The kelly cock valve ,which is a manually operated ball valve run in the I.D of drill string ,is one of effective tool for preventation from blowout .The kelly cock  is divided into upper kelly cock valve and lower kelly cock . The upper kelly cock is connected between lower end of swivel and upper end of kelly. The lower kelly cock  is connected between upper end of drillpipe and lower end of kelly or connected between lower end of kelly saver subs. The kelly cock features a simple operation, the kelly cock  can be opened on or closed off by only turning the special operating spanner at 90 degrees according to indicating direction.During drilling operation,in order to avoid serious well kick or blowout accident, the kelly cock  shall be connected at upper end and lower end of kelly.

Table 1. Upper Kelly Cock