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compression fitting threads?

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Unread postAuthor: Hawkeye » Mon Aug 06, 2007 8:39 am

No they actually don't. I'm not trying to be stubborn but compression fittings aren't the same threads. He may have used the next size up of compression fitting(one that is used to attach a half inch pipe ,probably 5/8 thread) and attained a reasonable fit.
However if you were to take a half inch pipe nipple then a NPT cap is the fitting that would go over it.
NPT don't tighten all the way as the thread is tapered. Compression fittings tighten all the way.
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Unread postAuthor: jimmy101 » Mon Aug 06, 2007 9:58 am

Hawkeye is 100% correct. Compression fittings have a standard thread. The threads are not part of the seal, they are strictly mechanical. The seal is created by the ferrule and the fitting's body, the threads have nothing to do with the ability of the fitting to seal.

Heck, think about it, it really does not matter if the threads seal or not, if the ferrule leaks, the gas (or liquid) will escape between the pipe and the hole in the nut, since that hole is not sealed there is absolutely no reason to use the more obscure NPT thread on the nut or to do anything else to get the nut's threads to seal. Indeed, you absolutely, positively would not want NPT thread on the nut. NPT threads are designed to seize up at a particular point. A compression fitting wouldn't work correctly with NPT threads since it is not supposed to stop turning until the pipe + ferrule is compressed to a certain point.

In some cases you can use an NPT with a standard thread, particularly if you are only screwing the two parts together a few turns. But that is really just a result of luck.
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Unread postAuthor: noname » Mon Aug 06, 2007 10:58 am

If I can get a compression fitting that says 1/4" comp x 1/4" NPT, it usually has NPT threads. :wink:
Here, I even drew a pretty little diagram:

Wow, I just saw my drawing, it really sucks! :P You get the general idea though.
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Attachments
compfitting.JPG
NPT threads are on the RIGHT, compression nut is on the LEFT.
compfitting.JPG (8.88 KiB) Viewed 2791 times
Last edited by noname on Mon Aug 06, 2007 12:59 pm, edited 1 time in total.
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Unread postAuthor: SpudBlaster15 » Mon Aug 06, 2007 11:58 am

Well, it appears that Hawkeye and jimmy101 are incorrect, and noname is correct, as the compression nut I used fits perfectly onto a 1/2" pipe nipple, and has tapered threads like a NPT fitting.

I will take a picture today when I go out to my shop.
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Unread postAuthor: Hawkeye » Mon Aug 06, 2007 1:01 pm

Sigh, I just about give up. Compression fittings can have NPT on one end which is why they are described as Compression X NPT. But the compression nut side is not NPT. Go to Home Depot and get one of these fittings and try to put a compression nut on the NPT side or vice versa and it won't work.
Why do you think they give it a different name? The two are actually divided up and labeled in different sections so people don't mix them up expecting them to fit because the numbers match.
Spudblaster, you may have a fitting of whatever sort that fits on the half inch nipple but broadly saying that all compression fittings will fit over a NPT is not really helping people who may buy them thinking that is true.
How do you actually know the nut is a compression nut?
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Unread postAuthor: noname » Mon Aug 06, 2007 1:05 pm

Hawkeye, you stated that compression fittings "aren't the same threads."
You were both wrong and right, and I was just saying what I said in the first place, which was correct.
I'm am now out of this discussion.
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Unread postAuthor: SpudBlaster15 » Mon Aug 06, 2007 1:07 pm

I didn't say that all compression fittings will fit on NPT threads, I said that MY compression fitting fits on NPT threads.

I know that it is a compression nut because it was taken off of a compression T fitting.
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Unread postAuthor: jimmy101 » Mon Aug 06, 2007 2:12 pm

Noname are you confusing the NPT threads on the NPT side of the coupler with the non-NPT threads on the compression side of the coupler?

Looking at a couple of compression fittings, NPT fittings and taps I have ...

1/4" compression and 3/8" compression, are not tapered threads. The nut will easily spin onto the fitting all the way up to the stop. Tapered pipe threads won't do that.

1/8" American standard tapered and straight pipe thread is 27 TPI.
A 3/8" tube compression fitting is 25 TPI.
My 1/8" NPT tap is 27 TPI.
A 1/4" NPT fitting is 18 TPI.

According to the Engineering Toolbox, http://www.engineeringtoolbox.com/npt-n ... d_750.html , the threads are tapered 1 degree 47 minutes.
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(above image from http://www.rjsales.com/products/nipples/index.html)

The pieces have the taper across both the crest and the roots of the threads. As a result, the height (distance from crest to root) of individual threads is constant but the OD of the thread for male threads increases, and the ID of female threads decreases, as you move from thread to thread. The pitch of the thread is constant.

Unlike a standard straight thread, NPT threads are directional, they have a "top" and a "bottom".

Screwing together a tapered male with a tapered female will jam eventually, exactly as it is supposed to.

Screwing together a tapered thread with an untapered thread (both with the same thread pitch) will also jam eventually.

Two straight threaded parts will never jam.

You might find an NPT part with the same thread as a non-tapered part. The two will screw together for some number of turns. That does not mean they are the "same thread". They can't be since the taper is part of the thread definition. Having the same TPI does not mean they have the same "thread".

In my experience, compression fitting manufacturers seem to pick unusual thread pitches. They can because they always supply both threaded parts as a set. If they use an unusual thread then parts from other manufacturers are not useable with their fittings. I can't find a standard anywhere for the threads of a compression fitting, only the range of pipe/tubing that the fitting must work with.
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Unread postAuthor: mopherman » Mon Aug 06, 2007 6:02 pm

well, i had another question so rather than make a new thread, Ill ask here?
do you guys think a 1/8 or 1/16 diaphram with flex enough in a one inch tee?
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Unread postAuthor: jrrdw » Mon Aug 06, 2007 8:32 pm

mopherman wrote:well, i had another question so rather than make a new thread, Ill ask here?
do you guys think a 1/8 or 1/16 diaphram with flex enough in a one inch tee?


Depends on what it's made of.
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Unread postAuthor: mopherman » Mon Aug 06, 2007 8:55 pm

jrrdw wrote:
mopherman wrote:well, i had another question so rather than make a new thread, Ill ask here?
do you guys think a 1/8 or 1/16 diaphram with flex enough in a one inch tee?


Depends on what it's made of.

orange gasket material
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Unread postAuthor: noname » Tue Aug 07, 2007 11:25 am

Orange gasket material isn't too helpful either. I've found many kinds of sheet rubber that look exactly the same, but have different flexibilities.
I think it'll work best with 1/16" sheet, usually I only use 1/8" rubber for 1 1/2" and up.
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