thread bigger than the pipe it is made on !?

[YuriB]

For instance
NPS 1 inch (DN 25) having outside diameter 33.4 mm will have BSPP thread OD 33.25mm;
NPS 2 inch (DN 50) having outside diameter 60.33 mm will have BSPP thread OD 59.61mm;
and so on.
But how come that NPS 2 1/2 inch (DN 65) having outside diameter 73.03 mm could (allegedly) have BSPP thread OD 75.18 mm !?

 

[pennpiper]

Maybe the answer to this mystery lies in the definition of the word "Nominal"

prognosis: Lead or Lag

 

[YuriB]

To make it more clear I put it as the table.
DN OD BSPP
6 10.29 9.73
8 13.72 13.16
10 17.15 16.66
15 21.34 20.95
20 26.67 26.44
25 33.4 33.25
40 48.26 47.8
50 60.33 59.61
65 73.03 75.18 Why this size stands out from the pattern indeed contradicting "common sense", so to say?

 

[YuriB]

Why "mystery" just on this size - not before neither after it ? And how definition of the adjective "nominal" can desolve this mystery ?

 

[pennpiper]

nom·i·nal
[nom-uh-nl] adjective 1. being such in name only; so-called; putative: a nominal treaty; the nominal head of the country.

prognosis: Lead or Lag

 

[racookpe1978]

A nominal 65 mm pipe is NOT 65 mm dia - inside, outside, or pipe thread (actual outside - (% thread engagement factor)x2) .

Nominal mm pipe sizes are based on arbitrary almost-metric equal "names" for actual ANSI/STD pipe diamters "translated" into near-metric equivilents "stated" diameters.
That is, a 100 mm "pipe" is really a 4 inch pipe, which isn't 4 inches diameter anyway.

 

[YuriB]

To put my qwestion an other way. Why on ALL sizes the OD of a pipe decreases slightly as the BSPP thread is made on it (in the form of fillings primarily) but only on THAT exactly "nominal" sise the pipe's OD increases mysteriously ?

 

[pennpiper]

There must be an error on the data you are looking at?
"That which cannot be, therefore is not" Simple!

prognosis: Lead or Lag

 

[YuriB]

The error must be creeping from one source to another as I have seen the numbers in many ones.
Anyway, Merry Christmas !

 

[europipe]

Found that the OD of dn65 pipe=76.1mm, maybe this explains it is in the same range.
Check british standard and DIN pipe OD's,
where did You get the 73.03mm?
Greetings

 

[YuriB]

Thank you, that one is credible. There are others, however less flagrant discrepencies in DN to actual mm conversions.

 

[europipe]

You looked at the ASTM range, not British Std.

 

[PavelManolev]

By the memory there could be different solutions for one DN size, with one called preferred size. I don't know if this is the case or probably as mentioned by europipe matter of diff standards. It is not possible to manufacture such a thread size by cutting "smaller" pipe (OK there are some technologies which could apply plastic deformation to increase the size by I don't think is practical for threads). and the last but not least, engineering companies try to avoid such a size DN 65 since it is not wide spread (could be hard to find it).

 

[europipe]

Pavel, like to concur with You, plastics came a long time after BSP thread and dn65 and 2.5" are still everywhere used especially in firewater- and foam systems.
We like to avoid it in new processlines-situations, but there are still many pumps and other equipment with
2.5" connections.
Here the table with difference in BS and USA pipe

http://www.marylandmetrics.com/tech/pipecomp.htm