Compounding of impulse steam turbines

Need for compounding

In a simple impulse turbine (De Laval), the whole pressure drop occurs in one nozzle, so the steam leaves at a very high velocity (1000 m/s or more). For best efficiency the blade speed must be about half the steam velocity component, so the rotor speed is very high (20000 to 30000 rpm). This causes severe centrifugal stresses, large vibration and bearing problems, and the exit kinetic energy loss is large. The speed must also be reduced by gearing.

Compounding is the method of dividing the total pressure drop or velocity drop into several stages, so that the steam velocity and the blade speed in each stage are moderate. It gives lower rotor speed and better efficiency.

Types

1. Pressure compounding (Rateau turbine)

The turbine has several stages in series. Each stage has a nozzle (fixed row) and a row of moving blades. The total pressure drop is divided among the nozzles; in each stage the steam is expanded in the nozzle, and its velocity is absorbed fully in the moving blades.

 P  \
     \___                     Pressure falls in nozzles
         \___                 only; constant over blades
  V   /\  /\  /\             Velocity rises in nozzle,
     /  \/  \/  \            falls in blades (saw tooth)
  [N1][B1][N2][B2][N3][B3]

Used in medium capacity turbines; each stage is separated by a diaphragm with nozzles.

2. Velocity compounding (Curtis turbine)

The whole pressure drop occurs in one nozzle. The high-velocity steam then passes through several rows of moving blades, with rows of fixed (guide) blades between them to redirect the steam to the next row. The velocity falls in each moving row.

 P  \____________________      Pressure drops in nozzle only
                                 and stays constant after
 V  /\                          Velocity rises in nozzle
   /  \  /\  /\                 and falls in the moving rows,
  /    \/  \/  \                 slightly in fixed rows
  [N] [M1][F][M2]

It gives a large work output from one stage, but efficiency is lower than pressure compounding because friction in the blades is high. It is often used as the first (governing) stage of a large turbine.

3. Pressure-velocity compounding

This combines both. The total pressure drop is divided into several stages, and in each stage a velocity-compounded Curtis wheel (two or three rows of moving blades) is used. It gives a shorter turbine than pure pressure compounding for a given pressure drop.

Comparison