Closure apparatus for a high pressure vessel includes a closure member having a hemispherically concave interior and a stepped cylindrical exterior portion. The stepped cylindrical exterior portion is adapted to complement the cross-sectional shape of flanged sector blocks so that the sector blocks are lockingly engaged in a circumferential groove in an access opening of the high pressure vessel when the closure member is installed, thereby eliminating any requirement for a wedge ring support for the blocks. The present closure apparatus may be retrofitted into high pressure vessels already equipped with a much heavier flat plate closure. The present closure apparatus may be of the partial access type, an elliptical manway being provided in the closure member. Access through the manway allows periodic maintenance of the vessel interior without having to remove the heavy closure member.