Metal finishing Q&As since 1989

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Joining steel and stainless pipe

2003

Dear Sir,

CO2 DOZING SYSTEM

Liquid CO2 is vaporised using Atmospheric vaporizer.
Inlet piping to Atmospheric Vaporizer
Service: Liquid CO2
Operating condition: 22 bar (g)
Pipe: Pipe size= 25mm, SS304, Seamless, Sch 80
valves and fittings: SS304, NPT female threaded ends.
All screwed fittings

# Client asking to provide welded joints.

Please advise. Which joint is stronger - screwed or welded joint?

Outlet piping from Atmospheric Vaporizer to Pressure reducing Valve
Service: Gas CO2
Operating condition: 21 bar (g)
Pipe size= 40 mm, Carbon steel, seamless, Sch 80
Valves: SS304, NPT female threaded ends.
Pipe fittings: Carbon steel and

Outlet piping from Pressure reducing valve
Service: Gas CO2
Operating condition: 3 bar (g)
Pipe size= 50mm, Carbon steel, seamless, Sch 80
Valves: SS304, NPT female threaded ends.
Pipe fittings: Carbon steel

# Client is saying that due to usage of dissimilar metals (Carbon steel piping, on to it Stainless steel valves are screwed fitted).Hence there is a chance for external corrosion at the screwed joint area.

# Please advise whether stainless steel 304 and Carbon steel are dissimilar metals. Will it lead to external corrosion at the screwed joint?

Your reply in this matter will be appreciated.

Thanks and Regards,

Mani Bhaskar
- Singapore

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2003

Mani,

Surely more erudite people can answer your questions better but I have a few comments.

l. A welded joint is or should be stronger than a threaded connection. Why? Because the given pipe pressure will diminish due to the threading of the male end. This is certainly the case for standard plastics like PVC.
2. A welded joint would surely be more prone to chemical attack than a threaded connection.
3. Yes, if you have dissimilar metals in certain environments, you will/can get preferential corrosion.
4. Another solution is to use flanges ...but as you didn't give one any idea whatsoever of the pipe sizing, this might prove to be uneconomical.
5. If you use threaded connections, then PLEASE do thoroughly wrap all connections with the appropriate tfe tape and, perhaps, even generously smear molybdenum disulfide grease over them to try to ensure that metal to metal contact is minimal.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).