Steam Turbine | Losses In Steam Turbine
LOSSES IN STEAM TURBINE
In this page,we are going to discuss different losses in steam turbine.Losses are all-time very important for manufacturing any machine. That's why Manufacturer takes special attention for manufacturing any machine.We know an ideal machine which has 100% gross efficiency will do the equivalent work to the isentropic enthalpy. It means turbine uses every single bit of heat drop produced by steam. But practically turbine's work done is much less than isentropic heat drop of the steam used. Because some internal losses occurred at the time of its operation. These losses are directly affected by the turbines output as well as its efficiency. Practically 100% gross efficiency is not possible for any turbine. When turbine works, some factors have reduced the output of the turbine, known as the losses in steam turbine or internal loss in the turbine. But this loss can be minimized through some process. Though there are several losses in a turbine, but here we will discuss some important internal losses in the turbine.
1. Nozzle Friction Loss :-
It is a very important loss for Impulse Turbine. When steam passes through the nozzles, friction loss occurs and the formation of eddies. Friction occurs in the nozzle due to the factor of nozzle efficiency and it is the ratio of actual enthalpy drop to isentropic enthalpy drop.
2. Blade Friction Loss :-
This loss is important for both Impulse and Reaction turbine . Blade friction loss is due to the steam's gliding over the blades and friction of the surface of the blades. The effect of turbine blades is considered as a blade velocity coefficient. The relative velocity of steam is reduced for this loss.
3. Wheel Friction Loss :-
When steam passes through the rotating turbine wheel, it produces some resistance on the turbine wheel. As a result, it rotates in lower speed from its original speed. It is the loss in both Impulse and Reaction turbine. The total frictional loss is about 10% of total turbine loss.
4. Losses due to mechanical friction :-
This loss is for turbine's bearing. Mechanical friction loss is due to the friction between the shaft and wheel bearing and also the regulating valve of the turbine. This loss may be reduced by proper lubrication of the moving parts of the turbine. This loss occurs both Impulse and Reaction turbine.
5. Losses due to leakage :-
Leakage loss is different in both Impulse and reaction turbine. In Impulse turbine, leakage loss occurs between the shaft, bearings, nozzles and stationary diaphragms. For Reaction turbine, it may occur at the blade tips. This loss is due to the leakage of steam on each stage of the turbine. Total leakage loss is about 1 to 2% of total turbine loss.
6. Residual Velocity loss :-
When kinetic energy of steam leaves from the turbine wheel, it happens. Actually, steam leaves from the turbine with some certain absolute velocity. That's why steam looses some kinetic energy. Residual velocity loss can be reduced by multistage turbines. This loss is about 10 to 12% in a single stage turbine.
7. Loss in regulating valves :-
Before entering the steam to the turbine, it passes through the boilers stop and regulating valve. Steam gets throttled in these regulating valves and as a result steam pressure will be less than the boiler pressure at the entry of turbine.
8. Loss due to wetness of steam :-
This loss is due to the moisture present in the turbine. When steam passes through the lower stage of the turbine, it becomes wet. At the lower stage, the velocity of water and steam are different and will not form a homogeneous mixture. That's why the velocity of water particle is less than that of steam and water particle has to be dragged with the steam and some part of kinetic energy of steam is lost. This loss occurs both impulse and reaction turbine.
9. Radiation Loss :-
This loss mainly takes place due to the temperature difference between turbine casing and its surrounding atmosphere. This loss can be minimized by the heavily insulated turbine. This loss is in also both impulse and reaction turbine.
10. Governing loss :-
It is the loss in both impulse and reaction turbine and this loss is due to the throttling of the steam at the main stop valve of the governor.
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