Steam turbine generator unit

A steam turbine is a machine that extracts heat energy from pressurized steam and uses it to perform mechanical work on a rotating output shaft. Its modern representation was invented by Charles Parsons in 1884.The manufacture of modern steam turbines involves advanced metalworking, using techniques first seen in the 20th century to form high-grade steel alloys into precision parts; continued advances in steam turbine durability and efficiency remain a hallmark of 21st century energy economics core.The steam turbine is a form of heat engine where most of the improvements in thermodynamic efficiency come from the use of multiple stages in the expansion of steam, which results in a reversible expansion process that is closer to ideal.

Because the turbine produces rotational motion, it can be coupled to a generator to convert its motion into electrical energy.This turbine generator is the heart of a thermal power plant that can be fueled by fossil fuels, nuclear fuel, geothermal or solar energy. In 2014, about 85 percent of electricity generation in the United States came from steam turbines.

As of 2021, one of the largest steam turbines in the world is the Arabelle, a turbine built by GE based on Alstom's original design.The Arabelle turbine has a diameter of 7 m, weighs 4000 tons and rotates at 1500 rpm. In a typical nuclear installation, 4,000 tons of supporting steel structures are also required, along with 1,000 tons of pumps, valves and piping.

Technical challenges include rotor imbalance, vibration, bearing wear and uneven expansion (all forms of thermal shock). In large installations, even the sturdiest turbines can shake themselves when run out of tune.

Types 

Steam turbines come in a variety of sizes, from small <0.75 kW (<1 hp) units used as mechanical drives for pumps, compressors, and other shaft-driven equipment (rarely seen) to 1,500 MW (2,000,000 hp) turbines used to generate electricity. There are several classifications of modern steam turbines.

Blade and stage design

There are two basic types of turbine blades, blades and nozzles. The blades move solely due to the effect of the steam on them, their profile does not converge.This results in a drop in steam velocity and essentially no pressure drop as the steam moves through the blades.Turbines consisting of alternating blades and fixed nozzles are called impulse turbines,Curtiss turbines, Lato turbines or Brown-Curtiss turbines.The nozzles look similar to vanes,but their profiles converge near the exit.This causes the steam to drop in pressure and increase in velocity as it moves through the nozzle.The nozzle movement is due to the impact of the steam on the nozzle and the reaction of the high-speed steam at the outlet.Turbines consisting of alternating movable and fixed nozzles are called reactive turbines or Parsons turbines.Except for low power applications, turbine blades are arranged in series in multiple stages, known as compounding,which greatly improves efficiency at low speeds.The reaction table is a row of fixed nozzles followed by a row of moving nozzles.Multiple reaction stages divide the pressure drop between the steam inlet and exhaust into many small droplets,creating a pressure compound turbine.Impulse phases can be pressure compounded, velocity compounded, Or pressure velocity compound.A pressure compound pulse stage is a row of fixed nozzles followed by a row of moving vanes with multiple compound stages.This is also known as the Rateau turbine, named after its inventor.The compound velocity pulse stage (invented by Curtis, also known as the "Curtis wheel") is a row of fixed nozzles followed by two or more rows of moving blades,alternating with rows of fixed blades.This splits the entire stage's speed drop into several smaller drops. [20] A series of speed-compound impulse stages known as pressure-velocity compound turbines.By 1905, when steam turbines began to be used in clipper ships (such as HMS Dreadnought) and land-based power applications,it had been determined that it was advisable to use one or more Curtiss wheels at the beginning of multiple stages (where the steam pressure was highest ),followed by the reaction phase.Using high pressure steam is more efficient due to reduced leakage between the turbine rotor and casing.This is illustrated in a German 1905 drawing of an AEG marine steam turbine.Steam from the boiler enters at high pressure on the right side through a choke valve,which is manually controlled by the operator (in this case, the sailor is called the choke valve).It goes through five Curtiss wheels and many reaction stages (small vanes on the edge of the two large rotors in the middle) before exiting at low pressure and almost certainly into the condenser.The condenser provides vacuum to maximize energy extraction from the steam and condenses the steam into feed water for return to the boiler.On the left are several additional reaction stages (on the two large rotors) which counter-rotate the turbines for reverse operation,with steam entering through a separate throttle.Since ships rarely run in reverse, efficiency is not a priority for reversing turbines, so only a few stages are used to save cost.

Blade design 

A major challenge in turbine design is reducing blade creep.Due to the high temperatures and stresses of operation,steam turbine materials can fail through these mechanisms. Creep becomes important as the temperature is increased in order to improve turbine efficiency.To limit creep, thermal coatings and superalloys with solid solution strengthening and grain boundary strengthening are used in the blade design.Protective coatings are used to reduce thermal damage and limit oxidation.These coatings are usually stabilized zirconia-based ceramics.The temperature exposure of nickel superalloys can be limited by the use of thermal protective coatings.This reduces the creep mechanisms experienced in the blade.The oxide coating limits the loss of efficiency from buildup on the outside of the blade, which is especially important in high temperature environments.Nickel-based inserts are alloyed with aluminum and titanium for strength and creep resistance.The microstructure of these alloys consists of regions of distinct composition.Due to the microstructure, the homogeneous dispersion of the gamma phase (combination of nickel, aluminum and titanium) increases the strength and creep resistance of the blade.

Refractory elements such as rhenium and ruthenium can be added to the alloy to increase the creep strength.The addition of these elements reduces the diffusion of the gamma phase, thereby maintaining fatigue resistance, strength and creep resistance.

Steam supply and exhaust conditions

Turbine types include condensing, non-condensing, reheat, extracting and induction.

Condensing turbines

Condensing turbines are most commonly found in power plants.These turbines receive steam from the boiler and discharge it to the condenser.The exiting steam is at a pressure well below atmospheric pressure and is in a partially condensed state, usually close to 90% by mass.

Non-condensing turbines

Non-condensing turbines are most widely used in process steam applications where the steam exits the turbine and is used for other purposes.The exhaust pressure is controlled by a regulating valve to meet the needs of the process steam pressure.These are commonly found in refineries, district heating installations, pulp and paper mills and desalination facilities where large quantities of low pressure process steam are required.

Reheat turbines

Reheat turbines are also used almost exclusively in power plants.In a reheat turbine, the steam flow exits the high pressure section of the turbine and returns to the boiler where additional superheat is added.The steam then returns to the intermediate pressure section of the turbine where it continues to expand. Using reheat in a cycle increases the work output of the turbine,and the expansion comes to an end before the steam condenses, minimizing erosion of the last few rows of blades.In most cases the maximum number of reheats used in a cycle is 2 as the cost of superheated steam offsets the increase in turbine work output.

Extracting turbines

Extraction turbines are common in all applications.In extraction steam turbines, steam is released from various stages of the turbine for use in industrial process needs or sent to boiler feed water heaters to improve overall cycle efficiency.The extraction flow can be valve controlled or uncontrolled.The extracted steam results in a power loss in the downstream stages of the turbine.Induction turbines introduce low-pressure steam in the middle stage to generate additional power.