Use of 6Mo Gaskets in lieu of Alloy 625 Gaskets 1

[MechEngJM]

Hello,

Looking for some insight into the following case...

Open Hazardous drains line has the following spec:

- Pipe: Alloy 625 (Ni Alloy, BE, ASTM B444 UNS N06625 Gr. 1, Seamless, ASME B36.19)
- Flange: Alloy 625 (Blind Flange, Ni Alloy, 150lb RF, ASME B16.5, ASTM B564 UNS N06625)
- Gaskets: Spiral Wound to ASME B16.5 CL. 150, Ni Alloy 625 outer / inner rings, Ni Alloy 625 winding w/ Inhibited Graphite filler, ASME B16.20
- Bolting: Alloy 718 (Studbolts ASTM B637 UNS N07718, Threaded full length with 2 heavy hex. nuts ASTM B637 UNS N07718, ASME B16.5 Table 1C)

Incorrect gaskets have been mistakenly ordered to remake two of the joints - 6Mo inner and outer rings with graphite filler. Current lead time for correct spec gaskets means that using the 6Mo gaskets is attractive. I don't see this as a concern, particularly as the gaskets would be changed out at next opportunity, but would appreciate a broad set of opinions.

Thanks for any help.

J.

 

[MagBen]

6Mo is inferior to 625 in terms of both corrosion resistance and strength level. Being said, 6MO can be a cost effective substitute for 625 in some applications. Not sure if not being the same material for pipe, flange and gasket matters in this case.

 

Current lead time for correct spec gaskets means that using the 6Mo gaskets is attractive.

Don't know if 6Mo is an acceptable substitute, it all depends on the service conditions, but this sentence raised a red flag. Never let schedule drive bad engineering.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MechEngJM]

Thank you both for your replies.

Relative strength and corrosion resistance of 625 versus 6Mo were less of a concern given the service compared to whether the 1:1 direct couple of the two metals would be an issue.

I also appreciate your comment re. letting schedule drive bad engineering -- I wish I could say that was not the case, but in reality, this was likely creeping in.

 

[SJones]

It's a material combination menu option for subsea systems, so it should be ok for topsides if the process conditions, which are not stated, are amenable.

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.

 

[MechEngJM]

Apologises, perhaps I should have been more clear: service is hydrocarbon liquids from open hazardous drains.

 

[SJones]

Will the conditions be within the allowable of Table A.8 in ISO 15156-3?

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.

 

[MechEngJM]

Yes, conditions satisfy Table A.8.

 

[EdStainless]

In most lower temp aqueous applications the 6% Mo grades will have just as good of corrosion resistance as 625.
As long as it is a real 6%Mo (not 254SMO which is less than 6%).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy