Confusion on ISO parallel thread naming convention 2

[Cheetos]

I recently looked up a compression fitting: 1/8" tube fitting x 1/8" male ISO parallel thread. The problem I have is with the 1/8" male ISO parallel thread part. I bought the part and I measured the thread OD and it's a little over 3/8". I got the same thing when I downloaded the 3D model from Swagelok. I then read up on internet and it says the nominal male thread diameter is 0.383. Can anyone help me understand where does the 1/8" come from? There's nothing on the thread side that resembles 1/8" thread.

 

[Pud]

Save you looking it up.
From Wikipedia:

The standard for Nominal Pipe Size (often abbreviated NPS, which should not be confused with the symbol NPS for the straight thread form standard) is loosely related to the inside diameter of Schedule 40 series of sizes. Because of the pipe wall thickness of Schedule pipe, the actual diameter of the NPT threads is larger than the Nominal Pipe Size outside diameter, and considerably so for small sizes. Pipe of a given size in a different Schedule than Schedule 40 provides a different wall thickness, while maintaining the same outside diameter and thread profile as Schedule 40. Thus the inside diameter of a given size of Schedule pipe differs from the Nominal Pipe Size, while outside diameters for a given nominal Schedule size are the same between Schedules.


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[TugboatEng]

When discussion pipe size under 14 inch, accept that the number denoting size is just telling you what row to look at on your dimensions chart and is otherwise arbitrary.

 

[EdStainless]

Not completely arbitrary, 3/8NPT is larger than 1/4NPT.
It is just that you don't know bigger by how much or what size either them really is until you look in the table.
Pipe sizes don't match the name until you get to 14".

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P.E. Metallurgy, consulting work welcomed

 

[Cheetos]

appreciate everyone's help. Still can't believe the number denoting size is telling you what row to look at until you reach a certain size. I'm glad to learn something new today.

 

[Jboggs]

Cheetos - There's actually some engineering history here. Engineers have to use the inside diameters of pipe for their flow calculations. So as iron pipes first came into use they were produced in standard ID sizes. A 1" pipe had a 1" ID. The job of connecting pipes was a challenge. As machines for cutting threads became available, the need for standardized thread sizes became obvious. In order to avoid reducing the effective flow ID of a pipe, connection threads are always on the OD. That meant that both pipe OD's and thread sizes had to be standardized. So the OD of a 1" ID pipe (made of iron or a low grade steel) was standardized at 1.315". That allowed the threads for a 1" pipe to also be standardized, and standard thread tooling became available. All's well.

Time passed. Better grades of pipe materials became available, which meant that wall thicknesses for standard pressure ratings could be reduced. But reducing the standard OD of pipes also meant changing all the standard thread sizes, and all related tooling, and all standardized labeling and inventory factors, etc etc. Or... they could just say that going forward the standard ID of a pipe that accepts a thread labeled as "1" NPT" is now 1.049. That was MUCH easier.

A similar argument applies to explain why the ID of sch. 80 pipe is less than the ID for sch. 40 pipe.

So, we are still living today with pipe thread sizes that were originally created for pipe made from lower grade materials.

That's why every young plumber or engineer encounters this same confusing situation when they learn that there is NOTHING about a 1" pipe that is actually 1".

 

[XL83NL]

Is this for a swagelok Rs or Rp fitting?
Better look up the standard to which conforms next time to be familiar with what you’re going to buy in case it’s new stuff for you.

Huub
- You never get what you expect, you only get what you inspect.