異徑管又稱大小頭，用于兩種不同管徑的連接，分為同心異徑管（同心大小頭）和偏心異徑管（偏心大小頭）兩種，如圖16.33所示。

   異徑管的質量應符合現行國家標準GB/T 12459《鋼制對焊無縫管件》及GB/T 13401《鋼板制對焊管件》的規定。異徑管的材質執行 SY/T 5037、GB/T 9711、GB/T 8163、ASTM A106/A53 GRB,API 5L、ASTM A105、ASTM A234、DIN 德國標準或按客戶要求。

   異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)(de)(de)(de)(de)選擇：根據(ju)等(deng)強度的(de)(de)(de)(de)(de)(de)(de)原則(ze)，異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)應采(cai)(cai)(cai)用(yong)與所連接的(de)(de)(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)子(zi)相同(tong)的(de)(de)(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)子(zi)表號。是(shi)選擇同(tong)心異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)還(huan)是(shi)偏心異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)時，應根據(ju)工藝流(liu)程和(he)配管(guan)(guan)(guan)(guan)(guan)布置的(de)(de)(de)(de)(de)(de)(de)要求而定。通(tong)常情況(kuang)下，對于DN≥50mm的(de)(de)(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)道上的(de)(de)(de)(de)(de)(de)(de)異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)，多采(cai)(cai)(cai)用(yong)對焊(han)異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)，而對于DNDN≤40mm的(de)(de)(de)(de)(de)(de)(de)管(guan)(guan)(guan)(guan)(guan)道，則(ze)采(cai)(cai)(cai)用(yong)承插焊(han)異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)。但鍍鋅管(guan)(guan)(guan)(guan)(guan)道上的(de)(de)(de)(de)(de)(de)(de)異(yi)徑(jing)(jing)管(guan)(guan)(guan)(guan)(guan)則(ze)要采(cai)(cai)(cai)用(yong)螺紋連接形式。

   異徑(jing)(jing)管件公(gong)稱(cheng)通(tong)(tong)徑(jing)(jing)的表示方法：對(dui)于異徑(jing)(jing)彎頭和(he)異徑(jing)(jing)雙接口管箍，首先給(gei)出大端(duan)的公(gong)稱(cheng)通(tong)(tong)徑(jing)(jing)，然(ran)后為小端(duan)的公(gong)稱(cheng)通(tong)(tong)徑(jing)(jing)。

   異徑管的表示方(fang)法如下：

        （RC或RE）DN100×50 20 GB 8163 BW GB/T 12459（B系列）Sch20（L）×Sch40（S）

   其中，RC為(wei)(wei)(wei)(wei)同(tong)心；RE為(wei)(wei)(wei)(wei)偏心；100為(wei)(wei)(wei)(wei)大端公稱(cheng)直徑；50為(wei)(wei)(wei)(wei)小(xiao)(xiao)端公稱(cheng)直徑；20為(wei)(wei)(wei)(wei)材料；GB 8163為(wei)(wei)(wei)(wei)標準號；BW為(wei)(wei)(wei)(wei)連接(jie)方式；GB／T 12459（B系(xi)列）為(wei)(wei)(wei)(wei)系(xi)列號；Sch20（L）為(wei)(wei)(wei)(wei)大端管(guan)子(zi)表號；Sch40（S）為(wei)(wei)(wei)(wei)小(xiao)(xiao)端管(guan)子(zi)表號。

   異徑管連(lian)接的管件選擇見(jian)表16.80。