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NPT Threads - Air Tight

03/03/2009 4:22 PM

I currently am having a problem sealing a 1/8 NPT SS CPC fitting in an AL manifold. There has to be a certain orientation for the fitting as well. So the problem comes when screwing the fitting in the manifold (using thread tape) that it is either over torqued or under torqued and leaks prevail at 80psi. Does anyone have any suggestions?

Thanks!

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#1

Re: NPT Threads - Air Tight

03/03/2009 4:57 PM

Hi mellesy,

How about using a pipe-thread sealant? Find a lot of different types for different applications at http://www.mcmaster.com.

Mike

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#2

Re: NPT Threads - Air Tight

03/03/2009 5:04 PM

Use a union in the line to orient the fitting.

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#3

Re: NPT Threads - Air Tight

03/03/2009 10:08 PM

NPT threads seal as a result of the deformation of the threads. Threading SS fittings into AL, you are deforming the AL threads more than the SS threads, and each time you remove the fitting and re-install it, you have to screw it in tighter. You can not be sure you have a seal if the fitting is backed off after initial installation to conform to required alignment. Teflon tape is more a lubricant than a sealant. Pipe dope (thread sealer) is much more appropriate than Teflon tape. A universal union of some sort would help with the alignment. You may want to consider changing the materials you are using, if that is an option. There is also a thread specification similar to NPT (I don't remember exactly what the name is) that provides tighter tolerance specifications for the threads than conventional NPT. If you are making your own manifolds, change your tapping tools frequently- they wear.

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#18
In reply to #3

Re: NPT Threads - Air Tight

03/05/2009 9:02 AM

Here is a list of other pipe threads. Is your suggestion any of these?

NPT American Standard Pipe Taper Thread

NPSC American Standard Straight Coupling Pipe Thread

NPTR American Standard Taper Railing Pipe Thread

NPSM American Standard Straight Mechanical Pipe Thread

NPSL American Standard Straight Locknut Pipe Thread

NPTF American Standard Pipe Thread Tapered (Dryseal)

BSPP British Standard Pipe Thread Parallel

BSPT British Standard Pipe Thread Tapered

...and from SAE:

Dryseal Pipe Thread Series are as follows:

NPTF for Dryseal American Standard Taper Pipe Thread.

PTF—SAE for Dryseal SAE Short Taper Pipe Thread.

NPSF for Dryseal American Fuel Internal Straight Pipe Thread.

NPSI for Dryseal American Intermediate Internal Straight Pipe Thread.

Where: N stands for American Standard [formerly American (National) Standard].

P stands for Pipe

T stands for Taper

F stands for Fuel

S stands for Straight

I stands for Intermediate

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#19
In reply to #18

Re: NPT Threads - Air Tight

03/05/2009 9:17 AM

I have been a fan of NST. Used on fire equipment. There was an old Navy barracks that has sprinklers built in that used "navy thread". I thought it was the same or very similar to 1.5" NPT.

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#4

Re: NPT Threads - Air Tight

03/03/2009 11:02 PM

Change the orientation of the manifold threads to accommodate the orientation of the fitting.

Somewhere in the course of the 360 degree rotation of the fitting there must be a lead thread location that will give the desired torque/orientation result.

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#5

Re: NPT Threads - Air Tight

03/04/2009 12:18 AM

Lock tite makes a product called PST. It come in small tube. There a couple different variants. Choose the white or silver tube. It will seal your threads and does not set up hard so you can remove it in the future if you like.

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#13
In reply to #5

Re: NPT Threads - Air Tight

03/04/2009 2:06 PM

I think this PST is the stuff I was referring to in my post below. It is the only thing I have ever found that works very good on stainless. Pretty expensive to use but if it will help s/s seal well worth it in my book. I had one job that had over 3000 threaded 304L s/s connections and it was a nightmare fixing all the leaks. I even brought in a company to have a school for my fitters (some with 30 years experience with pipe) to learn how to properly thread and work the pipe. I still wonder if it was just a bad thread deal from the factory or something like that and I never did figure out what caused all the leaks. I also spent a good bit of money checking into this too. Well I ended up not making a dime from the job after I finally replaced over 350 unions with flanges at my cost. In the end we had to back weld nearly all the fittings so it would have been cheaper by far to use socket weld fittings like I proposed to at the beginning of the job.

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#16
In reply to #13

Re: NPT Threads - Air Tight

03/04/2009 6:18 PM

Where were the ss fitings made?

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#21
In reply to #16

Re: NPT Threads - Air Tight

03/05/2009 10:18 AM

The fittings were made by CPC, not sure where (what country) they were made though. CPC model MCD-1002.

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#22
In reply to #16

Re: NPT Threads - Air Tight

03/05/2009 10:22 AM

Most of the fittings were made in the USA as I remember. I do remember having to get a few imports from time to time but there were not many of them. On this particular job we also ran many thousand feet of the Felker brand speed line stainless welded fittings. Speed line are basically a Sch. 10 type fitting that instead of a butt weld the fittings are belled on the ends so the pipe slides into them and you make a fillet weld. These fittings are generally higher than others but the labor to install them is very little compared to threading or butt welding. I had over 30-40 welder fitter teams on this job and we were all pretty much hating threaded s/s when we got done. I have done many threaded s/s jobs since then but never with this much trouble although always more trouble than threaded carbon steel. Nowadays if I am bidding a job I always add extra in my quote if it has threaded s/s and try and talk the client into using something besides threaded if I can.

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#23
In reply to #22

Re: NPT Threads - Air Tight

03/05/2009 2:25 PM

I recently did some retrofitting with 1.25- 2" SS pipe. All of the fittings were from China, and I thought they were of excellent quality.

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#6

Re: NPT Threads - Air Tight

03/04/2009 12:48 AM

I have used "thread sealant with teflon on fittings that I could not chance having a string of teflon tape enter the fluid. On hydraulic systems with more than 2-300 psi, it helps to allow it to dry overnight.

Another possibility is to install the fitting into the manifold until it is tight enough to not leak. Mark the orientation of the fitting. Now remove the fitting, install a pipe tap and bring the tap down till it is about to cut new threads.

Now put a tap wrench on the tap so that it aligns with the mark made previously. Tighten the tap until the tap handle aligns where you want your fitting to point, Then remove the tap, and reinstall the fitting, It should now align where you want. Good luck.

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#7

Re: NPT Threads - Air Tight

03/04/2009 9:05 AM

I am guessing that becouse of your orientation concern you are using an elbow or a tee configuration? The best alternative for a connection with orientation concerns is a tube stub and compression fitting. You can find these at www.Swagelok.com under the part numbers of a SS-400-1-4 and SS-400-2R-4. I have provided the part numbers for 1/4" as an example becouse these will come up on the website. You can order with part numbers SS-200-1-2 and SS-200-2R-2.

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#9
In reply to #7

Re: NPT Threads - Air Tight

03/04/2009 9:49 AM

Welcome to the insanity.

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#10
In reply to #9

Re: NPT Threads - Air Tight

03/04/2009 11:01 AM

Thanks Bob, don't worry...I am wearing my hard hat.

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#8

Re: NPT Threads - Air Tight

03/04/2009 9:12 AM

After fooling with pipe most of my adult life I had rather sandpaper a wildcat's butt than mess with S/S threads. I have a buddy that only uses the very expensive teflon pipe dope that comes in a tube and he has better luck than most getting S/S threads to seal. I can't remember the name of the dope but it has a very strong odor and it does not harden over time. The only thing I hate worse than stainless threads is stainless unions. I would suggest a set of flanges instead of a union. S/S threads just have a greater chance to leak IMO. If I had to guess there are 3 things that have probably happened causing the leakage in most cases.

Over tightening. Not much you can do here but you might try using a teflon pipe dope with a couple or layers of teflon tape over that

Under tightening. Take it back apart, apply the dope and tape and reassemble being careful not to over tighten.

Bad threads. I have seen where the threads are not cut deep enough or cut to deep or have a galled area in one of the pieces and it is very hard to get these to seal no matter what you do. If a S/S fitting gets tight after a few turns then stop, take it apart and figure out what is causing it. You will only gall the threads and ruin both pieces if you try to tighten them after it galls.

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#11

Re: NPT Threads - Air Tight

03/04/2009 12:26 PM

Hi,

Mikerho have reason - Use a pipe-thread sealant. The Loctite Vibraseal 516 is good.

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#12
In reply to #11

Re: NPT Threads - Air Tight

03/04/2009 1:50 PM

Welcome

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#14

Re: NPT Threads - Air Tight

03/04/2009 2:48 PM

If the manifold is a casting, there is always the chance of porosity causing some leakage. 

(I know... pretty unlikely, but these things cross my mind.)

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#15

Re: NPT Threads - Air Tight

03/04/2009 5:20 PM

Don't put tape or paste on the fitting. Get the fitting fairly tight. If orientation suggests too much torque may be required to drive the fitting to position back it out slightly to the orientation you choose. Use a thread locker/sealer such as loctite 277 or 290. The locktite will wick into the voids and set up to seal in the absence of air. Your connection will become bubble tight to well past 80 psig when the loctite drys.

If you ever want to take the fitting out at some future date read up on the other various loctite options. 277 will make your connection nearly permanent.

Many fittings are built to swivel so they can be positioned after proper installation.

An ISO G thread form face seals with a gasket or o-ring. Except for orientation it is superior in most cases if you can port your manifold for that thread form. Tom

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#17
In reply to #15

Re: NPT Threads - Air Tight

03/04/2009 6:24 PM

I think you have an excellent plan. If I understand the OP, this is a repetitive procedure n new parts. The Thread locker will offer the added advantage of mechanically holding the fitting in place during rough handling that may happen during shipping. GA to you.

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#20

Re: NPT Threads - Air Tight

03/05/2009 9:45 AM

Here is some standard info I dug up on tapered thread sealing. Hope this helps.

Snippets from SAE J1615

Male pipe threads, including male dryseal pipe threads, when made into assemblies or installed into ports, will generally leak if not covered with a sealant.

Types of Sealant

PTFE tape applied as joints are assembled.

Pre-applied paste.

Paste applied as joints are assembled.

Inspect threads to be sure they are not damaged nor contain slivers, burrs, dirt, or other contaminants.

TAPE: Looking from the leading end of the male thread, wrap the tape clockwise circumferentially around the thread. Overlap each spiral wrap of tape approximately 1/2 the width of the tape so that no more than two plies are applied. Be careful to leave the first 1/2 to 1-1/2 threads bare. Each wrap should be wound so that the tape is tight on the threads.

(Caution—During assembly, shredding of the tape can occur with consequent contamination

of the system.)

PASTE: Apply paste evenly around the circumference over the first four or five male pipe threads, being careful to avoid air pockets.

When assembling, each pasted (or taped) end should be put together two full turns past finger tight on sizes up to 1/2 in male pipe thread. On larger sizes, each threaded end should be put together 1-1/2 to 2-1/2 full turns past finger tight.

If positioning of a shaped part like an elbow or a tee must be accomplished, thread the shaped part in approximately 1 to 1-1/2 turns past finger tight and tighten further to the desired position.

To minimize the possibility of a leaking threaded joint after assembling male to female pipe threads, neither end should be backed out (loosened) once the assembly has been made.

From Loctite 554 TDS:

Directions for use for Assembly

1. For best results, clean all surfaces (external and internal)

with a LOCTITE® cleaning solvent and allow to dry.

2. If the material is an inactive metal or the cure speed is too

slow, spray all threads with Activator 7471™ or 7649™

and allow to dry.

3. Apply a 360° bead of product to the leading threads of the

male fitting, leaving the first thread free. Force the

material into the threads to thoroughly fill the voids. For

bigger threads and voids, adjust product amount

accordingly and apply a 360° bead of product on the

female threads also.

4. Using accepted trade practices, assemble and wrench

tighten fittings until proper alignment is obtained.

5. Properly tightened fittings will seal instantly to moderate

pressures. For maximum pressure resistance and solvent

resistance allow the product to cure a minimum of 24

hours.

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