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Nipple with hex center?

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Nipple with hex center?

Unread postAuthor: Technician Fry » Thu Dec 18, 2014 7:54 am

Suppose I have a steel pipe nipple with hex fittings on each end:
Image

I can tighten this section easily by screwing the hex fittings inwards towards each other. However, it seems I can't disassemble the section without a pipe wrench. I'm working with unregulated CO2 (800+ psi) so I'd like to tighten everything firmly without worrying about whether I can unscrew it later.

I'm familiar with hex nipples, but they tend to be only 1-2 inches long. Is there a name for a pipe section with a hex centerpiece? Like this:
Image

I assumed it would be a popular design but I can't find anything similar with Google. I checked McMaster's catalog but no luck.
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Re: Nipple with hex center?

Unread postAuthor: dajoro » Thu Dec 18, 2014 8:25 pm

I recommend thinking about what you just asked. You are messing with 800 psi. and your having trouble with pipe fittings? You might reconsider your whole approach.
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Re: Nipple with hex center?

Unread postAuthor: DYI » Fri Dec 19, 2014 1:38 pm

First, to answer your question: what you're looking for doesn't generally exist as a standard part. There is such a thing as a "long hex nipple", but they generally only exist in smaller sizes, are 3-4" long, and the entire unthreaded section has a hex profile.

I'm curious as to why using a pipe wrench to disassemble the section is a problem in your application. Are you worried about damage from the wrench causing stress concentrators which lower the burst pressure of the pipe? Because that sort of damage is accounted for in the working pressure, and is generally less severe than the stress concentration caused by the threads. Or is this just an aesthetic issue for you? Also, in case you didn't know, repeatedly assembling and disassembling tapered threads isn't recommended - 2 or 3 times is fine, but eventually there will be so much plastic deformation that they won't seal properly any more, or longitudinal cracks might develop.

A possible solution: in one of my applications, there's a DOM tube being used as a barrel which runs close enough to the rupture pressure that a deep gouge from a wrench could cause failure. So, instead of a pipe wrench, I use a strap wrench to tighten the barrel. It tends to take a bit of tinkering to get maximum torque out of a strap wrench, and this may not be viable for larger diameters. Another (albeit inelegant) alternative would be to attach couplers with hex nipples (or just a hex adapter, if you can find one) onto each end of the long section.
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Re: Nipple with hex center?

Unread postAuthor: Technician Fry » Sat Dec 20, 2014 7:01 am

dajoro wrote:I recommend thinking about what you just asked. You are messing with 800 psi. and your having trouble with pipe fittings? You might reconsider your whole approach.

I haven't had any trouble so far, I'm just wondering why many fittings have hex centers while pipes don't. :(

DYI wrote:First, to answer your question: what you're looking for doesn't generally exist as a standard part. There is such a thing as a "long hex nipple", but they generally only exist in smaller sizes, are 3-4" long, and the entire unthreaded section has a hex profile.

I'm curious as to why using a pipe wrench to disassemble the section is a problem in your application. Are you worried about damage from the wrench causing stress concentrators which lower the burst pressure of the pipe? Because that sort of damage is accounted for in the working pressure, and is generally less severe than the stress concentration caused by the threads. Or is this just an aesthetic issue for you? Also, in case you didn't know, repeatedly assembling and disassembling tapered threads isn't recommended - 2 or 3 times is fine, but eventually there will be so much plastic deformation that they won't seal properly any more, or longitudinal cracks might develop.

A possible solution: in one of my applications, there's a DOM tube being used as a barrel which runs close enough to the rupture pressure that a deep gouge from a wrench could cause failure. So, instead of a pipe wrench, I use a strap wrench to tighten the barrel. It tends to take a bit of tinkering to get maximum torque out of a strap wrench, and this may not be viable for larger diameters. Another (albeit inelegant) alternative would be to attach couplers with hex nipples (or just a hex adapter, if you can find one) onto each end of the long section.


I didn't know about the stress caused by reusing threaded parts - thanks for the heads-up. Since I can torque small sections tighter with a crescent wrench than a pipe wrench, I didn't want the thought "could I undo this later without damaging it?" to affect my construction. But if just reusing the parts is causing damage, and the pipe wrench isn't, then I guess I shouldn't prioritize ease of disassembly at all.

As a side question, do you happen to know if/how the stress from reusing tapered threads is different for NPT vs NPTF threads? Some Internet lore I found suggests that NPTF threads can only be used once, but some other disagrees.

To make things weirder, I ran into some of these at my local Lowe's:
Image
This of course doesn't help my situation, since it's SCH80 PVC, but I was surprised to find a physical example of what I'm describing. I only found this hex-center style in medium-small diameters/shorter lengths. (Pictured is 3/4" x 8", IIRC). The weird part is that they were mixed in 50/50 with the smooth pipe nipples, had no distinct label (the smooth style was the product image), and were priced identically with the same UPC. I can't even find any trace of this thing on the Lowe's website or anywhere else on the internet.

So the design is used, but why not for iron/steel or SCH40 PVC? After a quick look at steel pipe manufacturing, I can see how the curl-and-weld technique used for forming long, smooth steel pipes couldn't easily form the hex-center shape. (Edit: it would also be unnecessary, as steel pipe is designed to withstand being pipe-wrenched, whereas SCH80 PVC might not). As for SCH40 PVC, I suppose threaded pipe nipples of any kind aren't popular since long pipes are simply solvent welded. But I'm just speculating.
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Re: Nipple with hex center?

Unread postAuthor: DYI » Sat Dec 20, 2014 9:53 am

I didn't know about the stress caused by reusing threaded parts - thanks for the heads-up.


You may be oversimplifying something here - not quite sure what you're quoting me on. Regardless, it's generally good to not reuse them too many times.

But if just reusing the parts is causing damage, and the pipe wrench isn't, then I guess I shouldn't prioritize ease of disassembly at all.


Both are causing damage, but yes. Depends on the construction of the female ports whether the part is ruined first by accumulated damage from the wrench, or accumulated damage to the threaded connections.

As a side question, do you happen to know if/how the stress from reusing tapered threads is different for NPT vs NPTF threads? Some Internet lore I found suggests that NPTF threads can only be used once, but some other disagrees.


I don't know anything about NPTF threads. I've never used a dryseal tapered thread, to the best of my knowledge. Based on what little I've read about them, they're probably only designed to be used once.

To make things weirder, I ran into some of these at my local Lowe's:


You can run into all manner of weird things at your local Lowe's :roll: . Like you said, the manufacturing process for plastic pipe lends itself better to forming a hex on a pipe nipple. There just isn't any demand for a shape like that on steel pipes, certainly not enough to make it worthwhile to produce them. A pipe wrench is a more stable tool for assembly anyway. In industry, threaded sections are generally only used once, and nobody cares if the pipe doesn't look pretty.
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