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The circulating sub is connected between wellhead pipe string and ground circulating system. When casing is running or finished running, the circulating sub is connected to make fluid circulation. According to the application, the circulating sub is connected between Kelly and casing or between casing and hose. The former is used for fluid circulation during connecting of casing. The later generally is used after connecting of casing.
Construction
The circulating sub during running casing is equipped with casing pin thread on one end or DP thread and DP box thread or union thread on other end
 31/2″EUE.59×17.5F Mid Joint |  27/8″EUE.59×17.5F Mid Joint |  31/2″EUE.59×17.5FCable protector |
 27/8″EUE.59×17.5FCable protector |  Stuffing box |  Sucker Rod Series |
 Tubing |  Pumping Unit |