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 hose and case. The former is used for fluid circulation during connecting of casing. The later generally is used after connecting of casing.

drilling pipe box thread	casing thread	borehole	length(mm)
NC38	5″BTC	57	500
	5″LTC
	5″STC
NC38	5″BTC	57	580
	5-1/2″LTC
	5-1/2″STC
NC50	7″BTC	71	600
	7″LTC
	7″STC
NC50	9-5/8″BTC	71	620
	9-5/8″LTC
	9-5/8″STC
NC50	10-3/4″BTC	71	670
	10-3/4″LTC
	10-3/4"STC
NC50	13-3/8″BTC	71	670
	13-3/8″LTC
	13-3/8″STC