Steam Turbines and Condensers Questions and Answers

Answer: C

The blade channel area in an impulse steam turbine is constant. In an impulse turbine the steam expands in the nozzles, not in the blades, so the blade passage has the same area at inlet and outlet and the pressure is the same on both sides of the moving blades. In a reaction turbine the moving blade channel is converging, so the area decreases continuously, which is why the options of increasing or decreasing area are wrong.

Steam gains velocity only in the fixed nozzles. Across the moving blades of a simple impulse stage the relative velocity stays nearly the same, apart from friction, so no change of channel area is needed.

Answer: C

In a reaction steam turbine the pressure drops in both the nozzle (fixed blades) and the moving blade channel. The steam expands partly in the fixed blades and partly in the moving blades, which is why the moving blades work by reaction as well as impulse. In an impulse turbine the pressure drops only in the nozzles, so the options of drop in the nozzle only, or in neither, are wrong.

In the Parsons turbine the stage has 50 percent reaction, where the pressure drop is shared equally between the fixed and the moving blades.

Answer: B

Friction in a steam nozzle reduces the mass flow rate. Friction reheats the steam, increases the dryness and lowers the enthalpy drop that becomes kinetic energy, so the exit velocity is lower and the specific volume at the exit is larger, which reduces the mass flow through the throat. The specific volume increases and the dryness fraction increases, and the actual enthalpy drop decreases, so the other three options state the reverse.

Answer: D

The correct statement is the absolute exit flow angle is equal to inlet angle of rotor blade.

In a 50% reaction (Parsons) stage the enthalpy drop is split equally between the fixed and moving blades, so the two rows are identical in shape and the velocity triangles at inlet and exit are mirror images. As a result, the absolute inlet angle equals the relative exit angle (α₁ = β₂), and the relative inlet angle, which is the rotor blade inlet angle, equals the absolute exit angle (β₁ = α₂).

Symmetric blades, with equal inlet and exit angles, are the mark of an impulse stage, so the options calling the rotor or stator blade symmetric are wrong. The absolute inlet and exit flow angles are not equal in general; it is the cross pairs that match.

Answer: B

The correct statement is that in a reaction turbine the expansion takes place in guide vanes and moving vanes both.

In a reaction turbine the steam or water expands, and its pressure falls, both in the fixed guide blades and in the moving blades, so the moving blades receive a reaction force as well as an impulse. The Parsons turbine is the classic case, with half the drop in each, a degree of reaction of 0.5.

In an impulse turbine the whole expansion happens in the nozzles or guide vanes and none in the moving blades, so option (A) is wrong. Its degree of reaction is therefore zero, not one, and a reaction turbine's is above zero, so (C) and (D) are both wrong.

Answer: A

The degree of reaction of an impulse turbine is zero.

The degree of reaction is the fraction of the stage's enthalpy (pressure) drop that occurs in the moving blades. In an impulse turbine the entire drop happens in the nozzles, and the steam passes over the moving blades at constant pressure, so the fraction is zero.

A Parsons reaction turbine has a degree of reaction of 0.5. A value of one would mean all the drop in the moving blades, and values of two or three are impossible because the fraction cannot exceed one.