Pump Shaft Material for mild H2S/Ammonia Service 2

[Guest102023]

No formal failure analysis was done but fracture of 2" diameter SS316 shaft appears to be by (corrosion) fatigue.
We have had multiple failures, about one per year.
Conditions: 5ppm H2S, 30ppm NH3, 160°F.

I'm looking for material upgrade recommendations that give us significantly higher strength and no decrease in corrosion resistance, while meeting NACE MR0175 requirements. Duplex SS looks like a candidate.

.

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

 

[EdStainless]

Are there any chlorides?
What strength level are you looking for?

No chlorides, look at 2205.
Chlorides, you need to consider 625. Yes it is very expensive, but it works.

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

No chlorides reported, but it's in refinery, so I can't rule it out.

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

 

[metengr]

The 2205 duplex should be acceptable. Watch all transitions on the pump shaft to ensure proper radius to reduce local stress concentration.

 

[EdStainless]

But at what strength?
I am sure that the 316 was cold worked, will the 2205 need to be cold worked also in order to meet the strength requirements?

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Regardless, the 316 did not have sufficient fatigue strength based on a fatigue life of one year.. The 2205 will most likely have higher strength in comparison to the 316.

 

[EdStainless]

But if he needs it cold worked he may need to look for another option.
The duplex alloys loose ductility very quickly when cold worked.

If he need high strength he should look at Nitronic 50 or a 6%Mo alloy that has been cold drawn.
The nit50 is readily available since it is used for boat shafts (one trade name is Aquamet 22).

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

I considered a Nitronic alloy for the roughly 1.5 inch diameter shaft. It is proprietary, so is it even covered in NACE?

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

 

[EdStainless]

These alloys are not proprietary, there are numerous producers.
I believe that they are listed, Nitronic 50 specifically is commonly used in valve stems and similar components.

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

I've investigated mechanical properties of Nitronic 30 and 50, but they offer no advantage over duplex 2205, with lower corrosion resistance, so those are ruled out. 17-4PH is also a candidate and is permitted by MR0175 with the appropriate heat treatment.

Think I've got this sorted, so thanks to all.

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

 

[EdStainless]

If you need 150ksi or better and good corrosion resistance (pitting and CSCC) then either use a cold finished 6%Mo or cold finished 625.
17-4PH is less corrosion resistant than 304L. If a 316 shaft failed I wouldn't consider it.
Nit 50 can be cold worked to high levels and it is more corrosion resistant then 316L, but CSCC is still a risk.
Cold working 2205 does not hurt its good corrosion resistance (much better than 316L) but the ductility drops quickly when you cold work it.
The 6%Mo alloys (such as AL-6XN) can be easily cold worked to high strength, keep good ductility, have great pitting resistance, and they have reasonable CSCC resistance.
If you go to Ni based 625 you can cold work to very high levels and still be immune to CSCC and have good pitting resistance.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

All guesswork. Run a hardness check on the failed shaft material. I would not be surprised if the original shaft material was annealed. You can determine what strength level you need with margin and decide on an upgraded material.

 

[SJones]

Brim,

Nitronic 50 is UNS S20910 which has its own rows in ISO 15156-3, Tables A.2 and A.3. For your application, it would fall foul of the temperature limitation in A.2 for a straightforward qualification. Some people may try to weasel it in by virtue of the word 'shafts' in the title of A.3, where it has no environment restrictions.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

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