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NPT threads generating shavings and flakes during use

I have had good luck removing burrs like that from threads using a compliant non woven abrasive wheel like these:

I would also look into if it is an option to switch to a face seal based assembly. If your flow and pressure requirements are in scope, urethane push to connect tubing is great. SMC makes gasket sealed universal fittings that work with many female NPT connections. The only issue I have had is on fitting where then machine the countersink at the start of the female fitting too deep and it interrupts the seal.
 
While it looks like you found the problem, something else to consider when joining NPT threads is the use of plumbers putty; it is not what provides the sealing; NPT thread supposed to do that; but the plumbers paste will provide lubrication to prevent galling.
 
While it looks like you found the problem, something else to consider when joining NPT threads is the use of plumbers putty; it is not what provides the sealing; NPT thread supposed to do that; but the plumbers paste will provide lubrication to prevent galling.
Thank you, yes, that too is something we've tried. We've tested three different pipe dopes, including Tru Blu, and each results in similar chips. I've got a new NPT die coming that I'll attempt to clean-up the male fittings with and try that simply as a diagnostic measure.
 
While it looks like you found the problem, something else to consider when joining NPT threads is the use of plumbers putty; it is not what provides the sealing; NPT thread supposed to do that; but the plumbers paste will provide lubrication to prevent galling.

Actually some kind of sealing method is required with NPT threads. By design, the crests/root interface allows a small gap which must be filled with something to prevent leakage. The design purpose of the gap is to allow reuse of fittings, since no or little crest/root deformation occurs.

OTOH, dryseal threads, NPTF by designation, are intended to be used without any sealant. At assembly crest/root interference occurs and this area is deformed. NPTF threads are indicated in sensitive assemblies where loose sealant can compromise operation. NPTF fittings are considered single use, but in practice are sometimes reused with special precautions.

NPT and NPTF can be used together, but in this case sealant must be used.
 
Actually some kind of sealing method is required with NPT threads. By design, the crests/root interface allows a small gap which must be filled with something to prevent leakage. The design purpose of the gap is to allow reuse of fittings, since no or little crest/root deformation occurs.

OTOH, dryseal threads, NPTF by designation, are intended to be used without any sealant. At assembly crest/root interference occurs and this area is deformed. NPTF threads are indicated in sensitive assemblies where loose sealant can compromise operation. NPTF fittings are considered single use, but in practice are sometimes reused with special precautions.

NPT and NPTF can be used together, but in this case sealant must be used.
Good to know. I always used the pipe dope, Blue monster, thinking that its primary use is to avoid galling but was "hoping" it will improve sealing. I guess it was actually doing what i was hoping it will do. I have even made air hoses with NPT fasteners this way; even though it really should have been NGT fasteners (NGT has tighter tolerances than NPT).
 
Good to know. I always used the pipe dope, Blue monster, thinking that its primary use is to avoid galling but was "hoping" it will improve sealing. I guess it was actually doing what i was hoping it will do. I have even made air hoses with NPT fasteners this way; even though it really should have been NGT fasteners (NGT has tighter tolerances than NPT).

Interesting. I have never encountered NGT threads, so thank you for that. I just read about them and the indicated use is hazardous gasses. Why should you have used an NGT thread for air?

I'm still rattling around in this rabbit hole, but I think I'll climb out. I have encountered the omnipresent interweb problem of conflicting information on gas piping, and it's in print articles, not youboob vids. Since I've no plan to run any gas supplies it's time to leave!
 
Interesting. I have never encountered NGT threads, so thank you for that. I just read about them and the indicated use is hazardous gasses. Why should you have used an NGT thread for air?

I'm still rattling around in this rabbit hole, but I think I'll climb out. I have encountered the omnipresent interweb problem of conflicting information on gas piping, and it's in print articles, not youboob vids. Since I've no plan to run any gas supplies it's time to leave!
I don't think you are "required" to use them for air, but tighter tolerances will make a more air tight seal.
 
Good to know. I always used the pipe dope, Blue monster, thinking that its primary use is to avoid galling but was "hoping" it will improve sealing. I guess it was actually doing what i was hoping it will do. I have even made air hoses with NPT fasteners this way; even though it really should have been NGT fasteners (NGT has tighter tolerances than NPT).
If galling is a concern, use a tape that is impregnated with nickel powder. It is specifically made for stainless steel, but can be used for other materials. Blue Monster Nickel Guard Anti-Seize Thread Sealing Tape. It is expensive, but it works.
 
Not to be a nay-sayer here. But the fine shaving residue remaining in the female threaded part looks a bit suspicious. Could you post up a photo of the male part which had been threaded in there, and then removed?

Wondering if the plating (male part appears to be plated brass based on the photo showing damaged male threads) is breaking down upon assembly?
 
Not to be a nay-sayer here. But the fine shaving residue remaining in the female threaded part looks a bit suspicious. Could you post up a photo of the male part which had been threaded in there, and then removed?

Wondering if the plating (male part appears to be plated brass based on the photo showing damaged male threads) is breaking down upon assembly?
Yep, I can definitely do that. I'll be able get a photo and post it up tomorrow.
 
Well done plating should gouge/deform with the threads, not chip off. Very easy peeling usually indicates poor cleaning before plating.
 
I’m late to this thread and this doesn’t solve your problem, but NPT threads and high performance valves just don’t mix well. Even without damaged threads, NPT presents a system debris (FOD) risk due to pipe tape etc.

Many aerospace companies don’t allow NPT in any flight system because of its FOD risk and unreliable sealing.

TLDR: NPT is horrible, even when done right.
 
I’m late to this thread and this doesn’t solve your problem, but NPT threads and high performance valves just don’t mix well. Even without damaged threads, NPT presents a system debris (FOD) risk due to pipe tape etc.

Many aerospace companies don’t allow NPT in any flight system because of its FOD risk and unreliable sealing.

TLDR: NPT is horrible, even when done right.
As someone that works in aviation, flight test, i must totally agree with this. Never seen NPT connections... most disconnects that i have seen are a flare or something specialized.
 
As someone that works in aviation, flight test, i must totally agree with this. Never seen NPT connections... most disconnects that i have seen are a flare or something specialized.

No doubt NPT connections aren't for everything. But they do have their place for many, many applications. Inadequate deburr, plating over those burrs, and throwing a bunch of any kind of fittings in a bag will lead to dirt in the assembly. The real culprit here is poor workmanship, not the type of thread.

When NPT threads are cut in the field inspection goes out the window. If it screws in there is no deburr. If it works, it works. How many shavings and pipe dope have we all drank?

I wonder how much they talk about NPT thread form in plumber's apprenticeship programs?

The sad part about this particular problem is the fittings are brass and easily deburred, and the handling damage issue is easily overcome with a bit of care.
 
Some photos of the "used" NPT male QR fitting. There is noticeable damage to the threads, but not sure any significant amount has chipped off.


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And we do agree, NPT is not optimal for this application. The valves we use only come with NPT, so we've had to cope. Non-aerospace application, but still very sensitive to leaks (we can actively measure leak rates in millibar/hour).

I do appreciate all the feedback, it's driven some discussions here.
 
De-burring on a wire wheel or running them through NTP die should alleviate some of the install issues, but naturally a clean thread from the factory is better. The thread damage may have its largest impact not due to the amount of lost thread, but due to its burs preventing connection from threading on deep enough to seal. If push really comes to shove; you can probably just silver solder the joint (depending on temperature tolerance of the components).
 
Suggest you have the shavings analyzed to see if they are aluminum or chromium. If chrome, the plating is defective, and purchase the connctors with unplated brass threads. If aluminum, the male threads are abrading the female threads (chrome harder than aluminum). Another good easy experiment would be to make up a simple unplated brass NPT fitting into the alumium manifold, then remove it and inspect for similar FOD.
 








 
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