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)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)不是管(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)的(de)結(jie)構尺(chi)(chi)(chi)寸(cun)(cun)，但它是法(fa)蘭(lan)設(she)計和(he)使用(yong)中不可缺少的(de)配套參數，直接影響法(fa)蘭(lan)結(jie)構尺(chi)(chi)(chi)寸(cun)(cun)。我(wo)國(guo)目前生產和(he)使用(yong)的(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)，從外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)劃分有兩(liang)個系(xi)(xi)列(lie)：即標(biao)(biao)(biao)準中標(biao)(biao)(biao)注(zhu)的(de)“系(xi)(xi)列(lie)I”和(he)“系(xi)(xi)列(lie)II”。系(xi)(xi)列(lie)I的(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)為“英制(zhi)管(guan)(guan)(guan)(guan)(guan)”，俗(su)稱(cheng)“大(da)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)；系(xi)(xi)列(lie)I的(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)為“米(mi)制(zhi)管(guan)(guan)(guan)(guan)(guan)”，俗(su)稱(cheng)“小(xiao)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)。所謂“大(da)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)及“小(xiao)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)的(de)含義是：在相同(tong)管(guan)(guan)(guan)(guan)(guan)道公稱(cheng)通徑(jing)(jing)(jing)(jing)或公稱(cheng)尺(chi)(chi)(chi)寸(cun)(cun)的(de)條件(jian)下，管(guan)(guan)(guan)(guan)(guan)道元件(jian)（諸如法(fa)蘭(lan)、閥門、管(guan)(guan)(guan)(guan)(guan)件(jian)及管(guan)(guan)(guan)(guan)(guan)道過濾器(qi)等）所配用(yong)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)尺(chi)(chi)(chi)寸(cun)(cun)的(de)大(da)小(xiao)，尺(chi)(chi)(chi)寸(cun)(cun)大(da)的(de)叫“大(da)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)；尺(chi)(chi)(chi)寸(cun)(cun)小(xiao)的(de)叫“小(xiao)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)。例如：公稱(cheng)尺(chi)(chi)(chi)寸(cun)(cun)DN100的(de)法(fa)蘭(lan)，其配管(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)為114.3mm及108mm兩(liang)種，外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)114.3mm的(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)稱(cheng)為大(da)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)或系(xi)(xi)列(lie)I管(guan)(guan)(guan)(guan)(guan)；外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)108mm的(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)稱(cheng)為小(xiao)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)或系(xi)(xi)列(lie)II管(guan)(guan)(guan)(guan)(guan)。鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)系(xi)(xi)列(lie)是管(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)尺(chi)(chi)(chi)寸(cun)(cun)的(de)基礎。兩(liang)個體系(xi)(xi)管(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)（即用(yong)PN標(biao)(biao)(biao)記的(de)法(fa)蘭(lan)和(he)用(yong)Class標(biao)(biao)(biao)記的(de)法(fa)蘭(lan)）的(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)見表3-1。對(dui)于公稱(cheng)壓力PN250、PN320和(he)PN400的(de)管(guan)(guan)(guan)(guan)(guan)法(fa)蘭(lan)的(de)公稱(cheng)尺(chi)(chi)(chi)寸(cun)(cun)和(he)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)見表3-2。