DN50 (Gasket path)
PN2.5 (gasket pressure, equivalent to 25kg pressure)
D1221 indicates that the gasket is with inner and outer ring, and is a steel strip graphite filled with inner and outer ring 304 of carbon steel.
If you get a winding pad (two methods to measure size)
1: With calipers (ordinary ruler also line) measuring the inner diameter and outside diameter of the gasket, plus a gasket thickness can be, the general gasket thickness is 3.2mm and 4.5mm
2: With the diameter of the gasket and pressure, can be found in accordance with the standards in the book, the general standard is HG20610 or GB GB4622.2