在管道系統中，管道元件（含法蘭及墊片）公稱通徑或公稱尺寸是管路的一個重要參數。管法蘭及墊片公稱通徑的定義、標記及數值應遵循GB/T 1047-2005《管道元件DN（公稱尺寸）的定義及選用》的規定。法蘭及墊片公稱通徑或公稱尺寸的標記用DN及NPS兩個系列的標記表示：公稱直徑或公稱尺寸用DN標記的法蘭及墊片，DN的單位為“mm”；公稱通徑或公稱尺寸用NPS標記的法蘭及墊片，NPS的單位為“in(英寸）”，1in=25.4mm.標準鋼制管法蘭的公稱通徑或公稱尺寸系列見表3-1。

 鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)不(bu)是(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)的(de)(de)(de)(de)(de)結(jie)構尺(chi)(chi)(chi)寸(cun)(cun)，但(dan)它是(shi)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)設(she)計和使(shi)(shi)用中(zhong)不(bu)可缺少的(de)(de)(de)(de)(de)配套參數，直接影(ying)響(xiang)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)結(jie)構尺(chi)(chi)(chi)寸(cun)(cun)。我(wo)國(guo)目前生產和使(shi)(shi)用的(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)，從外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)劃分(fen)有兩(liang)個系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)：即(ji)標準中(zhong)標注的(de)(de)(de)(de)(de)“系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)I”和“系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)II”。系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)I的(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)“英(ying)制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)；系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)I的(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)“米制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱“小(xiao)(xiao)(xiao)(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)。所謂“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)及(ji)“小(xiao)(xiao)(xiao)(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)(de)(de)含(han)義(yi)是(shi)：在相同管(guan)(guan)(guan)(guan)(guan)(guan)(guan)道公(gong)(gong)稱通徑(jing)(jing)(jing)(jing)(jing)或(huo)公(gong)(gong)稱尺(chi)(chi)(chi)寸(cun)(cun)的(de)(de)(de)(de)(de)條件下(xia)，管(guan)(guan)(guan)(guan)(guan)(guan)(guan)道元件（諸如(ru)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)、閥門、管(guan)(guan)(guan)(guan)(guan)(guan)(guan)件及(ji)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)道過(guo)濾器(qi)等）所配用鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)尺(chi)(chi)(chi)寸(cun)(cun)的(de)(de)(de)(de)(de)大(da)(da)小(xiao)(xiao)(xiao)(xiao)，尺(chi)(chi)(chi)寸(cun)(cun)大(da)(da)的(de)(de)(de)(de)(de)叫(jiao)“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)；尺(chi)(chi)(chi)寸(cun)(cun)小(xiao)(xiao)(xiao)(xiao)的(de)(de)(de)(de)(de)叫(jiao)“小(xiao)(xiao)(xiao)(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)。例如(ru)：公(gong)(gong)稱尺(chi)(chi)(chi)寸(cun)(cun)DN100的(de)(de)(de)(de)(de)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)，其配管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)為(wei)114.3mm及(ji)108mm兩(liang)種，外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)114.3mm的(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱為(wei)大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)I管(guan)(guan)(guan)(guan)(guan)(guan)(guan)；外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)108mm的(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱為(wei)小(xiao)(xiao)(xiao)(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)II管(guan)(guan)(guan)(guan)(guan)(guan)(guan)。鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)系(xi)(xi)(xi)(xi)(xi)列(lie)(lie)是(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)尺(chi)(chi)(chi)寸(cun)(cun)的(de)(de)(de)(de)(de)基礎(chu)。兩(liang)個體系(xi)(xi)(xi)(xi)(xi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)（即(ji)用PN標記(ji)(ji)的(de)(de)(de)(de)(de)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)和用Class標記(ji)(ji)的(de)(de)(de)(de)(de)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)）的(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)見(jian)表3-1。對于公(gong)(gong)稱壓力PN250、PN320和PN400的(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)(fa)(fa)蘭(lan)(lan)(lan)的(de)(de)(de)(de)(de)公(gong)(gong)稱尺(chi)(chi)(chi)寸(cun)(cun)和鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)見(jian)表3-2。