Steam Turbines

In its first twenty years, the steam turbine reduced coal consumption of coal-fired electric generating stations by 75 percent. Part of this efficiency is due to turbines suiting the high speed and the constant load of electrical generators. Today the steam turbine still plays an important role in generating electricity worldwide.

 The turbine of Generator G9 during an overhaul. The turbine shaft and blades have been removed by removing the top half of the turbine casing. The generator is still in place in the green housing. The steam control system is on the left – Image: Heurisko Ltd.  The turbine shaft of a Mixed Pressure Turbine sits in a giant lathe while being rebladed . Note the 7 stages of blading on the left and 8 on the right – Image: Heurisko Ltd.

The Science of Steam Turbines

 Fitting new blades to the central shaft of Mixed Pressure turbine 11. This end of the shaft will finally carry 7 stages of blading – Image: Heurisko Ltd.

The earliest use of a steam turbine in a power station was in 1888 in the North of England. It is believed that Charles Parsons sketched the original design of this turbine on the back of an envelope. It took 25 years and $300, 000 of research for engineers to improve its efficiency by just 2%, an indication of Parsons true genius.

All of the turbine designs prior to Parsons’ invention were inefficient, and he realised that this was because the steam was expanded in a single step.

“I therefore decided to split up the fall in pressure of the steam into small fractional expansions over a large number of turbines in series, so that the velocity of the steam nowhere should be great” – Charles Parsons.

An aerial view of the shaft of Mixed Pressure steam turbine 11 which is being rebladed. Notice the 8 stages on the left and 7 on the right. The person gives an idea of the scale as does the lathe which occupies the whole floor – Image: Heurisko Ltd.

Within Parsons’ turbine each succeeding turbine blade was made larger. This enlargement was done in three ways by:

  1. increasing the height of each blade
  2. increasing the diameter of the succeeding drums
  3. altering the angles and openings between the blades and twisting the blade along its’ length to change its’ shape (before this blades had a constant profile).

Between each set of turbine rotor blades are diaphragms. The diaphragm has blades that twist in the opposite direction. Steam expands and speeds up through the diaphragms. The steam hits the turbine blades which then rotate.
These principles remain part of modern steam turbine design.

Heat to Kinetic Energy

A single blade from the low-pressure steam turbine that drives Generator 9. Others are stacked behind – Image: Heurisko Ltd.

The conversion of heat to kinetic energy occurs in the turbines of the Wairakei power station. Each of the 10 turbines is

  • made up of a central shaft with blades around the outside
  • made efficient by careful design of the blade shape to capture the energy of the expanding steam
  • designed to operate with a specific pressure of steam
  • designed to spin at a precise speed.