The Mechanical Design of an Experimental Plant-Type Gas Turbine

Abstract

Problems of thermal expansion and insulation encountered in the structural design of steam turbines are accentuated in gas turbines owing to their generally higher operating temperature. This paper describes the construction of a four-stage 8000-b.h.p. gas turbine designed and manufactured at the National Gas Turbine Establishment, and embodying several features offering a solution of these problems. A tubular rotor reinforced by radial discs is required if material is to be used with economy and, in addition, the rotor must be maintained at a temperature which allows full use of the material properties. Heat should be confined as closely as possible to the gas flow annulus to obtain the maximum output of work from the blading and to avoid undesirable thermal stress in the rotor. The insulation of the bearings and the provision of some flexibility between these and the shaft to meet the possibility of damage from excessive heat soakage in an emergency is also important. The methods used to attain the above objectives include rotor discs connected by threaded side-rims of large diameter and cooled by an axial flow of air under the blade roots. Flexible sleeves are fitted between the shaft journals and the roller-type bearings, to inhibit heat flow and to accommodate differential radial expansion. The stator casings are treated as gas-carrying ducts, relieved of mounting constraints and bearing alignment duty by being freely mounted on radially disposed pegs and keys giving freedom for thermal expansion in all directions. Preliminary experiments and rig tests on the bearing mounting sleeves, and on surface treatment to prevent seizure of the screw-thread connections between the rotor discs are described. More than usual attention is given in the design to methods of facilitating inspection and maintenance, and provision is made for replacing individual rotor blades and bearings without separating rotor and stator.