Mud saver valve

A mud saver valve, which comprises:

• a tubular body connectable between the kelly and the drill string;
• an annular seat ring mounted within said body, said seat ring having a central opening;
• a piston axially movably disposed within said body for engagement with said seat ring, said piston having a bore substantially coaxially aligned with said central opening and a flange extending radially outwardly from said piston to slidingly engage the interior of said body, said piston having a port above said flange communicating the exterior of said piston with said bore;
• a plug removably mounted in said piston above said port to normally close said bore, said plug including a shear ring, removably inserted in said bore and a spear axially movably mounted in said shear ring for movement between a first position wherein said spear sealingly engages said shear ring and a second position to allow fluid to flow upwardly between said spear and said shear ring, said shear ring including a central aperture, and said spear including a first portion sealingly enageable with said central aperture of said shear ring, a second portion below said first portion forming a flow passage between said spear and said central aperture, and means for limiting the upward travel of said spear with respect to said shear ring;
• and means for urging said piston into engagement with said seat ring.