Reduce steam velocity and blade velocity 

In order to reduce steam velocity and blade velocity.  

The following methods may be used:

• Pressure Compounding
• Velocity Compounding
• Pressure-Velocity Compounding

Pressure Compounding :

The expansion of steam from boiler pressure to exhaust pressure is carried out a number of steps or stages. Each stage has a set of nozzles and a row of moving blades the row of moving blades are separated from each other by partitions or diaphragms into which the nozzles are set as only a portion of the velocity available is developed in each set of nozzles the blade velocity is kept down to a reasonable amount.

This type of com founding is known as the Rate all method and the nozzle and blade arrangement for a pressure compounded impulse turbine. In this arrangement, the pressure of the steam drops in each of set nozzles as indicated by the pressure graph the steam velocity is increased by each pressure drop and then increases again in each row moving blades.

Velocity compounding :

This design consist of one set of nozzles in which the steam is expanded from initial to exhaust pressure the velocity of the steam resulting from this expansion is absorbed in two or more rows of moving blades. Rows of fixed or guide blades attached to the casing are set between row’s of moving blades are receive and redirect the steam to the next row of moving blades. As the velocity is absorbed in more than one row of moving blades the blade speed is less than If the velocity was all absorbed in one row of blades.

The pressure drops form inlet pressure to exhaust pressure in the single set of a nozzle as the pressure graph shows this large single pressure drop produces high steam velocity which is absorbed in the two rows of moving blades Not that there is no pressure or velocity drop in the fixed guide blades.

Pressure-velocity compounding :

This is a combination of the first two methods of compounding namely pressure compounding and velocity compounding.

The steam is expanded is two or more sets of nozzles in series each set having velocity compounded blades to receive the steam issuing from the nozzles.

The method of reducing rotor speeds namely pressure compounding velocity compounding and pressure velocity compounding have all applied to impulse turbine.

In the case of the reaction turbine it is not necessary to make special blade arrangements to reduce rotor speed this is because the pressure drops across each row of moving blades as well as across each row of fixed blades and consequently, the pressure drops in the even stages and small amounts all through the machine the requires. However, a large no. of an alternate row of fixed and moving blades resulting in a long machine therefore in order to reduce the no. of blade rows necessary reaction turbines frequently have a velocity compounded impulse stage it the inlet end of the machine.