Working principle and design of steam turbine

An ideal steam turbine is considered an isentropic process, or constant entropy process, where the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine.However, no steam turbine is truly isentropic and typical isentropic efficiencies range from 20% to 90% depending on the turbine application.The interior of a turbine consists of several sets of blades or blades.A set of stationary blades is attached to the housing and a set of rotating blades is attached to the shaft.The devices intermesh with certain minimum clearances and the devices vary in size and configuration to efficiently utilize the expansion of the steam at each stage.The actual thermal efficiency of a steam turbine varies with turbine size,loading conditions,clearance losses, and friction losses.They peak at around 50% in 1,200 MW (1,600,000 horsepower) turbines; smaller ones are less efficient.To maximize turbine efficiency, steam expands in multiple stages, doing work.These stages are characterized by how energy is extracted from them, known as impulse or reaction turbines. Most steam turbines use a mix of reactive and impulse designs: each stage behaves like one or the other, but the entire turbine uses both.Usually, the low pressure part is reactive type and the high pressure part is pulse type.

Impulse turbines 

Impulse turbines have fixed nozzles that direct the flow of steam into high-velocity jets.These jets contain a lot of kinetic energy,which is converted into shaft rotation by the barrel-shaped rotor blades when the steam jet changes direction.The pressure drop occurs only over the fixed blades, and there is a net increase in steam velocity across the stages. As the steam flows through the nozzle, its pressure drops from inlet pressure to outlet pressure (atmospheric pressure, or more commonly condenser vacuum).Due to this high expansion rate of the steam, the steam leaves the nozzle at a very high velocity.The steam leaving the moving blades has a substantial fraction of the maximum velocity of the steam when it leaves the nozzle.Energy losses due to this higher exit velocity are often referred to as carryover velocity or exit losses.The law of moment of momentum states that the sum of the external moments acting on a fluid temporarily occupying a control volume is equal to the net time change in the flux of angular momentum through the control volume.

Reaction turbines

In reaction turbines, the rotor blades themselves are arranged as converging nozzles.This type of turbine utilizes the reaction force created when steam is accelerated through a nozzle formed by rotors.The steam is directed onto the rotor through the stationary vanes of the stator.It leaves the stator in the form of a jet that fills the entire circumference of the rotor.The steam then changes direction and increases its speed relative to the speed of the blades.A pressure drop occurs across both the stator and the rotor, the steam accelerates through the stator and decelerates through the rotor, there is no net change in steam velocity across the stage, but both pressure and temperature are reduced, reflecting the work done in the stage by the drive of the rotor.

Operation and maintenance

Due to the high pressures used in the steam circuit and the materials used, steam turbines and their casings have high thermal inertia.When the preheat turbine is used, the main steam shutoff valve (after the boiler) has a bypass line that allows the superheated steam to slowly bypass the valve and continue to heat the line and turbine in the system. Also, when there is no steam, the turning gears engage to slowly spin the turbine to ensure even heating to prevent uneven expansion.The turbine is first turned by turning the gear, giving the rotor time to assume a straight plane (no bending), then the turning gear disengages and the steam enters the turbine, first into the rear wheel blades, then slowly into the front wheel blades at 10–15 RPM (0.17– 0.25 Hz) to slowly heat the turbine Warm-up procedures for large steam turbines may exceed ten hours.

During normal operation, rotor imbalance causes vibrations which, due to high rotational speeds, cause the blades to break away from the rotor and pass through the casing.To reduce this risk, a lot of effort is spent balancing the turbines.In addition, the turbines operate with high-quality steam: superheated (dry) steam or saturated steam with a high dryness.This prevents rapid blade impact and erosion (moisture carryover) that occurs when condensate is sprayed onto the blade.In addition, liquid water entering the blades can damage the thrust bearings of the turbine shaft.To prevent this, along with controls and baffles in the boiler to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.Modern steam turbines have simple maintenance requirements and low cost (typically around $0.005 per kWh); their service life is typically in excess of 50 years. 

Speed regulation

Controlling turbines with a governor is essential because turbines need to run slowly to prevent damage,and some applications,such as the generation of alternating current, require precise speed control.Uncontrolled acceleration of the turbine rotor can cause an overspeed trip, which causes the governor and throttle valves that control the flow of steam to the turbine to close. If these valves fail, the turbo can continue to rev until it disintegrates, usually disastrously. Turbines are expensive to manufacture and require precision manufacturing and special quality materials.During normal operation synchronous to the grid, the power plant uses a 5% droop speed control.This means 100% full load speed and 105% no load speed.This is necessary for the stable operation of the network, without power plant searches and dropouts.Usually the change in speed is small.Power delivery is adjusted by slowly increasing the droop curve by increasing spring pressure on the centrifugal governor.Typically, this is a fundamental system requirement for all power plants, as old and new power plants must be compatible to respond to momentary changes in frequency without relying on external communications.