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)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)不(bu)是(shi)(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)(lan)的(de)(de)(de)(de)(de)結構尺(chi)(chi)寸(cun)(cun)(cun)，但(dan)它是(shi)(shi)法蘭(lan)(lan)設(she)計(ji)和(he)(he)使(shi)用(yong)中不(bu)可缺少的(de)(de)(de)(de)(de)配套參數，直接(jie)影響法蘭(lan)(lan)結構尺(chi)(chi)寸(cun)(cun)(cun)。我國目前生產(chan)和(he)(he)使(shi)用(yong)的(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)，從外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)劃分有(you)兩個系(xi)(xi)列：即標(biao)準中標(biao)注的(de)(de)(de)(de)(de)“系(xi)(xi)列I”和(he)(he)“系(xi)(xi)列II”。系(xi)(xi)列I的(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)(wei)“英制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱“大(da)(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；系(xi)(xi)列I的(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)為(wei)(wei)“米制管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱“小(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。所謂“大(da)(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)及(ji)“小(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)(de)(de)含(han)義(yi)是(shi)(shi)：在相同管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道公(gong)稱通徑(jing)(jing)(jing)(jing)(jing)或(huo)公(gong)稱尺(chi)(chi)寸(cun)(cun)(cun)的(de)(de)(de)(de)(de)條件(jian)下，管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道元件(jian)（諸如法蘭(lan)(lan)、閥門、管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)件(jian)及(ji)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)道過濾器(qi)等(deng)）所配用(yong)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)尺(chi)(chi)寸(cun)(cun)(cun)的(de)(de)(de)(de)(de)大(da)(da)小(xiao)，尺(chi)(chi)寸(cun)(cun)(cun)大(da)(da)的(de)(de)(de)(de)(de)叫(jiao)“大(da)(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；尺(chi)(chi)寸(cun)(cun)(cun)小(xiao)的(de)(de)(de)(de)(de)叫(jiao)“小(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。例(li)如：公(gong)稱尺(chi)(chi)寸(cun)(cun)(cun)DN100的(de)(de)(de)(de)(de)法蘭(lan)(lan)，其(qi)配管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)為(wei)(wei)114.3mm及(ji)108mm兩種(zhong)，外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)114.3mm的(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱為(wei)(wei)大(da)(da)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)系(xi)(xi)列I管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)；外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)108mm的(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)稱為(wei)(wei)小(xiao)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)系(xi)(xi)列II管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)。鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)系(xi)(xi)列是(shi)(shi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)(lan)尺(chi)(chi)寸(cun)(cun)(cun)的(de)(de)(de)(de)(de)基礎。兩個體系(xi)(xi)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)(lan)（即用(yong)PN標(biao)記的(de)(de)(de)(de)(de)法蘭(lan)(lan)和(he)(he)用(yong)Class標(biao)記的(de)(de)(de)(de)(de)法蘭(lan)(lan)）的(de)(de)(de)(de)(de)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)見(jian)表3-1。對于公(gong)稱壓力PN250、PN320和(he)(he)PN400的(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)(lan)的(de)(de)(de)(de)(de)公(gong)稱尺(chi)(chi)寸(cun)(cun)(cun)和(he)(he)鋼(gang)管(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)見(jian)表3-2。