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Old 20-08-2013, 11:07   #16
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Re: Seacock/thruhull help needed

Yeah, I think some of the problems are today's crappy NPT threads. They seem to vary all over the map, some you can only get a couple of threads buried and some go way in. But you're right, in theory NPT should be a tight enough fit to not need sealant. I have two houses that are complicated plumbing nightmares and got real tired of redoing things that I had used teflon tape on.. that want to drip. (2 wraps up to 5 wraps!) I went to pipe dope and problems solved....
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Old 20-08-2013, 14:07   #17
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Re: Seacock/thruhull help needed

Sailorchic - I believe you're confusing NPT (National Pipe Tapered Thread) with NPTF (National Pipe Tapered Fuel Thread) AKA. Dryseal. NPT threads designed to require a paste or tape to create a low pressure seal.

The NPT thread is not a completely self-sealing thread and needs either paste or tape to close the seal and are for lower pressures, where as the NPTF thread is self-sealing and designed for higher pressures.

Both threads have the same pitch and taper, the difference is in what is called the root (trough) or crest (peak) of the thread.

The NPT threads are designed to allow for some clearance between the root and crest, it is that clearance that the tape of paste is designed to fill. And it does provide lubrication.

The NPTF threads are a closer tolerance thread where the root of the thread is designed to interfere with the crest of the the mating thread. The mechanical seal is created by the root crushing the end of the crest so that it conforms to the roots shape.
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Old 20-08-2013, 14:27   #18
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Re: Seacock/thruhull help needed

Cheechako - I believe the issue regarding thread quality is, as you've probably guessed the attempt to squeeze those extra parts out of a tool and the saving of time.

In metal, among other things its the cutting blade that's pushed beyond its tolerance limits.

In plastic, it's pushing the mold beyond its capability. Molds loose their shape after x number of parts. If the plastic material has glass in it the mold degrades at a faster rate. Also if the plastic used in the molding process or a constant process is not adhered to, the product will vary. The elimination of one second in the hold time (the time after the plastic is injected into the mold and before the mold is opened and the part released can produce a "useable" but dimensionally inaccurate part.

What that 1 second does over millions of parts is provide increased profits.
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Old 20-08-2013, 16:59   #19
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Re: Seacock/thruhull help needed

Quote:
Originally Posted by Watercolor View Post

The NPT threads are designed to allow for some clearance between the root and crest, it is that clearance that the tape of paste is designed to fill. And it does provide lubrication.
All of the seacock are connected to the boat bonding system. If I use dope or paste on the fittings won't this cause these pieces to be disconnected? Or does enough of the threading touch to provide a good connection?
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Old 20-08-2013, 20:05   #20
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Re: Seacock/thruhull help needed

Quote:
Originally Posted by Watercolor View Post
Sailorchic - I believe you're confusing NPT (National Pipe Tapered Thread) with NPTF (National Pipe Tapered Fuel Thread) AKA. Dryseal. NPT threads designed to require a paste or tape to create a low pressure seal.
You know, I am not at all confused.

NPT Like NPTF both have tapered threads and both provide a metal to metal seal. The NPTF is better machined for higher pressure applications and does not need lubrication.

The pipe dope and Teflon tape are for lubrication only and provide no sealing if the npt joint is properly tighten. Think 125 pound steam running at 350 degrees F or higher, where the pipe dope would flow.

I agree that some websites list the tape or pipe dope is used for sealing, but they are not quite correct. The pipe dope or tape is required to provide the lubrication required for the metal threads to slightly deform, but the tape or dope does not provide the actual seal. As I said cooking oil or even motor oil will provide enough lubrication the the NPT fittings to deform and provide a water or air tight seal. Try it!

From wikipedia

"The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr free and lubricated using a lubricating material like lubricating paste or tape. "
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Old 20-08-2013, 20:37   #21
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Re: Seacock/thruhull help needed

It's been awhile since I last installed a seacock. It seems to me that I put 4200 on the thru-hull mushroom base and the threads, as well as on the bottom of the seacock flange. In a manner similar to the one suggested in the following.

Installing A Seacock & Thru-Hull Fitting In A New Location Photo Gallery by Compass Marine at pbase.com

Your question about the sealer isolating the seacock and thru-hull is an interesting one.

The thru-hull is NPS thread and seacock lower unit is the same. This is not a sealing thread, it relies on compression often with an O ring or gasket to seal. If the top of the thru-hull and the lower internal seacock rim don't compress then water can leak back through the threads.

The individual who wrote the article is especially knowledgeable in grounding and electrical issues. And I'm sure some forum members will respond.

N.B. On NPT threads both sides of the threads lock into one another, the gap is at the peak and valley of the threads.

Hope that helps
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Old 20-08-2013, 22:29   #22
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Re: Seacock/thruhull help needed

Sailorchic -

Your comment, "Think 125 pound steam running at 350 degrees F or higher, where the pipe dope would flow." got me thinking about a time back in the late 50's when I was working for the water department and we had to work around some guys doing some steam pipe work. They were using some nasty black sealant on the pipe threads that I'm sure were NPT. So I decided to look up steam heat sealant, here's one of the products I found.

Copaltite Threaded joints holding 10,000 PSI 1,200 ° F

I also checked to see if NPT threads are used on with steam heat. It is.

I'm happy to be corrected on this issue, but it's my understanding that NPTF (Dryseal) threads, a variation of the NPT thread, were introduced to overcome the problem of spiral leaking, in particular, the gap between the root (valley) and the crest (peak) of the threads, that gap appears to be part of the NPT standard. This seems to coincide with my reading of the ASTM thread standard, specifically for thermoplastic fittings.

The following is from National Pipe Thread vs. National Pipe Thread Fuel. I've also included a few similar articles that seem to reinforce this position.

"Both NPT and NPTF have the same threads-per-inch, pitch diameters, and taper-per-inch. The differences come in the major and minor diameters, the root and crest of the threads."

"The NPTF thread crests fall with in the parameters of the NPT requirements, but have a smaller range than the NPT."

"The NPTF thread roots are different than the NPT thread roots. NPTF thread roots are designed to interfere with the crest of the mating thread with the intention of creating a mechanical seal through thread form deformation at assembly. NPTF threads have two classes identified: Class 1 and Class 2. NPT thread roots are designed to allow clearance with the mating thread crests on assembly."

"NPT threads are designed to screw together. In most cases there will be no interference between the root and crest of the threads at assembly. The thread is designed to be assembled with some form of sealant to assure a leak free joint."

"NPTF threads will screw together with NPT threads and should have no noticeable assembly problem. There will most likely be an interference fit between the root and crest on either the major or minor diameter of the thread, depending on which part is NPTF. To accomplish a seal on the joint, a sealant will be required."

NPT Vs. NPTF Taper Pipe Threads
http://www.colder.com/portals/0/libr...whitepaper.pdf
http://www.interstatepneumatics.com/...Threadtype.pdf

Even if we can't agree on this more esoteric issue I think we can agree on the more important issue, when joining NPT threads always use teflon tape or paste.

Fair winds
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Old 21-08-2013, 10:32   #23
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Re: Seacock/thruhull help needed

Oh I don't think we're that far apart. Both NPT and NPTF seal by deformation of the threads. NPTF being of higher tolerance does not need any lubrication to form the metal to metal seal. NPT needs lubrication so that the last 1-1/2 threads (IE the thin wall threads) deform enough to seal metal to metal, including at the root of the threads.

Yes if the threads are poorly formed as is much the case now a days or the fittings are not torqued enough, you do get an spiral leak. Pipe dope / thread sealer does make the poorly tighten joint leak proof. But if done correctly, it is not the seal.

NPT threads are rated for 5000 psi or so. A sealant at a few thousand psig would also tend to flow. A seal at high pressure requires a full metal to metal seal. Pipe dope is not going to cut it as a seal.

Some day, I'll have to get my way old (1930's) engineering handbook out of storage and photocopy a few pages.

Yes there are a lot of web sites that list pipe dope as a sealant and the stuff nowadays is. Its faster to install that way and needs less training and muscle.

But the original intent of the design was for the threads to be lubricated so that the thinwall threads at the ends of the pipe would deform, sealing wall to wall, including the lower cavity. Done properly that's how it works. It works better with the NPTF as the tolerances are smaller and it needs far less pressure to seal/deform the threads.

So while thread sealer does work at low pressures, in a pinch using an oil to lubricate the threads can also give a leak free seal.

I do believe what your saying is 100% correct up to a point and is taken as the gospel now a days. Lord knows its all about speed of installation and pipe sealant does make it easier / faster to install.

But the design standard of the NPT threads only needs lubrication of the threads and proper torquing to deform the threads for a full metal to metal seal.

Well I think I beat that dead horse long enough...

Namaste.
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Old 21-08-2013, 12:17   #24
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Re: Seacock/thruhull help needed

Quote:
Originally Posted by SeaSloth View Post
All of the seacock are connected to the boat bonding system. If I use dope or paste on the fittings won't this cause these pieces to be disconnected? Or does enough of the threading touch to provide a good connection?
Without getting into the question of whether you should, or should not, be bonded.... NPT should make contact regardless of pipedope used...
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Old 21-08-2013, 20:06   #25
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Re: Seacock/thruhull help needed

Sailorchic - you're right we have flogged this dead horse pretty well. I really enjoy the discussion. Stuff like this always makes me think. Your arguments are too well thought out, persuasive and informative, I'm sure other members of the forum will find this of some value.

Forgetting about NPTF and stick with NPT threads only, my new bottom line on the basic issue (Do you use tape and/or paste on NPT threads?) is:

Always use teflon tape or paste when connecting NPT threaded parts because:

1. They help lubricate the threads allowing them to reach their "finger-tight" engagement potential, and
2. The tape or paste acts as a sealant to fill the commonly found imperfections in today's threaded fittings, and
3. Allows for the suggested goal of 2 additional turns beyond finger-tight for plastic fittings and on X turns for metal.

You mentioned the issue of improper torquing which I agree is critical, and it got me thinking so I added 3. The ASTM F 1498 standard for plastic is 2 additional turns past finger-tight. I believe the metal standard is 1 turn past finger tight. Is this correct.

Again thanks for the thoughtful discussion.
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Old 21-08-2013, 22:42   #26
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Re: Seacock/thruhull help needed

Metal is 1 to 2 turns, though probably more 1 to 1-1/2 turns. It all depends on tolerances. Which is the problem with NPT as the fitting standards tend to vary somewhat.

No matter what material, tape or dope should be used, "specially" with plastic on plastic fittings.

Its been fun and yes I've enjoyed it to. Gets me thinking, which is not always a good thing.. Really impressed with your knowledge base. Keeps me on my toes.
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Old 11-02-2014, 04:41   #27
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Re: Seacock/thruhull help needed

I finally was able to replace all of the plastic tailpieces with quality bronze (Groco) and new Trident hose.

The last one gives me the chills.

The first picture shows the entire rig that was below the waterline. The horizontal hose on the right led to the seacock. The vertical hose on the right was the head sink drain. The vertical hose on the left was the port deck/cockpit drain.

The second picture zooms in on the T. That rusted clamp on the right really stands out but if you look close you might notice that the right hose clamp is firmly clamped around the nylon fitting while the hose has no clamps holding securing it to the barbs.

It is a nice AWAB stainless clamp too. And the ridges in the clamp show that it was intentionally tightened around the nylon and not that the hose slipped free.

Scary.

Anyway, this setup had 6 below waterline connections that were very hard to get to and inspect. I have changed it to a single hose that extends above the waterline. The only below-waterline connection to inspect is at the tailpiece which is easily accessible.
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