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)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)徑(jing)(jing)(jing)(jing)(jing)不是(shi)(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)(lan)(lan)的(de)(de)(de)(de)(de)(de)結構尺(chi)(chi)(chi)(chi)寸(cun)，但它(ta)是(shi)(shi)法(fa)蘭(lan)(lan)(lan)(lan)設計和(he)(he)使(shi)用中不可缺(que)少的(de)(de)(de)(de)(de)(de)配(pei)套參數，直接影響法(fa)蘭(lan)(lan)(lan)(lan)結構尺(chi)(chi)(chi)(chi)寸(cun)。我國(guo)目(mu)前生(sheng)產和(he)(he)使(shi)用的(de)(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)，從外(wai)徑(jing)(jing)(jing)(jing)(jing)劃分有兩(liang)個(ge)系(xi)(xi)列(lie)：即(ji)標準中標注的(de)(de)(de)(de)(de)(de)“系(xi)(xi)列(lie)I”和(he)(he)“系(xi)(xi)列(lie)II”。系(xi)(xi)列(lie)I的(de)(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)(wei)(wei)“英(ying)制(zhi)(zhi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱“大(da)外(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；系(xi)(xi)列(lie)I的(de)(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)(wei)(wei)“米(mi)制(zhi)(zhi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱“小(xiao)外(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。所謂“大(da)外(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)及(ji)“小(xiao)外(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)(de)(de)(de)含(han)義是(shi)(shi)：在相同管(guan)(guan)(guan)(guan)(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)(chi)寸(cun)的(de)(de)(de)(de)(de)(de)條(tiao)件下(xia)，管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道元(yuan)件（諸(zhu)如(ru)法(fa)蘭(lan)(lan)(lan)(lan)、閥門、管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)件及(ji)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道過濾器(qi)等）所配(pei)用鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)徑(jing)(jing)(jing)(jing)(jing)尺(chi)(chi)(chi)(chi)寸(cun)的(de)(de)(de)(de)(de)(de)大(da)小(xiao)，尺(chi)(chi)(chi)(chi)寸(cun)大(da)的(de)(de)(de)(de)(de)(de)叫“大(da)外(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；尺(chi)(chi)(chi)(chi)寸(cun)小(xiao)的(de)(de)(de)(de)(de)(de)叫“小(xiao)外(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。例如(ru)：公(gong)(gong)稱尺(chi)(chi)(chi)(chi)寸(cun)DN100的(de)(de)(de)(de)(de)(de)法(fa)蘭(lan)(lan)(lan)(lan)，其配(pei)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)徑(jing)(jing)(jing)(jing)(jing)為(wei)(wei)(wei)114.3mm及(ji)108mm兩(liang)種，外(wai)徑(jing)(jing)(jing)(jing)(jing)114.3mm的(de)(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱為(wei)(wei)(wei)大(da)外(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)系(xi)(xi)列(lie)I管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；外(wai)徑(jing)(jing)(jing)(jing)(jing)108mm的(de)(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱為(wei)(wei)(wei)小(xiao)外(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)系(xi)(xi)列(lie)II管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)徑(jing)(jing)(jing)(jing)(jing)系(xi)(xi)列(lie)是(shi)(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)(lan)(lan)尺(chi)(chi)(chi)(chi)寸(cun)的(de)(de)(de)(de)(de)(de)基礎。兩(liang)個(ge)體系(xi)(xi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)(lan)(lan)（即(ji)用PN標記(ji)的(de)(de)(de)(de)(de)(de)法(fa)蘭(lan)(lan)(lan)(lan)和(he)(he)用Class標記(ji)的(de)(de)(de)(de)(de)(de)法(fa)蘭(lan)(lan)(lan)(lan)）的(de)(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)徑(jing)(jing)(jing)(jing)(jing)見表(biao)3-1。對于公(gong)(gong)稱壓力PN250、PN320和(he)(he)PN400的(de)(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)(lan)(lan)的(de)(de)(de)(de)(de)(de)公(gong)(gong)稱尺(chi)(chi)(chi)(chi)寸(cun)和(he)(he)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)徑(jing)(jing)(jing)(jing)(jing)見表(biao)3-2。