A compressor assembly, comprising: a compressor casing, wherein an inner surface of the compressor casing defines an interior chamber and an outer surface opposes the inner surface, the compressor casing defining an access opening extending between the inner and outer surfaces (2, 3); and a closure device at least partially disposed within the access opening, the closure device comprising: a cylindrical plug at least partially disposable within the access opening, the outer surface of the compressor casing disposed radially outward from the inner surface, wherein the cylindrical plug is configured to substantially obstruct the access opening; a shoulder extending radially outward from the cylindrical plug, and characterized in that the shoulder is at least partially disposable against the inner surface of the compressor casing, the shoulder being biasable against the inner surface by pressure within the compressor casing such that pressure within the compressor casing forces the shoulder into contact with the inner surface to retain the cylindrical plug at least partially disposed within the access opening; and a sealing member disposed circumferentially about the cylindrical plug and configured to substantially prevent fluid flow between the cylindrical plug and the access opening, wherein the access opening comprises a main bore section and a counterbore section extending into the main bore section from the inner surface of the compressor casing, and a retention surface extending radially between the main bore section and the counterbore section, wherein the access opening has a centerline and the retention surface extending circumferentially about the centerline; and wherein the closure device has a central axis, the shoulder extending circumferentially about the central axis and being disposable against the retention surface such that the central axis and the centerline are substantially collinear. a compressor casing, wherein an inner surface of the compressor casing defines an interior chamber and an outer surface opposes the inner surface, the compressor casing defining an access opening extending between the inner and outer surfaces (2, 3); and a closure device at least partially disposed within the access opening, the closure device comprising: a cylindrical plug at least partially disposable within the access opening, the outer surface of the compressor casing disposed radially outward from the inner surface, wherein the cylindrical plug is configured to substantially obstruct the access opening; a shoulder extending radially outward from the cylindrical plug, and characterized in that the shoulder is at least partially disposable against the inner surface of the compressor casing, the shoulder being biasable against the inner surface by pressure within the compressor casing such that pressure within the compressor casing forces the shoulder into contact with the inner surface to retain the cylindrical plug at least partially disposed within the access opening; and a sealing member disposed circumferentially about the cylindrical plug and configured to substantially prevent fluid flow between the cylindrical plug and the access opening, wherein the access opening comprises a main bore section and a counterbore section extending into the main bore section from the inner surface of the compressor casing, and a retention surface extending radially between the main bore section and the counterbore section, wherein the access opening has a centerline and the retention surface extending circumferentially about the centerline; and wherein the closure device has a central axis, the shoulder extending circumferentially about the central axis and being disposable against the retention surface such that the central axis and the centerline are substantially collinear. a cylindrical plug at least partially disposable within the access opening, the outer surface of the compressor casing disposed radially outward from the inner surface, wherein the cylindrical plug is configured to substantially obstruct the access opening; a shoulder extending radially outward from the cylindrical plug, and characterized in that the shoulder is at least partially disposable against the inner surface of the compressor casing, the shoulder being biasable against the inner surface by pressure within the compressor casing such that pressure within the compressor casing forces the shoulder into contact with the inner surface to retain the cylindrical plug at least partially disposed within the access opening; and a sealing member disposed circumferentially about the cylindrical plug and configured to substantially prevent fluid flow between the cylindrical plug and the access opening, wherein the access opening comprises a main bore section and a counterbore section extending into the main bore section from the inner surface of the compressor casing, and a retention surface extending radially between the main bore section and the counterbore section, wherein the access opening has a centerline and the retention surface extending circumferentially about the centerline; and wherein the closure device has a central axis, the shoulder extending circumferentially about the central axis and being disposable against the retention surface such that the central axis and the centerline are substantially collinear.