sealing of threaded joints 1

[nickdock]

Having a wee spot of bother with the reliability of threaded joints , particulary stainless steel 2" pipe. Trying to find a reliable sealant to do the job , any ideas ?

 

[pipedream]

what is in the pipline?
what temperatures, highs and lows?
how is this pipe supported, and how often? (vibration?)
how are you threading the pipe....what kind of cutting fluids?
are the dies in good shape and made for stainless?
how are the threads turning out, nice and sharp or torn and rough?
are the threads to the correct depth? (a quick rule of thumb is to make the fitting onto the pipe by hand until it binds up, then back it off and if the dies are set correctly it should be 3 to 3 1/2 rounds. proper fitting tightness will usually be five rounds. this does not substitute for ring and plug guages.)
what kind of fittings are you using?
what type of stainless?

just a few questions.

what kind of pipe dope are you now using?
you might try using an anaerobic type of pipe dope, clean the threads to remove the cutting fluids, dope the male threas, assemble the joint, and depending on type of dope and conditions it might need to cure before putting into service. loctite makes some very good pipe dope.
most pipe dopes will state that they are only good upto two inch.

if you are already using an anaerobic type of dope do not apply teflon tape to the threads, the teflon will act as a barrier between the threads and will not let the dope get to both sets of threads. this kind of dope hardens in the presence of metal and the absence of air, and if it can't touch the metal it will not work properly.

just my two cents worth.

G3

 

[JohnBreen]

Nickdock,

Maybe "seal welds"?

http://www.coade.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=1;t=000028#000000

 

[nickdock]

The pipesystem will carry chemicals that are used in the timber treatment industry, some sections of pipe will have the concentrate of the chemical used (acidic in nature) the vast majority will be a solution with water. Operating conditions are subject to the manner in which the system has been installed usually inside a steel framed building with allowance for adequate ventilation. Vibration on the pipes is minimal. Kong sung threading machine not sure of model number , 4 piece die headstock used. There is a question of if the headstock is worn due to amount of play when the die is located . Threads produced are generally nice and sharp and also tapered but to the correct amount is debatable. Type of stainless is 316 , type of fittings :- usual supplied by BSS , plumb center etc. Variety of dopes tried :- stag & ptfe , fernox ls-x & ptfe , fernox ls-x & hemp .Most successful was the ls-x & ptfe until the other week when on a new install leaks occured on a variety of joints . look forward to your reply .

 

[JohnBreen]

Hello,

Have you looked at previous threads? A few posts back there was a very good discussion on threaded stainless steel joints:

thread378-154496

 

[shopper1732]

NPT threads seal via interferance and deformation. Typical method to accomplish is to use teflon to lubricate in order to achieve proper seating.
But, stainless NPT threads can be problematical and teflon does not always lube enough to prevent galling, or seizing of the two parts before'seating' is accomplished.
A better lube is a nickel based compound such as "Never-Seixe". It eliminates galling and allows tightening until a proper 'seat' and sealing are achieved.

 

[TBP]

Unless NPTF dies & fittings are being used, you'll be left with a spiral leak-path because the crests & roots of the mating threads won't exactly mate. Dopes & tape need to fill this leak path, in addition to providing lubrication.

If the threading machine being used has an adjustable head, it needs to be set such that the fitters can get 3 to 4 turns to hand-tight, on clean dry threads. It's not very often that you can just set the die head exactly to the increments marked on the scale, and have it where it needs to be. Also, are the same dies being used to thread carbon steel pipe? Ridgid makes dies & cutting oil specifically for stainless. I had a Ridgid rep tell me that if the dies for SS are being used for carbon steel, that will cause problems when you go back to threading stainless.

 

[Ashereng]

If you absolutely can't leak, and threads are not doing the job, how about welding?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[moltenmetal]

NPT threads, unless cut in very malleable and lubricious materials like leaded brass, should always be though of as a spiral leak path between what's inside the pipe and the outside world. Fill this path with a suitable thread sealant and you're okay- try to do without at your peril!

My recommendations: first, get dies with the correct cutting angles for stainless steel, otherwise you won't get decent surface finish. And the threading machine needs to be very rigid so it won't deform under the stresses involved (no surprise, Ridgid still makes the best threading tooling on the market IMHO). Use plenty of a good cutting fluid. As to sealants, for 2" stainless, the only thing we've found which works reliably is teflon tape top-dressed with a good quality anaerobic pipe thread sealant- Henkel Loctite has quite a few formulations to offer but there are other brands as well. There's plenty of metal on the female pipe threads to cure the sealant so the teflon tape won't really interfere with this too badly, and the teflon provides better galling resistance than paste alone can provide. And a good anaerobic sealant never sets up hard anyway- it crosslinks and forms a gel but never really solidifies. And don't over-tighten- it won't make the joints more leak resistant, all it will do is increase the likelihood of galling.

 

[TBP]

A further problem I've seen with screwed piping in general is lazy fitters using the threading machines to tighten the fittings on the pipe they've just threaded. The last one I saw doing this wasn't even taking the pipe out to clear the chips & cutting oil. He wasn't reaming after cutting to length, either. It wasn't my job, and he wasn't working for me. Anybody who passes themselves off as skilled fitter who does work like that would be better employed rounding up shopping carts in the parking lot of big box store.

 

[unclesyd]

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