Efficiency
Efficiency in turbine is defined as the ratio of the amount of work done by the turbine to the amount of heat transferred to the turbine by the working fluid and the efficiency at which the heat is transferred to the turbine rotating blades is dependent on the ratio of moving blade velocity and the steam velocity.
In case of impulse stages, the maximum efficiency occurs at the ratio of 0.5, which means if the moving blade speed is half the speed of the steam, then it will have maximum efficiency.
Losses in Turbine
Profile Loss:
This happens due to the formation of layers on the blade surfaces, which deteriorates the profile of the blade. The working fluid means the steam will have a viscous force acting against it, which will slow it down. At a particular flow rate this will result in a very larger amount of pressure drop across the rotating blades. Since the pressures in case of a turbine are fixed, the flow rate becomes less .This means the heat drop obtained across the stages is less than that for an expansion and is therefore less efficient.
Secondary Loss:
This happens due to friction on the casing wall and on the blade root and tip. It is same as that of Profile loss.
Tip Leakage loss:
As it’s the name signifies, this loss arises due to the steam passing through the small clearances required between the tip of the moving blade and the casing, or between the end of the fixed blades and rotating shaft. The problem is sorted out by the use of inter-stage seals. Generally we go for labyrinth Seal for the sealing of turbine. The amount of the leakage depends on whether the turbine is of an impulse type or a reaction type. Because In case of the Reaction turbines, there is a pressure drop across the moving blades, so this leads to the flow of steam through the clearances. Similarly, it is more important for a reaction turbine to have good tip sealing than for an impulse turbine.
Loss due to Lacing Wires:
This loss specially occurs in case of the LP turbines. The long blades of the LP cylinder are tightened towards the tip with lacing wires. Their purpose is to damp the vibrations and increase the resonant frequency of the blade so that it will not coincide with the any existing frequencies present in the turbine. The presence of these wires produces losses in efficiency because of the passage area that they block
Wetness Loss:
This loss is incurred by the presence of moisture, In case of the low-pressure steam towards the exit of the LP turbine. The loss occurs due to two reasons; the first is the reduction in efficiency due to the absorption of energy by the water droplets present in the steam and the second factor is the erosion of the final moving blades. Erosion occurs because the water droplets result in an impact on the blades, in particular at the tip of the blade.
