试着翻一下，不对请指正： the size/ diameter of control valve body shall be reduced by two size cla ... 常见管道公称直径分为下列等级 公制 15 20 25 40 50 65 80 100 150 200 250 300 350 400 450 500 600 英制 1/2 3/4 1 1-1/2 2 2-1/2 3 4 6 8 10 12 14 16 18 20 24\ 公制单位为mm，英制单位寸" 以上为管道公称直径，实际外径不同 缩两级就是往前数两个档那个等级的公称直径，比如dn80的缩两级就是dn50 至于缩两级翻译成英语我觉得应该是“the nominal diameter ( dn ) of control valve should be two level less than the pipeline ”

试着翻一下，不对请指正： the size/ diameter of control valve body shall be reduced by two size classes than pipeline mounted on.

控制阀的口径比管道口径缩两级 the control valve port size shall be two grade less than the pipe size.