INSIDE B.O.P. VALVE 
The Don Mashburn, Inc one- and two-piece I.B.O.P. Valves are dart style positive check valves in the drill string. These are non-return check valves, allowing pumping through the valve into the drill string and offering a positive one-way seal that prevents a well from an uncontrollable kick flowing up the drill string, protecting your swivel, standpipe, and pumps.