ASME B16.21-2011 pdf download – Nonmetallic Flat Gaskets for Pipe Flanges.
2.6 Size
NPS, followed by a dimensionless number, is the designation for nominal pipe size, as described in ASME B36.1OM and is related to the reference nominal diameter, DN, used in international standards. The relationship is as follows:
GENERAL NOTE: For NI’S  4, the related DN = 25 x NIS
2.7 Pressure Class Designation
Class, followed by a dimensionless number, is the designation for common flange pressure-temperature ratings as given by the referenced flange standards.
3 MATERIALS
3.1 Composition
Gaskets shall be made of resilient or pliable materials. Metal or nonmetal composites may be incorporated as reinforcement or filler material.
3.2 Service Requirements
Selection of a material suitable for a given service application is the responsibility of the user subject to the requirements of any applicable code or government regulation. The material selected shall be compatible with the fluid and suitable for the pressure-temperature conditions of the service.
4 DIMENSIONS AND TOLERANCES
4.1 Dimensions
Gasket dimensions shall be in accordance with Tables 1 through 9 (Tables I-i through 1-9 of Mandatory Appendix I) for the flanges’ standards, sizes, and classes indicated. Selection of gasket thickness is the responsibility of the user, considering the properties of the gasket material as well as the intended service application.
(4) center to center of adjacent bolt holes, ±1.0 mm (±0.03 in.)
5 MARKINGS
5.1 Material Marking
Unless otherwise specified by the purchaser, gasket material shall be marked with the material manufacturer’s name or trademark and material desigration.
5.2 Gasket Marking
Gaskets shall be packaged with NPS, class, gasket manufacturer’s name, material designation, thickness, md ASME B16.21 marked or tagged on the package or gasket. At the option of the manufacturer, the DN designation may be included.