sizing pipe threaded elements 2

[Guideon]

Hi to everyone,

I have a problem with sizing pipe threaded elements. I am looking for guide rules that with simple external meassurement I'll be able to determine the element size such as safety valves,threaded pipes,pressure gages,caps etc.
To clarify my question, hereby an example, when I meassure 1/2" safety valve, I can't relate the 1/2" to the actual possible meassurements (ext/int dia)of this valve.
The same problem, when meassuring a threaded hole in a vessele..
I hope that I need not to take several meassurements(ext&int dia, thickness, pitch, number of threads per inch...) and than look at the tables to which size it fits.

Thanks, Guideon

 

[Compositepro]

Pipe sizes are nominal values only. There are numerous sources for these dimensions on the net. The Machinery Handbook and McMaster-Carr catalog have tables. Memorize these numbers if you work with pipe.

 

[MintJulep]

It's really very simple.

If you measure a pipe, and no dimension is 1", then it's 1" pipe.

 

[JohnRBaker]

For NPS schedule 40, often referred to as 'Standard Iron Pipe', the nominal 'size' is based on the EFFECTIVE flow capacity, NOT the OD, nor even the ID for that matter, of the pipe's cross-section.

But if you're just looking for an quick & easy chart, this one seems to cover at least the American standards:

http://www.engineersedge.com/pipe_schedules.htm

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[racookpe1978]

"I hope that I need not to take several meassurements(ext&int dia, thickness, pitch, number of threads per inch...) and than look at the tables to which size it fits."

Get the OD. If a threaded (tapped) hole, use the max diameter of the tapped threads - NOT the inside dia of the threads.

Then look it up in the table. After 363 look-up's in the table, you will have memorized the table. Ain't no easier way.

...


You're trying to apply measurements and logics to a pipe nominalclature system that has grown up from the initial cast iron pipe and wooden fitting molds of the earliest days of the industrial revolution. Then, a 1/2 inch pipe WAS 1/2 inch diameter. The cast fittings were then sized to fit the OD of the iron pipe.

As pressures got higher - from 20 psi to 40 to 60 to 80 for example, the fitting molders and casting businesses refused to change their fittings and molds (since that change would be expensive), so the OD stayed the same and the ID got smaller. Then, as steel pipes came up and became popular, the pipe walls could be thinner than they were with the original iron pipes, but (again) the fittings stayed the same, so the ID became larger than the nominal ID (the ID of a 1/2 inch pipe got bigger than 1/2 inch) so engineers could get more flow for the same size pipe. Pressures went up again, but now nobody wanted to change OD, so new "schedules" were invented locking the OD and allowing ever smaller ID's to allow different pressure ratings.

After a few more years, steam pressures had to go over 150 psi (the reason we call them 150 "lb" rating for example) but nobody wanted to change OD's. So, the fitting manufactoring industry invented cast fittings with the same ID of the fitting (= OD of the old original iron pipes) but with ever-greater fitting wall thickness for 300 lb, 600 lb, 900 lb 3000 lb etc steam pressures.

Tubing, on the other hand, is specified by OD and wall thickness. Pipe? Nope. 8<)

 

[Guideon]

I could not ask a better answer than that. Thank you for the useful info.
Guideon

 

[TBP]

Pipe in the later 1800's was measured off the ID. When it was determined that thinner walls could be used, the extra ID was indeed beneficial for increased flow. But the main reason for NOT changing the OD, was the huge installed base of screwed piping in ships, locomotives and factories. To change the OD would mean that ALL of the installed fittings & valves wouldn't match the new OD of the pipe. In addition, NONE of the threading (external & internal) tools would work anymore.

I don't know exactly when the change was made, but I have a steamfitters book published in 1890 that lists the same pipe & thread dimensions that are still used today.

Schedules for wall thickness came about with the advent of welded fittings. Since the wall thickness of pipe to be welded did not have to factor-in a thread being cut, it could be thinner. This lead to a kind of "wild west" situation for a few years, until the ASME people standardized things in 1935.

 

[Guideon]

Thank you all for the info.
Guideon