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)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)不是(shi)管(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)的(de)(de)(de)結構(gou)尺寸(cun)(cun)(cun)，但它是(shi)法蘭(lan)設計和(he)使(shi)用(yong)(yong)中(zhong)不可缺少(shao)的(de)(de)(de)配(pei)套參數，直接影響法蘭(lan)結構(gou)尺寸(cun)(cun)(cun)。我國目前(qian)生產和(he)使(shi)用(yong)(yong)的(de)(de)(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)，從(cong)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)劃分有兩個系(xi)(xi)(xi)列(lie)(lie)(lie)：即標(biao)(biao)準中(zhong)標(biao)(biao)注的(de)(de)(de)“系(xi)(xi)(xi)列(lie)(lie)(lie)I”和(he)“系(xi)(xi)(xi)列(lie)(lie)(lie)II”。系(xi)(xi)(xi)列(lie)(lie)(lie)I的(de)(de)(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)為(wei)(wei)(wei)(wei)“英制管(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱(cheng)(cheng)“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)；系(xi)(xi)(xi)列(lie)(lie)(lie)I的(de)(de)(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)為(wei)(wei)(wei)(wei)“米(mi)制管(guan)(guan)(guan)(guan)(guan)(guan)”，俗稱(cheng)(cheng)“小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)。所(suo)謂“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)及“小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)含義(yi)是(shi)：在相同管(guan)(guan)(guan)(guan)(guan)(guan)道(dao)(dao)公(gong)(gong)稱(cheng)(cheng)通徑(jing)(jing)(jing)(jing)(jing)或(huo)(huo)(huo)公(gong)(gong)稱(cheng)(cheng)尺寸(cun)(cun)(cun)的(de)(de)(de)條件下，管(guan)(guan)(guan)(guan)(guan)(guan)道(dao)(dao)元(yuan)件（諸如(ru)法蘭(lan)、閥門、管(guan)(guan)(guan)(guan)(guan)(guan)件及管(guan)(guan)(guan)(guan)(guan)(guan)道(dao)(dao)過濾器等）所(suo)配(pei)用(yong)(yong)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)尺寸(cun)(cun)(cun)的(de)(de)(de)大(da)(da)小(xiao)，尺寸(cun)(cun)(cun)大(da)(da)的(de)(de)(de)叫(jiao)“大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)；尺寸(cun)(cun)(cun)小(xiao)的(de)(de)(de)叫(jiao)“小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)”管(guan)(guan)(guan)(guan)(guan)(guan)。例(li)如(ru)：公(gong)(gong)稱(cheng)(cheng)尺寸(cun)(cun)(cun)DN100的(de)(de)(de)法蘭(lan)，其(qi)配(pei)管(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)為(wei)(wei)(wei)(wei)114.3mm及108mm兩種，外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)114.3mm的(de)(de)(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)稱(cheng)(cheng)為(wei)(wei)(wei)(wei)大(da)(da)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)(huo)(huo)系(xi)(xi)(xi)列(lie)(lie)(lie)I管(guan)(guan)(guan)(guan)(guan)(guan)；外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)108mm的(de)(de)(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)稱(cheng)(cheng)為(wei)(wei)(wei)(wei)小(xiao)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)或(huo)(huo)(huo)系(xi)(xi)(xi)列(lie)(lie)(lie)II管(guan)(guan)(guan)(guan)(guan)(guan)。鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)系(xi)(xi)(xi)列(lie)(lie)(lie)是(shi)管(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)尺寸(cun)(cun)(cun)的(de)(de)(de)基礎。兩個體系(xi)(xi)(xi)管(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)（即用(yong)(yong)PN標(biao)(biao)記的(de)(de)(de)法蘭(lan)和(he)用(yong)(yong)Class標(biao)(biao)記的(de)(de)(de)法蘭(lan)）的(de)(de)(de)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)見(jian)表(biao)3-1。對(dui)于公(gong)(gong)稱(cheng)(cheng)壓力PN250、PN320和(he)PN400的(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)(guan)法蘭(lan)的(de)(de)(de)公(gong)(gong)稱(cheng)(cheng)尺寸(cun)(cun)(cun)和(he)鋼(gang)(gang)(gang)(gang)管(guan)(guan)(guan)(guan)(guan)(guan)外(wai)(wai)徑(jing)(jing)(jing)(jing)(jing)見(jian)表(biao)3-2。