NPT Threads

It's the size of the pipe. Take a look at this Standard Pipe Schedules and Sizes - Engineers Edge

You will note that it is not the major diameter of the pipe, as it would be if it were a machine screw or bolt.

It's just one of those things... like... why 12 inches to a foot? It's nominal, which means, it just is what it is... you cant find a pipe that measures exactly what it is in the physical world. varying pipe thickness for the same size pipe also throw a wrench into things.

Traditionally it was a 1/2 inch. It was then cast iron. Then they changed to steel. Stainless came along. There has been just so much improvement over the steel. They no longer need to make it as thick to handle the same pressure. So the standard when to the OD and thread size.

If they did not do this then different manufactures would have pipes with different OD's As improvements are made.

Huh????

Lyn has it right -- Specifications for NPT = nominal pipe I.D. (inside-diameter of Schedule 40) and number of threads per inch. Also, all NPT sizes use a taper of 3⁄4 inch per foot.

Specifications for NPT = nominal pipe I.D.

exactly, it's nominal...

nom·i·nal

[nom-uh-nl] Show IPA-adjective

1.being such in name only; so-called; putative: a nominaltreaty; the nominal head of the country.

2.(of a price, consideration, etc.) named as a mere matter ofform, being trifling in comparison with the actual value;minimal.

3.of, pertaining to, or constituting a name or names.

It's just a name, there is no measurement associated with it.

That's not what the OP asked! He is asking why in pipe sizes that there is no dimension that correlates to the size. On 1/2 inch pipe size there is nothing that measures 1/2 inch.

Exactly...

He is asking (if i'm understanding correctly) why he can't take out his tape measure and find a 1/2" measurement anywhere on a 1/2" NPT pipe. The answer is simply because it's "nominal", which simply means it's only a name given to that pipe. There are a small handfull of pipes that can be measured relatively closely to their given "nominal" name, but that's more of coincidence than anything. Nominal is simply a name/title given to something, and as far as NPT is concerned, the pipes are all nominal.

No measurement value directly correlates to the pipe size... it just is what it is, and has evolved that way over the decades of materials improvements as stated earlier.

FYI this only works for pipes under 14" OD... Above 14" OD they can be measured by the OD.. EX: 14" NPT OD =14" 18"NPT OD= 18" etc etc etc... but all the ones under 14" are Nominal, and do NOT have a dimension the same as the name.

They used to... some old turn of the century houses/barns have real unfinished 2"x4" pieces of wood... now a 2"x4" is more like a 1.5"x3.5" after finishing.

That, right there, is probably the best explanation in this entire thread. You can't find a 1/2" dimension on a 1/2" pipe or fitting for exactly the same reason that you can't find a 2" dimension on a 2x4.

2x4 is just the name of that size of lumber. It's not its measurement.

1/2" NPT is just the name of that size of thread. It's not its measurement.

You can either memorize the actual odd sizes of the various fittings, or just have your chart handy.

The origin of the name (nominal) used to be the inside of the pipe, back in the day. But that is now more or less meaningless.

The point is... pipe used to be measured by the ID, just like tubing is today. That's where you get the nominal names. The wall thicknesses changed, but the names didn't.

Sorry. I got that switched around. Plumbing 101. Tubing is measured on OD of course. What I was trying to say was that piping ID used to match it's name. Now it doesn't.

Ok, I'm leaving this thread before I stick three feet in my mouth.

They do at the saw mill. Then they are planed.

RVZ717, according to you, we can affirm that there isn't any logical link between the name and dimensional for this kind of thread...

Yes, exactly... for all pipes under 14" there is no logical link between a measured value and the name of the pipe..

It took me a long time to "get it"... Take out your tape, and go to your local steel yard and measure away... You'll "get it" too.

"... no logical link between a measured value and the name of the pipe."

IMHO, that's not precisely true. The "logic" is perfectly acceptable by the specification's origin -- plumbing. It's simply a consequence of the profession's terminology.

As long as you don't go out with a tape measure and take a measurement of an unknown pipe without knowing that it's nominal...

A 1/4" NPT pipe will measure out to .54 on the OD without knowing any better you might tend to think that must be a 1/2" pipe... which would obviously be wrong.

A 1-1/4" pipe will measure out to 1.66 on the OD.

A 2" NPT will measure exactly 2-3/8" on the OD

2-1/2" will measure exactly 2-7/8" on the OD

A 4-1/2" NPT pipe will measure exactly 5" on the OD. So if a guy grabs his tape and measures 5", gets an order for some 5" pipe, the pipe he will receive will actually measure 5.563" OD... whoops.

As far as I know there is no trend, or formula, or any other nice thing to explain it... it simply just is what it is. That's why there are nice charts like The one that was already linked to by Lyn.

Pipe is only measured by the OD and wall thickness... The OD does not change for different schedule pipe, but the wall thickness will. There is absolutely no reason to measure the ID of Pipe, unless it's to figure out the wall thickness.

Now tube on the other hand is the exact opposite. All tube measurements are taken on the ID, and the ID WILL be the same as the name... tube is NOT nominal, Pipe IS nominal.

Yeah, I went to buy some wood clamps that use plumbing pipe for the bar, and Sears said that their latest shipment had come in mis-labelled. Someone had complained. I made them sell me some anyway, promising to return them if there was anything wrong. :-)

All tube measurements are taken on the ID, and the ID WILL be the same as the name

Sorry, but tubing is identified by outside diameter. Inside diameter changes with wall thickness. (Wall thickness is generally specified by gauge.)

http://www.engineeringtoolbox.com/pipes-tubes-d_347.html

http://www.ryerson.com/stocklist/l-1114-Tubing-Pipe-Round-Mechanical-Tubing.html

whoops... yeah goofed that one up...correct... tube is measured OD not ID... but the name and measurement actually do match.

Why i'm trying to say as historically there was. The bore or ID on pipe sizes was true. That improvements on materials have let them produce pipe with less material. That material was remove from the inside to keep the outside dimensions the same.

Your a pipe manufacturer you come up with a new material to make pipe out of. Which lets your reduce wall thickness 15% to save production cost. Your design to keep the ID the same. So the OD has to change and be smaller.

Now how do you sell that to all the companies making fittings. That they have to change their size to fit your pipe. Or you decide to produce your own fittings to fit this new pipe. Your now have a stand alone product that does not match up with anyone else.

How do you sell that to every contractor or plumber out there to make the change. They knowing will be lock into a size that is not standard. May not be able to get repair part readily at any supply house.

If one were trying to design a comprehensive piping specfication system, you have to consider the entire piping system. It isn't just the pipe, but the fittings that the pipe has to fit into.

Piping systems are designed to have application across a great range of pressures, temperatures, flows, and fluids. Obviously the same ID of pipe can not deliver fluid in a linear volume, based on a linear increase in pressure. There are many factors.

In order to make all the pieces continue to fit together, the OD of the pipe can not increase in size for a given 'size' of pipe, and this gives rise to the notion of 'nominal' pipe size. (also because the pipe thread is not exactly a diametrically measurable feature either, as it is tapered)

The ID of the pipe changes with the Schedule of the pipe... so really, Nominal pipe size makes the most sense, but is incomplete as a specification, without the Schedule of the pipe being given with it. (and the grade of material and connection method)

Part of the problem though, arises from the evolutionary nature of (lack of) piping standards through the steam era. Typically the manufacturer who makes fittings is probably not the manufacturer who makes the pipe.. but obviously they have to work together.

I don't think I'm doing a good job of explaining it all, but in general, the pieces of the system have to work together, and 'nominal pipe size' is a reasonable solution to the complex problem.

Chris

aah these steam days.

Pipe used to be made by centrifugal casting, i.e. the od of the pipe was set and the the schedule was the amount of metal poured into the form, thus defining the wall thickness, the weight per length, strength, etc.

there isn't a 1/2" dimension on a 1/2" pipe unless you saw a bit that length off (which would make it a washer or a stand-off rather than a pipe) but after a few years it just feels like a 1/2" pipe ..........

I guess there's still a little bit of the black art left in engineering .....

Getting there guys...pipe is measured ID, while tube is measured OD.

We don't need to consider 14" and up, since those sizes will be flanged/ welded. The query reference is iro screwed pipe..

'NOMINAL' can be assumed to mean "in the close order of"

Been there, done that!

Quite right. My thinking was dyslexic tonight. Embarrassing.

The OP asked the origin of NPT size. The answer of course is the ID of the pipe; which used to match the nominal size, but has changed over the decades when wall thickness changed and was standardized, but still retains its original nomenclature.

When they made walls thinner, they kept the OD the same so that newer pipes would still fit into older fittings. But this meant that the ID no longer matched the size that it was called. Thus, the modern day mismatch between what the pipe is called, and what the actual ID is.

"When they made walls thinner"

Not just thinner.. I think Sch 160 is thicker. I'm not sure what the schedule was in the steam days.. probably 40?

chris

The higher the schedule number, the greater the wall thickness.

A thicker wall gives rise to a reduction in the actual pipe ID, as some comments have noted, so that pipes and screwed fittings may still be matched/ married.

Thus, 2" schedule 40 pipe may measure 2" ID, but 2" sched 80 pipe will have a slightly smaller ID, and 2" sched 160 pipe will have an even smaller ID.

That is why the ID is described as nominal...

(These standard variations in wall thickness only occur in seamless pipe)

yes, that is what I meant... thanks for the clarity.

Before different schedules were adopted, 1/16" wall was standard.

I do not have my piping books at this work site. But, the question refers to origin ? Not sure what origin he is querying: pipe, size, schedule, threads ? We sould also mentioned in the past, pipe (steam days) came in strong, xs, xxs, etc. let's throw more mud on a clear answer. I would like to mention that when calling out pipe, one should say 1/2" pipe, 2" pipe, or 6" pipe; never 1/2" diameter pipe. Diameter refers to a messurement, and that can cause some questions.

"...one should say 1/2" pipe, 2" pipe, or 6" pipe; never 1/2" diameter pipe."

Good point.

I'm referring to the thread. For taper metric threads we find the nominal dimension in own thread, i.e, for a thread M12x1,5k (DIN 158-1) at a given distance "L = 3 mm" from the tip of the thread, the largest diameter is 12 mm (normative). I think maybe it would be clearer if the nomenclature was similar to that or was given by OD, so 1 / 2 - 14 NPT would be 0.84 - 14 NPT and so on...that's why the standards need to be reviewed periodically...

Impossible to change at this point... it's all too imbedded in industry. To change the standards at this point would cause many people a headache. In my feeble mind, we need to slowly be moving away from NPT all together (it's nasty, leaky, once used stretched out stuff). Many new connection technologies have come to light in the past 20 years or so. I know in Hydraulics, any time we can not use NPT, we take it, and go with one of the other dozen options.

its frustrating i know. but, too many times a newby runs up a flag and thinks they got it solved. i seen a new person throw away all our catalogs because we have internet. we use the old info to understand what was used. another newby refiled our intire system so you could extrapolate one file quicker and faster; yet 10 times slower if you wanted a complete serch. just learn, grasshopper, from your elders. and accept the old ways.

This is a simple topic, becoming vey technical...sounds like lathe work now

Threading NPT pipe is usually done with adjustable dies, either by hand or with threading machine (eg Rigid Tools markets a great unit). The dies are adjustable in order to cope with varying wall thicknesses, so that for a thicker wall one might have to make 2-3 cuts in order to match the thread to the fitting being used ie the thread in the fitting is NPTF (female) and does not vary in diameter against the pressure rating of the fitting.

The measurements and tolerances you mention are theoretical only, and have little to do with actual practice. One makes a cut, check the fit, and cut a little smaller if required. The optimium size is indicated buy a mark on the dies.

All this talk of OD is irrelevant...the pipe "size" is it's 'nominal' bore or ID.

Your concern with dimensions is more appropriate parallel threads, where precision is factor.....'nuff said.

In my experience, the threads (for 3000 psi applications) were cut with essentially a depth guage.

Anyone having to acheive certification for their piping systems, really must know all this 'dimensional' stuff. and yes, it is that complicated. straight threads are easy by comparison, imho.

but what you are describing is great for electrical conduit.

thanks

Chris

Mmmm.... I don't dispute the requirement for a well cut thread in 3000psi actual process applications (eg hydraulic service), as opposed to 3000psi rated screwed fittings. Those applications will all be small-bore in any case...you will not use 2" NPT screwed pipe on a 3000psi process.

The point is that the adjustable dies are manufactured to consistently produce the ideal NPT thread, including all parameters ie length, taper OD, depth and pitch.

Technical knowledge learnt from first principles is a beautiful thing, but the overwhelming majority of applications for NPT thread are quite mundane. The precision threads I referred to previously were, for example machine bolts in high- torque applications, where a sloppy fit will strip the thread under tension.

NPT is a relatively low precision thread due to the taper, and until the joint is actually tightened, the fit is akin to a pencil in a shirt pocket...

yes. very true. ga.

"you will not use 2" NPT screwed pipe on a 3000psi process."

I have done, and most likely will again use 2"NPT pipe on a 3000psi process in industrial hydraulics. Although If I have any say in the matter I don't use any NPT stuff... but sometimes the customer want's it for one reason or another (usually to cut costs).

I don't like NPT, but have to deal with it on a daily basis. I'll take an SAE O-ring connection any day over an NPT.

Buy a Machinery's Handbook!!!!!!!

The 1/2 inch refers to the nominal pipe size. NPT sets the standards for taper and threads per inch in different sizes and styles of pipe threads like NSPT.

Nominal in engineering drawing means the theoretically perfect or standard size.

I remember reading once, and I will look around for it to see if I still have it, that pipe thread sizes originated by the dies of the most successful foundries of times gone by.

Apparently the dies of one foundry got to be so common all the other foundries had to start copying them.