| 摘要 |
<p>A resilient annular packer, is in an annular recess in the bore of the preventer housing and sealingly engages the upper end of the recess. A resilient annular piston is in a chamber which surrounds the recess and is separated therefrom by a wall. A disc extends sealingly through the wall to transmit inward movement of the piston to the packer to contact the latter into engagement about an object in the bore or upon itself when the bore is empty. There is provision for exposing the outer surface of the packer to the pressure in the bore. The preventer is relatively short and of small radial extent and is relatively inexpensive to mfr. and the pressure opposing closure of the preventer acts on a relatively small area.</p> |
