Losses in Steam Turbines
The steam turbine is not a perfect heat engine. Energy losses tend to decrease the efficiency and work output of a turbine. This inefficiency can be attributed to the following causes.
- Residual Velocity Loss. The velocity of the steam that leaves the turbine must have certain absolute value (vex). The energy loss due to absolute exit velocity of steam is proportional to (vex2/2). This type of loss can be reduced by using multistage turbine.
- Presence of Friction. In real thermodynamic systems or in real heat engines, a part of the overall cycle inefficiency is due to the frictional losses by the individual components (e.g. nozzles or turbine blades)
- Steam Leakage. The turbine rotor and the casing cannot be perfectly insulated. Some amount of steam leaks from the chamber without doing useful work.
- Loss Due to Mechanical Friction in Bearings. Each turbine rotor is mounted on two bearings, i.e. there are double bearings between each turbine module.
- Pressure Losses in Regulating Valves and Steam Lines. There are the main steam line isolation valves (MSIVs), the throttle-stop valves and control valves between steam generators and main turbine. Like pipe friction, the minor losses are roughly proportional to the square of the flow rate. The flow rate in the main steam lines is usually very high. Although throttling is an isenthalpic process, the enthalpy drop available for work in the turbine is reduced, because this causes an increase in vapor quality of outlet steam.
- Losses Due to Low Quality of Steam. The exhausted steam is at a pressure well below atmospheric and the steam is in a partially condensed state, typically of a quality near 90%. Higher content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades.
- Radiation Loss. Steam turbine may operate at steady state with inlet conditions of 6 MPa, t = 275.6°. Since it is a large and heavy machine, it must be thermally insulated to avoid any heat loss to the surroundings.