
B: Metric & Imperial Nominal Sizes
ST18 & ST22 (U.S.A. & Mexico) Operation & Maintenance
Appendix B:
88291013-137 R05
68
Subject to EAR, ECCN EAR99 and related export control restrictions.
Metric & Imperial Nominal Sizes
Metric nominal size (DN) is the nominal size for valves,
flanges and fittings as defined by ISO 6708-1980. DN
designates the size of all components in a piping system
other than components designated by outside diameters
or by thread size. For example, steel tubes are desig-
nated and ordered by outside diameter and thickness.
A metric nominal size is indicated by the letters “DN” fol-
lowed by a number
without
any unit of measure. For met-
ric to imperial nominal size equivalents, refer to
1: Metric to Imperial nominal sizes
NOTE
Because DN is a round number that
cannot
be
measured and is only
loosely related
to manu-
facturing dimensions, you should
only
use the
DN number for reference and
never
for calcula-
tions.
Table B-1: Metric to Imperial nominal sizes
Metric DN
(Nominal Size)
Imperial NS
(Nominal Size)
A dimensionless designation
Designated in inches
sometimes as ND, NB or NPS
6
⅛″
8
¼
″
10
⅜″
15
½
″
20
¾
″
25
1
″
32
1¼
″
40
1½
″
50
2
″
65
2½
″
80
3
″