55]

Thank you for your replies.

MJCronin - The molten salt is for a heat storage application.

EdStainless - Thank you. For Cl, chlorides are specified as <0.1%wt. Fl? In the application the thermal conditions are highly cyclic (approx 1 cycle per day). Can you elaborate on the non-suitability of austenitic SS for these cyclic conditions as we had been looking at SS304, SS321 and SS347 as options, which appear to show acceptable corrosion performance at temperatures up to 460 Deg.c. The ferritic SS grades you mention - do these perform better for cyclic service, in your experience?

Any further advice much appreciated.

 

[EdStainless]

The ferritic grades have half of thermal expansion and slightly higher thermal conductivity than the austenitics.
This reduces local variation in temperature and greatly reduces the thermal stresses from expansion and contraction.
I have seen some of these systems that can start heating (or cooling) fairly quickly. This is where issues arise.
If this can't happen in your system then 304L is likely fine, just make sure that they design for a lot of movement of the coils. With temperature cycling you would rather have things move and distort than be held rigid and build up very high stresses.
439 is a stabilized version of 430 (similar to 321 vs 304).
Any moisture (ppm levels) and traces of halides (Cl and Fluorine) will destroy any SS in short order.
Good design to minimize stresses at welds is one key to keeping these systems working.

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P.E. Metallurgy, consulting work welcomed

 

[Smith55]

EdStainless - Thank you for the feedback. As we do have rapid thermal ramp-up / ramp down, it sounds as though we ought to study further the ferritic grades you mention. What would be the drawbacks, if any, of using the ferritic grades in terms of engineering, availability, constructability, etc, for application to piping, heat exchangers, storage tanks, etc. I am not familiar with these grades having not used them in previous projects. We intend to directly weld all piping connections to avoid problems with molten salt leaks / creep at flanges - therefore, were possible would you recommend selecting the same material for piping, valves, equipment etc for welding compatibility?

 

[EdStainless]

For piping and valves people either use lower alloy ferritic grades or they move to austenitic.
The alloys that I listed are somewhat limited in availability (product forms).
You will be able to find tubing and some small pipe sizes.
Choosing where to put flanges is a big decision.
You need some for system maintenance reasons but limiting them have benefits.

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P.E. Metallurgy, consulting work welcomed

 

[Guest102023]

This sounds too advanced and too critical to be designed by crowdsourcing.
Recommend you retain a specialist metallurgist.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[dik]

This site is not crowdsourcing... and I really like the topics, even if they are beyond my reach.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[MJCronin]

"We intend to directly weld all piping connections to avoid problems with molten salt leaks / creep at flanges -"

Soooooo.... None of your mechanical equipment (including pumps instrumentation and valves) will ever require any maintenance ?

Whenever any repairs or any maintenance is required, your equipment will be cut from the piping system to enable valve seat repair and pump seal/bearing replacement ... Do I have that right ????

Have you ever designed process piping systems before ?

What kind of pumps and valves do you intend to use ?

Can you explain ??

BTW, I strongly agree with Brimstoner .... who says:

This (materials selection)sounds too advanced and too critical to be designed by crowdsourcing.
Recommend you retain a specialist metallurgist.

MJCronin
Sr. Process Engineer

 

[Smith55]

All - thank you for the advice.

MJCronin - There will be some flanges for maintenance purpose, but these will be minimized. It is common practice in this application to directly weld the valves in. The valves are top entry to enable in-situ maintenance. The pumps are vertical pumps installed in the molten salt tank - flanged to enable removal for maintenance.

 

[shvet]

Looking for advice on piping material selection for high temperature molten salt service.

See para. 2.10 Shreir&Jarman's Corrosion 3d ed.

 

[SJones]

Just read Materials Performance, September 2022, pages 18-19. Looks like N07740 is the way to go.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[MJCronin]

Will not be cheap !!!!

https://www.specialmetals.com/documents/technical-bulletins/inconel/inconel-alloy-740h.pdf

MJCronin
Sr. Process Engineer

 

[EdStainless]

For molten salt service 740 is overkill.
In general any Ni based alloy would be overkill.
I would want to see data under your conditions but I would suspect that straight 13Cr might be a good option.
We did supply 625 tubing for use in solar furnace absorber tubing.
And 439 tubing for primary heat exchanger applications.

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P.E. Metallurgy, consulting work welcomed