NPT Thread Sealant & Gauge Rings 1

[MDAust]

Hi All,
I have reviewed many threads on this forum and have concluded the consensus is that sealant is required on NPT threads when used on pressured piping up to 300ANSI. An associated thin layer of thread tape is debatable.
My question is - what is the best sealant to be used for NPT threads, 3" or less, Carbon Steel in 300ANSI oil service at less than 100 Deg C (212 Deg F)?
The following materials have been suggested on the forum - X-Pando, McMaster-Carr 1965K1, Loctite 515, 542, 565 or 567, Permatex, Megaloc or Rectorseal. We are currently using Loctite 565 but is only available in small tubes and fairly expensive - but does work well.
I would be also be interested to know peoples experience in including the use of gauge rings as a pass/fail criteria of NPT threads within the installation standard. Another company mentioned to me they had a 30% failure rate of purchased NPT materials when gauging of all fittings commenced.
Thank you. Mark

 

[moltenmetal]

Gauging of component assemblies with field cut threads is useless. I'd say that gauging even with machined threaded components is useless except to detect gross misassembly.

X-Pando is a last resort material for high temperature service.

Our best success has been with 1.3 + s.g. Teflon tape top dressed with Loctite 598 or 565 sealant. This combo originated from a Dow spec 20+ years ago and has proven to be the most idiot resistant and thermal expansion/vibration resistant sealant for temps below 350 F based on our experience. Skip the tape and you will encounter severe problems above 1" NPS with threaded pipe fittings. Skip the paste and your initial failure rate will be greatly higher too- as will your problems later on potentially if there is thermal expansion cycling.

 

[EdStainless]

excellent reply MM,that is the only practice we found to give acceptable performance.
You have to remember that since pipe threads are tapered and truncated if you put them together dry they must have a leak path. This is why the use of a compound is required.


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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

Absolutely EdStainless- an NPT joint should be thought of as a spiral leakage path to atmosphere that is plugged with sealant. You need a large gap filler and something which flexes with thermal expansion, as well as a high pressure lubricant to prevent galling as the parts are tightened against one another. The tape- paste combination does all these things, whereas tape or paste alone leave one or more lacking.