U.S. ISO Pipe Designations—DN Pipe Sizes
There’s yet another pipe-size naming system.
It is ISO pipe-size designations for U.S. IPS
or NPS pipe. This is DN pipe size as opposed
to DIN pipe standard. DNstands for diame-
ter nominal. Under this system, U.S. NPS
pipe is named for the closest metric dimen-
sion to the nominalpipe size. For instance,(^3) /
4 - inch IPS pipe is DN 20 mm pipe. It’s the
exact same pipe, with real dimensions from
Table B-4. If this were standard weight or
schedule 40 pipe, it would be 1.050-inch OD,
0 .113-inch wall (26.7 mm OD, 2.87 mm wall).
The closest DIN metric pipe would be DIN
2448, 26. 9 2. 9. NOTE: DN and DIN pipe
standards appear the same but they are dif-
ferent.Table B-2 presents DN pipe sizes and
their English-unit equivalents.
U.S. PIPE-THREAD
STANDARDS
Just as with the pipe itself there are several
standards for the threads on pipe and pipe fit-
tings. In the United States, the most common
pipe threads are National Pipe Thread (NPT),
which is a tapered pipe thread, and National
Standard Free-Fitting Straight Mechanical
Pipe Thread (NPSM), which is straight—not
tapered. Tapered threads are used for sealed
piping, while straight threads are for mechan-
ical connections only. The dry-sealed, taper-
thread standard is NPTF. This is tapered
thread used for joining pipe to seal liquid tight
without using sealants. NOTE: NPTF is the
pipe thread that should be used for fuel-sys-
tem piping. The NPTF and NPT threads are
interchangable, but only the NPTF is self-seal-
ing. All the other pipe threads are incompati-
ble with each other. Even with NPTF fittings
on fuel piping, I recommend using plumber’s
sealant (dope) on the threads. You will find
NPT fittings used in fuel systems in conjunc-
tion with plumber’s sealant on the threads.
This is acceptable, but NPTF is preferable for
all fuel-system applications.
Be aware that there are still other pipe-
thread standards such as Garden Hose
Thread (GHT), Fire Hose Coupling (NST),
and British Standard Taper Pipe Thread
(BSPT). Don’t confuse these with NPTF.APPENDIX B
Figure A-B-1. Pipe
threads (Courtesy
http://www.plumbing-
world.com)
TABLE B-2. NOMINAL U.S. IPS OR NPT
PIPE SIZE TO NOMINAL METRIC “DN”
CONVERSION CHART
Nominal Pipe Size Nominal Pipe Size
(Inches/U.S.) (Metric)(^1) /
8 "6 mm
(^3) /
16 "7 mm
(^1) /
4 "8 mm
(^3) /
8 " 10 mm
(^1) /
2 " 15 mm
(^5) /
8 " 18 mm
(^3) /
4 " 20 mm
1" 25 mm
11 / 4 " 32 mm
11 / 2 " 40 mm
2" 50 mm
21 / 2 " 65 mm
3" 80 mm
31 / 2 " 90 mm
4" 100 mm
41 / 2 " 115 mm
5" 125 mm
6" 150 mm