NPT pipe thread sealant for steam... 1

[quizzical1]

Hi All,

We're experiencing leaking pipes - mainly in the 304 SS material - at the threaded connections over time as the piping is exposed to thermal cycling between 338F and room temp daily.

We have tried Loctite 567, Rectorseal, X-Pando and MegaLoc sealants but all have eventually leaked. Short of socket welding joints is there a sealant out there?

TIA

Q~

 

[EdStainless]

Tapered pipe threads are tapered and truncated, therefore there will always be a leak path.
What is the working fluid?
What is the temp range?

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Hi Ed,

Saturated steam up to 150 psig 366F that is used ~ 8 hrs / day then shut off overnight and left to cool off.
we can add traps to the line ends to keep the line near 200F if that helps in reducing the huge temp swing to within 166F.

 

[btrueblood]

Did you apply a primer before using loctite - stainless is notorious for being difficult to get methacrylate adhesives to cure with? Also have you tried loctite 577, its a somewhat gummier material when cured, we've had some luck with it (although at lower temps than yours)? Yes to welding, and yes to trying to limit depth and number of thermal cycles if possible.

 

[TugboatEng]

You're really pushing the limits of anaerobic sealants at that temperature. The PTFE tapes and pastes would be better suited. Copaltite is a long established sealant for steam service but overkill at this temperature and is hazardous until cured.

I have had a lot of failures with Loctite 567 in various applications despite its claims of being tolerant of contamination. Loctite 577 has been far superior in every instance in my experience.

I don't agree that stainless is particularly difficult to seal with anaerobics. The trouble is that stainless may gall and appear tight before the clearances are small enough to cure. The 577 has PTFE for lubrication and cures in wider gaps than most other sealants. It works very well for stainless steels.

Tapered pipe thread has poor performance in applications with cyclic pressure and temperature.

If you are going so far as to consider socket welding, have you considered a system such as Swagelok? For sizes 1" and under, it's a little more expensive per fitting but saves a great deal on installation and tubing costs. They are very tolerant of temperature and pressure cycles.

 

[quizzical1]

Thanks everyone,

yes, very familiar with Swagelok fittings but never thought to use it here.
also never tried 577 but willing to see if there's an improvement.

 

[XL83NL]

Well said tugboat. We do this scope all the time. Also for steam. Swagelok is a good, or actually any brand that manufactures twin ferrule compression type tube fittings.

We’ve successfully foliac manganese (from rocol) for NPT up to 500 deg Celsius. I think it’s also suited for steam up to somewhere around 300 deg C? It requires a lot of craftsmanship to apply but it does the trick. Its the only sealant we have successfully for temperatures that are beyond the typical 230 C range of PTFE.

Huub
- You never get what you expect, you only get what you inspect.

 

[r6155]

Wrong design. Best to butt weld.
You can now use seal weld.

Regards

 

[TugboatEng]

I agree, the Swagelok knockoffs can be perfectly suited for these less corrosive, less severe applications and have very palatable pricing. Look for silver plating in the threads of the nut.

 

[EdStainless]

You can only seal weld if everything is clean.
With all of the sealants he has tried he is better off taking it apart and cutting and facing the pipe.
Hire someone that knows how to use automated orbital welders.
It will take more time to weld prep than to weld.
And you will need to make up the lost length.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Jeffrey W]

I would highly recommend Loctite 5770

Loctite 5770 Maximum Temperature Maximum Pressure
(Paste) 530F 2000 PSI

Ideal for SS and steam lines.
Recommended primer: Loctite 7088 or Loctite SF7649 or Loctite SF7471

 

[quizzical1]

Thanks everyone.

 

[DAVIDSTECKER]

Years ago, I was working with aluminium NPT piping for a hydraulic system, and we were having trouble with air being sucked into the system.
The pipe supplier told me to use Anti-seize instead of pipe dope because aluminium galls about the worst of any pipe so we were not getting the full metal to metal seal the is NPT.
We did that and never had another leak.

 

[3DDave]

Anaerobic adhesives don't cure because of "small gaps," though small gaps help. They cure because of contact with a chemically active surface, one that removes dissolved oxygen, hence "anaerobic."

The bottles that they are stored in are required to be oxygen permeable. Since stainless is non reactive to oxygen, curing is inhibited. They work great on carbon steel and benefit from primers on aluminum, stainless, brass. et al.

The large gap observation is that it allows oxygen to diffuse into the material faster than the dissolved oxygen can react with the metal and stores more oxygen than the reaction at the surface can mop up. Probably oxygen has a weak attachment to the cross-linkng sites on the molecules and removing the oxygen allows for the cross-linking to happen.

 

[TugboatEng]

Thanks, I appreciate your contribution to deeper understanding. I am an anaerobic fan. I have lots of experience and limited knowledge. Primers are generally recommended but I have, in my experience, found them unnecessary. I have never found a sealing situation Loctite 577 alone couldn't solve. My application are sub 250F engine fluids (coolant, diesel fuel, lube oil, and salt water). I did build a threaded stainless steel piping system for salt water and 577 worked well enough. It never leaked to the point of wetness but had a lot of staining at some of the joints. I assumed it was crevice corrosion. Thread sealant wasn't going to be the solution to the problems that piping system was going to experience.