Despite the conservation efforts of industrialized nations, rising energy consumption remains a global concern, and one intensified by the economic advances of emerging countries. Numerous fossil fuel and nuclear power plants are under construction or committed and have to be equipped with the appropriate steam turbines.

Fig. 1: Tilting a rotor after vertical TIG welding. (Photo by courtesy of Polysoude.)

For better cost-effectiveness and environmental protection, the efficiency of these steam turbines has improved continuously, either by increasing their size or by raising working temperatures; often, both measures are applied simultaneously.

Larger turbines require adequately designed rotors, but there is limited capacity for finishing these huge parts.

Fig. 2a: TIG welding station for joining vertically positioned rotor parts. (Photos by courtesy of Polysoude.)

To gain more production flexibility, a major turbine manufacturer decided a few years ago to try an entirely different manufacturing method.

An order for several large rotors had to be completed on a short lead-time. With no possibility of completing the order fast enough in the conventional way, an alternative solution had to be found. The new approach started with using available capacity to forge smaller rotor segments in order to join them together later.

Assembly took place in two steps: initial joining was carried out by manual welding on vertically superimposed segments, followed by mechanized submerged arc welding as soon as sufficient stability made it feasible to turn the built-up rotors into a horizontal position. (See Fig. 1.)

Fig. 2b: A vertical TIG rotor welding station with two torches. (Photos by courtesy of Polysoude.)

From a technological point of view, the outcome of the operation had been successful and established the possibility of overcoming the limited forging capacities. However, integrating it into common production lines failed because of the extensive amount of manual welding to be done, and the need for careful craftsmanship.

Later, the idea of producing assembled rotors came up again. Based on the previous experience, turbine manufacturers had prepared a general outline of the expected features: assembly of two or more rotor segments with a total length of up to 12 m in vertical position; mechanized welding with increased performance; the highest level of weld quality; and a state-of-the-art technique. (See Figs. 2a and 2b.)

Then, planning, development, and implementation of the station was carried out in close cooperation with Polysoude, the French company that specializes in mechanized and automated TIG welding solutions.

To guarantee consistently high joint quality, mechanized TIG welding was chosen as the process of choice. Here, welding parameters are specified in advance and completely controlled by microprocessor-equipped devices; any programmed welding sequence can be repeated exactly, as often as desired. For enhanced process performance, TIG hot-wire welding was selected.