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 Table B-
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″
8¼″
10 ⅜″
15 ½″
20 ¾″
25 1″
32 1¼″
40 1½″
50 2″
65 2½″
80 3″
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