the gravel pack screen a flapper valve is mounted for movement about a horizontal pivot axis into engagement with the annular valve seat the flapper valve and the cooperating valve seat are both provided with spherical segment sealing surfaces so as to prevent leakage through the valve due to any misalignment of the pivot axis of the flapper valve with respect to the annular valve seat with this apparatus the withdrawal of the gravel packing apparatus at the completion of the gravel packing operations prevents the entry of undesired fluids and contaminates into the producing formation application_number US 64527184 A description referring to fig1 a and 1b there is shown a combined well completion and gravel packing apparatus 10 shown in inserted relationship within the well casing 1 having a plurality of perforations 1a disposed in vertically spaced relationship adjacent a production formation prior to insertion of the apparatus 10 a sump packer 11 may be installed by wireline at a position below the perforations 1a the well completion apparatus includes a settable packer 12 which is carried into the well by a conventional hydraulic packer setting mechanism 14 which in turn is run into the well on the bottom end of a tubular work string 5 packer 12 is threadably and sealably secured to the top end of a liner assemblage 20 which includes in descending order a ported liner element 21 a seal bore sub.