NPT Leakage in cryogenic condition 1

[LSUUR]

Hi, I have a problem about leakage issue at NPT thread in cryogenic condition(operating temp. is -143 degree C)
Our filter has NPT(316L) and will be installed in tubesheet(316L) of vessel by hand tightening.
Normally we use teflon tape or sealant for ensuring tightening but it is not allowed by customer due scraps of teflon or sealant can be flowed down to next process.

Can you help me to ensure tightening of NPT in cryogenic condition ? Except teflon or sealant, is there any solution??

 

[MJCronin]

IMHO, the client is forcing you into a welded fitting ....

Have you considered a seal weld on the the NPT fitting ? (Yes, I know that there are additional problems with seal welds)

What is the code of construction ?

MJCronin
Sr. Process Engineer

 

[EdStainless]

Without a sealant NPT will always leak, these are a truncated thread so there is a leak path.
Either a different style of thread, or a different style of fitting, or sealant, those are the options.
Teflon tape should never get into a system, the first couple of threads should never be taped, only the larger portion of the threads.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[hacksaw]

NPT fittings are not suited to your service, threaded fittings with o-rings (metal or suitable elastomer) perhaps, but not NPT threaded fittings.

 

[mk3223]

Per my experience, the sealant or teflon tape are normally not allowed for the thread connections in the process. And, the threaded connection on the process side is typically required to be seal welded.

 

[Compositepro]

As has been stated NPT thread always require a sealant. This may be tape or paste, with teflon or not. In some plants tape is banned, particularly where the process can get plugged by tiny pieces of tape such as instrument air lines. Otherwise threaded connections will work if done properly by experienced pipefitters. But there are many ways to get the installation wrong. That is why amateurs run into problems and then swear off using NPT for life.

You mention a tubesheet. That implies that both ends of the thread are inside your process.

 

[SnTMan]

There are certain pipe threads classified as "dryseal", i.e., no tape needed. Usually called out as "NPTF" if I recall correctly. Your filter may or may not be available with such. Your tubesheet may or may not be tapped as such.

Regards,

Mike





The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[1gibson]

To get pedantic, if tape and sealant are banned, Loctite makes a thread sealing cord, looks like giant dental floss. Loctite 55.

 

[r6155]

LSUUR
"...by hand tightening". ¿Why?

Regards
r6155

 

[racookpe1978]

Cryogenic conditions cycle the pipes, nozzles/nipples, vessels and all of their seals every time the vessel is filled and refilled because the temperature difference between air and liquid is so great. The filled vessel does "stabilize" somewhat with its insulation, but the pipes (if not kept completely filled with liquid gas) warm up when the liquid is not flowing, then chill again, then warm up.

With that movement (stretch and relax), you will find it incredibly difficult to pretend a threaded fitting will stay sealed for the life of any product.

The very cold temperature will harden plastics (seals and tapes) - making the stretch and relax movement between the threads and vessel walls even more problematic.

Give it up. NPT/NPTF ain't gonna work.

 

[Vikko]

Why does the customer not want teflon tape on an NPT thread ? It will not be flowed down to the next process at all, that seems an unreasonable explanation to me. It would be hardly possible even with liquid/gel sealant. In any case, at this point use hemp strings, which is similar and was made in the past before than teflon which is usually much better. The sealing technique is not independent from the joint type, so I think that if you want an o-ring or other type of gasket you have obviously also to change the kind of connection (with a flange and not the thread). O-ring does not always work on a thread fittings (or are really hard to be selected/designed and found, it is better you design the flange profile instead, according to the o-ring specifications), because it depends on the size and dimensions of the chamfer, the surface roughness, etc. Especially if the presure and temperature are severe.