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)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)不是(shi)(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)蘭(lan)的(de)(de)(de)(de)(de)(de)結構尺寸，但(dan)它是(shi)(shi)法(fa)(fa)蘭(lan)設計和(he)(he)使用中(zhong)不可缺少的(de)(de)(de)(de)(de)(de)配套參(can)數，直(zhi)接(jie)影響(xiang)法(fa)(fa)蘭(lan)結構尺寸。我國目前生產和(he)(he)使用的(de)(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)，從外(wai)(wai)徑(jing)(jing)(jing)(jing)劃分有(you)兩(liang)個系(xi)列：即標準中(zhong)標注的(de)(de)(de)(de)(de)(de)“系(xi)列I”和(he)(he)“系(xi)列II”。系(xi)列I的(de)(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)為“英制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗(su)稱(cheng)(cheng)“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；系(xi)列I的(de)(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)為“米制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗(su)稱(cheng)(cheng)“小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。所謂(wei)“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)及(ji)(ji)“小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)(de)(de)(de)含義是(shi)(shi)：在相同管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道公稱(cheng)(cheng)通徑(jing)(jing)(jing)(jing)或公稱(cheng)(cheng)尺寸的(de)(de)(de)(de)(de)(de)條件下，管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道元件（諸如法(fa)(fa)蘭(lan)、閥(fa)門、管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)件及(ji)(ji)管(guan)(guan)(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)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)尺寸的(de)(de)(de)(de)(de)(de)大(da)(da)小(xiao)，尺寸大(da)(da)的(de)(de)(de)(de)(de)(de)叫“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；尺寸小(xiao)的(de)(de)(de)(de)(de)(de)叫“小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。例如：公稱(cheng)(cheng)尺寸DN100的(de)(de)(de)(de)(de)(de)法(fa)(fa)蘭(lan)，其(qi)配管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)為114.3mm及(ji)(ji)108mm兩(liang)種，外(wai)(wai)徑(jing)(jing)(jing)(jing)114.3mm的(de)(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱(cheng)(cheng)為大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)或系(xi)列I管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；外(wai)(wai)徑(jing)(jing)(jing)(jing)108mm的(de)(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱(cheng)(cheng)為小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)或系(xi)列II管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)系(xi)列是(shi)(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)蘭(lan)尺寸的(de)(de)(de)(de)(de)(de)基礎(chu)。兩(liang)個體系(xi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法(fa)(fa)蘭(lan)（即用PN標記的(de)(de)(de)(de)(de)(de)法(fa)(fa)蘭(lan)和(he)(he)用Class標記的(de)(de)(de)(de)(de)(de)法(fa)(fa)蘭(lan)）的(de)(de)(de)(de)(de)(de)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)見(jian)表(biao)3-1。對(dui)于公稱(cheng)(cheng)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)法(fa)(fa)蘭(lan)的(de)(de)(de)(de)(de)(de)公稱(cheng)(cheng)尺寸和(he)(he)鋼(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)見(jian)表(biao)3-2。