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Leakage in 3/8" NPTF Thread

12/12/2013 3:57 AM

3/8" NPTF thread in a component is leaking in assembly.L1 gauge is OK in components. Mating part also checked with gauge found OK.Any idea please

Mohanadas

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#1

Re: Leakage in 3/8" NPTF thread

12/12/2013 3:58 AM

What is the fluid, and at what temperature and pressure?

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#2

Re: Leakage in 3/8" NPTF thread

12/12/2013 4:12 AM

What is the length of thread that is mating? (how many threads are engaged?)... heck if they are mating..they should get married not just engaged... but hey anything goes these days

Del

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#3

Re: Leakage in 3/8" NPTF thread

12/12/2013 4:42 AM

NPTF = National Pipe Thread for Fuels, and is supposed to be a sharp V form, not needing sealing compounds. If either the male or female part is incorrect (e.g., regular pipe thread by mistake), there may be leakage in a helical path following the threads.

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#4

Re: Leakage in 3/8" NPTF thread

12/12/2013 4:53 AM

A little JB weld or similar two part epoxy on the threads before assembly will fix everything.

I do it to fuel and hydraulic lines all the time.

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#5

Re: Leakage in 3/8" NPTF thread

12/12/2013 7:14 AM

For general use

For controls i would use

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#6
In reply to #5

Re: Leakage in 3/8" NPTF thread

12/12/2013 8:15 AM

mmmmm blue gunk
Del

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#7

Re: Leakage in 3/8" NPTF thread

12/12/2013 8:34 AM

Tighten the fitting properly. No leaks!

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#8

Re: Leakage in 3/8" NPTF Thread

12/12/2013 1:23 PM

...and replace the pipefitter with someone who is appropriately qualified.

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#9
In reply to #8

Re: Leakage in 3/8" NPTF Thread

12/12/2013 3:01 PM

Why, is he leaking too ?

Del

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#10
In reply to #9

Re: Leakage in 3/8" NPTF Thread

12/12/2013 5:34 PM

Yeah! He's another Snowden

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#11
In reply to #9

Re: Leakage in 3/8" NPTF Thread

12/12/2013 5:49 PM

No, just covered in blue gunk.

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#12

Re: Leakage in 3/8" NPTF Thread

12/12/2013 8:20 PM

In my opinion NPTF is good only for one time. If the fitting is taken apart, teflon or sealant will be necessary. Once the peak on the threads has been crushed in the initial fit, it never goes back the same, leaving a channel for a leak.

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#13
In reply to #12

Re: Leakage in 3/8" NPTF Thread

12/12/2013 9:49 PM

I have to agree with you Mike. And not knowing how many times this joint has been disassembled, you have a valid point. GA from me. Here's a little info for the OP if they're not that familiar with the thread difference between NPT and NPTF

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#14
In reply to #13

Re: Leakage in 3/8" NPTF Thread

12/12/2013 10:36 PM

Leaking in 1st assembly itself

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#15
In reply to #14

Re: Leakage in 3/8" NPTF Thread

12/12/2013 10:59 PM

Not to insult your intelligence, but you said you checked the threads with a L1 gauge. Is it possible that the leaking joint have NPT jointing to NPTF? Here's 3 quotes from the link I posted;

"Visually, both threads appear to be identical. Both have a ¾" taper over one foot of length. Both have the same pitch diameter at the top of the hole of internal threads or end of the pipe on external threads, and both have the same thread lengths or depths. However, there is a subtle difference in the thread form that differentiates the two. The major and minor diameters of both threads differ slightly. With NPT threads, after a wrench is applied, slight spaces at the major and minor diameters may exist that would allow the assembly to leak and therefore a sealing compound is used to fill any gaps. On the other hand, NPTF threads are designed to ensure that sufficient crushing of the entire thread form will take place to produce a mechanical seal."

".... it is acceptable to use an NPTF tap for NPT applications. However, NPT taps cannot be used for NPTF applications, as it will likely produce a thread that will leak. The same is true of external threads. In most cases the tap drill is the same for both forms."

"The most significant difference in the two threads is the inspection required. Since sealing compounds will be used for NPT threads, only a single plug with a step, known as an L1 plug (internal thread) or a single thin L1 ring (external) are required to check size. However, since the taper and the position of major and minor diameters are so critical to the sealing of NPTF threads, the additional threads in the assembly known as L2 and L3, and the major and minor diameters are inspected with either special plug or ring gages.

Good luck, dj


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#16

Re: Leakage in 3/8" NPTF Thread

12/12/2013 11:44 PM

Mohanadas-

You probably have a damaged thread on one fitting or a mix of NPTF and NPT fittings. Really doesn't matter though, it can be easily fixed.

Place some "SELECT UNYTE thread sealing compound w/Teflon" made by J.C. Whitlam Mfg. Co., Wadsworth, Ohio, on the male threads. Put enough on that there will be no doubt that it enough, don't be stingy. Assemble as you would either a NPTF or NPT thread. The compound will not hurt the performance of the NPTF-NPTF, NPTF-NPT or NPT-NPT joints. Some might say no compound on a NPTF but this is OK. I have seen it done hundreds of times. When all else fails this stuff can seal it. It could probably seal a frog's a__s and that's water tight!

SELECT UNYTE will be available at any good plumbing supply house. It is approved for all sorts of materials including: acids, caustics, kerosene, Jet Fuel, castor oil, carbon dioxide, natural gas, hydrogen gas, gasoline, Ammonia, diesel fuel, water, liquid sugar, petroleum solvents, LPG, corrosives and too many more to list here. This stuff can seal about thread assembly, even those of an apprentice pipe fitter.

It is simple to use and very effective. That's why I like it!

Good Luck, Old Salt

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#17

Re: Leakage in 3/8" NPTF Thread

12/13/2013 7:51 AM

One other thing to look at is to make sure that the two mating threads are of the same type. While NPT and NPTF are somewhat compatible, they are not fully compatible and thread sealant will only help under certain conditions, of which, most were explained already.

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#18
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Re: Leakage in 3/8" NPTF Thread

12/13/2013 8:46 AM

enger-

I have found that if the fittings leak, it is either due to damaged threads or an incorrect match. If all else fails the most effective product for sealing them is to use epoxy but this does not lend itself to taking the joint apart in the future. If used, apply heat to assist in the mechanical force usually used. If all else fails, weld the joint if possible. This is a definite not take apart method.

I have never seen a joint not seal with the use of the Select Unyte mentioned above except those that have been mauled and no attempt to join them should have been made initially. Another excellent thread compound is "Loctite Thread Sealant with PTFE #30534". It has the effectiveness of the Select Unyte but not as compatible with as many products. Important properties of each that make them more suitable than others are: they have a very high viscosity compared to others; they have a high concentration of Teflon in them; they are non-hardening; and they are thixotropic.

The failure of a joint due to mechanical leakage is the type of situation which these products excel. As stated above, I have always seen both these sealants work to seal the leak if the threads are identical or a mismatch. These products work and are quick to seal, two things I like about them.

Good Luck, Old Salt

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#20
In reply to #17

Re: Leakage in 3/8" NPTF Thread

12/13/2013 12:46 PM

Thread differences are covered in post #15

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#19

Re: Leakage in 3/8" NPTF Thread

12/13/2013 10:07 AM

Hi Mohanadas, welcome to CR4. You will get some good advice here, but some can be cranky. Also watch out for us jokers. Here's my answer:

I never heard of threads leaking. If that keeps up, your pipe will be as smooth as a baby's but.

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Users who posted comments:

Del the cat (3); dj95401 (4); enger (1); JohnDG (1); lyn (1); mike k (1); Mohanadas (1); old salt (2); ozzb (1); PWSlack (2); StandardsGuy (1); tcmtech (1); Tornado (1)

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