Stage for a steam turbine

Claim 1

A stage of an axial flow turbine for converting at least a portion of energy available from an elastic fluid into mechanical energy, comprising: a plurality of buckets affixed to and circumferentially aligned around a rotor of said turbine, each bucket including an aerodynamic region intermediate an outer tip section and a inner root section, wherein the turbine includes a shell having an inner surface for circumferentially surrounding said plurality of buckets: a plurality of bucket covers, each of said plurality of covers respectively connecting the tip secction of adjacent buckets and each of said plurality of covers including an outer surface, wherein each of said plurality of covers permits untwisting of each respective bucket of said plurality of buckets during turbine operation; one rib respectively extending radially outward from the outer surface of each of said plurality of covers, respectively, each said rib tangentially aligned with respect to the ribs on adjacent covers, the radially extensive edge of said rib in close proximity to yet spaced from the inner surface of the shell to form a radial clearance gap between the inner surface of the shell and said rib, said rib being the only impediment to flow of the elastic fluid between the tips of said plurality of buckets and said inner surface of the shell; and a diaphragm axially spaced from said plurality of buckets and circumferentially disposed around the rotor for directing the elastic fluid into the plurality of buckets, said diaphragm including a plurality of spaced apart nozzle partitions having a root proximate the rotor, said nozzle partitions forming a respective plurality of channels therebetween and an inner ring for fixedly securing at the root said plurality of nozzle partitions including a leading edge and a trailing edge and disposed to include both an axial lean and a tangential lean, each of said axial lean and said tangential lean with respect to a radial reference from the axis of rotation of the rotor, said inner ring including a greater outward radial extent adjacent the leading edge of said nozzle partitions than the outward radial extend adjacent the trailing edge of said nozzle partitions, each of said plurality of nozzle partitions spaced from an adjacent nozzle partition such that the channel therebetween includes a maximum throat and a trailing edge throat, wherein the minimum throat is disposed between the leading edge of the nozzle partition and the trailing edge throat at the root of the nozzle partition and the minimum throat is disposed monotonically more proximate the trailing edge throat at increasing radial distance from the root of said nozzle partition, whereby the margins of the channel define a converging-diverging passageway at least over a portion of the radial extent of the nozzle partition.