Chlorides under acidic conditions: what is the most cost effective piping material? 2

[guerky]

I have highly agressive streams that causes frequent pitting corrosion problems in the piping. I would like to change it in order to extend the lines lifespan, as well as improve the process safety and reliability.

The streams ranges from basic pH (12) to highly acidic (5% HCl). An aqueous phase has high chlorides concentration (>10%), and an organic phase has some key components that are believed to enhance the corrosion.

All the equipmente uses SS316L (for some reason, the reactors seems to be least affected), and holes in the piping are very common.

The problem is:
Glass could not be employed due to NaOH use around the process.
PTFE lined carbon steel pipes would be ideal, but cant be used due to operations under vacuum conditions.

There are some "cheap" stailess steel alternatives better than the 316L for hydrochloridric acid/chlorides?
All the "recomended" options reserached so far fell into special alloys range. Those are not very cost effective options.

Thank you all in advance.

 

[moltenmetal]

PTFE lined pipe is vacuum resistant up to 12", so you need to give this a second look.

If you need metallics, you're into a very expensive solution indeed.

 

[Compositepro]

It is possible to connect the weep holes in teflon or kynar lined pipe to vacuum, so you do not collapse the liner.

 

[guerky]

Moltenmetal,

Never using lined pipes for vacuum aplications is a general rule of thumb so, indeed, it can be wrong in some situations.
Where this diameter value come from? I would like to read more about it.


Compositepro,

Indeep, equalizing the pressure would eliminate the collapse problem, but it would involve many small auxiliary pipes to cover all the lines. Its a multipurpose plant with a huge amount of diferent process lines already installed. I cant see how i could do this without creating a severe hindrance (and making the maintenance crew curse me for five generations).

 

[guerky]

Someone has any experience with Ultra 654 SMO and Ultra 904L?

They're austenitic stainless steels with higher amounts of Cr, Ni, Mo and N (Ultra 654SMO).

 

[MJCronin]

Consider the use of C276; Sched 10S with CI backing flanges.

C276 offers the best resistance to a witches brew of hot chloride solutions where 316SS does not last. It's not cheap.....

But it should be easier to obtain and priced about the same as similar high-Ni alloys such as Ultra 654 SMO and Ultra 904L.

What is your design temperature, design pressure and your maximum pipe size ?

Talking with the operators, will your pipe actually undergo vacuum conditions ? (I hope that you are not simply trusting an old pipe line spec)

Please inform us of what you find out, your prices and your final decisions......

MJCronin
Sr. Process Engineer

 

[MJCronin]

http://www.eng-tips.com/viewthread.cfm?qid=368762

MJCronin
Sr. Process Engineer

 

[bimr]

Have you considered FRP with a corrosion resistant coating? What temperature does this piping have to handle?

http://www.smt-epocal.com/document/Guide_corrosionE.pdf

 

[guerky]

MJCronin

There are 6 reactors and vacuum is used for distillations in 4 of them. The process is not fully automated, so we have to assume that almost all the lines will operate under vacuum at some moment.

The operating temperature stays bellow 90°C in the main process. But in another process, it can reach 140°C maximum.

The maximum pipe size is 3".

 

[MJCronin]

ahhh.... At last, We have a tiny bit more information ! !

IMHO....

1) If your maximum pipe size is 3", you have no problem with vacuum protection for the piping. (No, I wont tell you why..)

- This makes the high nickle C276 (also known as Hastealloy C and N10276 ) a good candidate to consider for replacement in this service.

2) You have vacuum protection FOR YOUR PIPING with 3" metallic piping at schedule 10S at the temperatures you describe.

3) What is the vacuum protection for the larger diameter components of the adjoining systems ? Are they rated for full vacuum ?
( You probably cannot develop a vacuum in the piping system without developing a vacuum in the adjoining process equipment)

4) Why do you refuse to tell us the maximum operating pressure ? Is there something to hide ?

Getting information from a demanding yet confused OP is sometimes discouraging ....

MJCronin
Sr. Process Engineer

 

[guerky]

MJCronin,

Sorry for the incomplete info. By the way, here goes another one missing in the original post: most lines needs to be heated up to 60°C (trace heating)to avoig product crystalization. PTFE lined pipes, for its lower thermal conductivity, tends to make the plugging problems more frequent (the temperature gradient of a trace make the corrosion problem still worse, but there is nothing to do about it for now).

You stated that there is no need for vacuum protection for piping under 3" (sch10). Do you refer to full metal piping or lined piping?
Yes. All the equipments are rated for full vacuum (the operational vacuum dont go way bellow 100 mbar, though).
The maximum operating pressure is pretty low (2 kgf/cm²), not worth mentioning (i swear, i'm not hidding anything!!!).

About the materials, maybe i dont need all this nickel that the Hastelloy has to offer. For pitting corrosion resistance, the nickel is the least useful element of the alloy (and the most expensive). A lower nickel and higher chromium/molibdenum alloy would fare better under those circunstances (teoretically). What i intend to do before selecting any material is putting some bodies of proof (of diferent materials, of course) inside the reactors to gauge the resistance in the real media.

The Duplex 2205 would be my first choice. It has the characteristics that i seek (higher chromium, molibdenum and nitrogen), and its lower nickel fraction make it just slightly more expensive than the 316L. Its not the ideal MOC, but could increase the lifespan of the pipes (that is actually about 3 years).

 

[EdStainless]

I don't think that 2205 will give you enough, and 904L is a waste of money.
Perhaps a 6%Mo alloy (654smo, AL-6XN, 1925hMo, 25-6Mo and so on) is a better alternative.
Yes a Ni-Mo-Cr (c276, C22, 686, 59) would work, but be very expensive.
Do you need sch10? Would sch5 meet the pressure requirements? These are strong alloys.
I don't know where you are located but there are people that stock some of these in tubing sizes.

https://www.atimetals.com/products/Pages/al-6xn.aspx

look at the data sheet.

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P.E. Metallurgy, Plymouth Tube

 

[MJCronin]

In my opinion, you must not use opinion or what I like to call "Whatdoyathink" engineering.

The commonly used parameter that you should use to compare between candidate materials in hot chloride solutions is the pREN.

Of the materials mentioned above, 686 and possibly 59 alloy will perform slightly better than C276.... but at5 significantly higher cost.

C276 will give you the best bang for the buck.... IMHO

Please let us know of you final decision....

MJCronin
Sr. Process Engineer

 

[MJCronin]

https://en.wikipedia.org/wiki/Pitting_resistance_equivalent_number

MJCronin
Sr. Process Engineer

 

[MJCronin]

http://www.sandmeyersteel.com/images/Alloy-276-Spec-Sheet.pdf

MJCronin
Sr. Process Engineer

 

[guerky]

MJCronin,

Its not an opinion. Its a filtering criteria, based on financial restrictions. The C276 would take almost four times the available investment, with a payback >3 years (50% higher than the actual policy allows for minor process modifications).

I stated that i would run tests under the real conditions with several materials (including C276, by the way). I dont see how this falls under the "Whatdoyathink" engineering case.

Thanks for the tips, anyway.

 

[EdStainless]

The Ni in Ni based alloys is there for microstructural stability. The Ni itself does not contribute to the corrosion resistance (neither does the absence of Fe). You can't run 16% Mo and 25% Cr in an Fe based alloy, it forms weird phases. In order to get enough Cr and Mo for the levels of corrosion resistance you want you need a Ni base alloy.
Yes they cost money (the various alloys are all about the same price, but some you will only find in some product forms), maybe twice the price of a 6%Mo. That makes them about 10 times the price of 304.
The power of some of the more advanced Ni-Mo-Cr alloys (C type) is that they can handle swings in pH that would be very destructive to other alloys. Moving from chemically oxidizing to reducing conditions limits the selection of alloys to very few, and throwing in Cl only makes it worse.
So if you can't afford the correct material you can afford to continue having very frequent leaks?
Changing materials to one that lasts longer, but still fails in unpredictable ways is no improvement at all.
That isn't engineering.

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P.E. Metallurgy, Plymouth Tube

 

[1gibson]

Does your investment payback period consider the real costs of having to deal with leaks and repairs using the existing materials?

 

[moltenmetal]

Crane Resistoflex catalog - I don't have it handy, but recall that their PTFE lined pipe is good for full vacuum up to something around 12". The liners are very thick and they are not loose in the shells- there is some preload on the liners which helps them to resist vacuum.

Corrosion immunity beats corrosion resistance.

 

[moltenmetal]

Just looked it up- Resistoflex Thermalok pipe can be purchased with or without full vacuum resistance at 10" and 12"' but is fully vacuum resistant at sizes smaller than 10"