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)(wai)徑(jing)(jing)(jing)不(bu)是管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)的(de)(de)(de)結構尺寸(cun)(cun)，但它是法(fa)蘭(lan)(lan)設(she)計(ji)和(he)使(shi)用(yong)(yong)中(zhong)不(bu)可缺少的(de)(de)(de)配(pei)套參數，直接影響法(fa)蘭(lan)(lan)結構尺寸(cun)(cun)。我國目前生產(chan)和(he)使(shi)用(yong)(yong)的(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)，從外(wai)(wai)(wai)徑(jing)(jing)(jing)劃分有(you)兩個(ge)系(xi)(xi)列：即標(biao)準中(zhong)標(biao)注的(de)(de)(de)“系(xi)(xi)列I”和(he)“系(xi)(xi)列II”。系(xi)(xi)列I的(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)“英制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗(su)稱(cheng)(cheng)“大(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)；系(xi)(xi)列I的(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)“米(mi)制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗(su)稱(cheng)(cheng)“小(xiao)(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)。所(suo)謂“大(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)及(ji)“小(xiao)(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)含義是：在相同管(guan)(guan)(guan)(guan)(guan)(guan)(guan)道公稱(cheng)(cheng)通徑(jing)(jing)(jing)或公稱(cheng)(cheng)尺寸(cun)(cun)的(de)(de)(de)條件下，管(guan)(guan)(guan)(guan)(guan)(guan)(guan)道元件（諸如法(fa)蘭(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)濾器等）所(suo)配(pei)用(yong)(yong)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)尺寸(cun)(cun)的(de)(de)(de)大(da)小(xiao)(xiao)，尺寸(cun)(cun)大(da)的(de)(de)(de)叫“大(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)；尺寸(cun)(cun)小(xiao)(xiao)的(de)(de)(de)叫“小(xiao)(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)。例(li)如：公稱(cheng)(cheng)尺寸(cun)(cun)DN100的(de)(de)(de)法(fa)蘭(lan)(lan)，其配(pei)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)為(wei)114.3mm及(ji)108mm兩種，外(wai)(wai)(wai)徑(jing)(jing)(jing)114.3mm的(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱(cheng)(cheng)為(wei)大(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)或系(xi)(xi)列I管(guan)(guan)(guan)(guan)(guan)(guan)(guan)；外(wai)(wai)(wai)徑(jing)(jing)(jing)108mm的(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱(cheng)(cheng)為(wei)小(xiao)(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)或系(xi)(xi)列II管(guan)(guan)(guan)(guan)(guan)(guan)(guan)。鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)系(xi)(xi)列是管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)尺寸(cun)(cun)的(de)(de)(de)基礎。兩個(ge)體系(xi)(xi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)（即用(yong)(yong)PN標(biao)記(ji)的(de)(de)(de)法(fa)蘭(lan)(lan)和(he)用(yong)(yong)Class標(biao)記(ji)的(de)(de)(de)法(fa)蘭(lan)(lan)）的(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)見(jian)表(biao)3-1。對于公稱(cheng)(cheng)壓力PN250、PN320和(he)PN400的(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)(lan)的(de)(de)(de)公稱(cheng)(cheng)尺寸(cun)(cun)和(he)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)見(jian)表(biao)3-2。