Steam turbines are a significant constituent of the utility system of a process plant. These turbines are both energy efficient and reliable for optimizing appropriate steam supply. The power generating industry mostly uses Steam Turbines to generate power. Function of Steam Turbine-
To know the basic process of how a steam turbine generates electricity one must know about the following Steam Turbine Parts and Function of Steam Turbine.
The basic parts of an efficient Steam Turbine and their functions:
Steam Chest & Casing: Steam Chest and casing is the outermost part of a turbine. The steam chest is connected to both the steam supply line and the steam exhaust line respectively. The casing covers the rotor and nozzles.
The steam chest and casing is the protection over the moving parts of the turbine.
Rotor: This part of the turbine carries disc assemblies, a shaft through the assembly and some buckets. The shaft goes through bearing cases, extending beyond the casing of the turbine. One of the two ends of the shaft is coupled to the driven pump and the other end connects to the speed governor and the over speed system.
The main function of the rotor is to convert the thermal energy of the incoming steam into kinetic energy which in turn generates the electricity later on.
The bearing cases: The bearing cases contain the rotating oil seals and journal bearings. The cases protect these seals and bearings from water, steam, and dust particles. The bearing cases extend to the steam end bearing housing and contain moving parts of the speed-governor system.
Casing sealing gland: The function of this part is to seal the casing and the shaft so that unnecessary dust particles do not harm the shaft. This is made of Spring-backed segmental carbon rings and also spring-backed labyrinth seal to sustain in high-pressure exhaust steam.
Governor system: The basic function of a governor system in any speed-sensitive rotary machinery is to control the speed limit. The governor valve plays the key role in varying the steam flow into the turbine, which in turn controls the speed of the turbine. The governor may connect with the turbine shaft directly or through warm gears or magnetic impulse. A setpoint of speed is predetermined and the turbine speed is compared to that and the governor output.
Over speed trip system: The function of a trip system is to stop the functioning of
a rotary machine as soon as the machine draws more power than the predetermined set limit. It consists of a quick-closing valve and a spring-loaded pin mounted in the shaft of the turbine.
The over speed trip system is instructed to trip at 10% over the set point of the speed. At this time a quick closing stop valve closes and the steam supply gets interrupted.
Labyrinth seal: There is always chances of leakage when the machinery functions between a high-pressure side and a low-pressure side. Labyrinth allows a small amount of leakage to bring down severe leakages in these points. This is why a minimum clearance is given between the labyrinth and the shaft.
When the gas enters the narrow clearance between the labyrinth and the shaft, it gets expanded and pressure drops down. As soon as it passes out of the clearance passage the pressure becomes slightly more than the atmospheric pressure. This phenomenon causes less amount of leakage.
Reversing blade assembly with nozzle ring- Function of Steam Turbine: The nozzle ring consists of nozzles. It is bolted inside the bottom half of the casing. The function of the nozzles is to direct the stream flow towards the first row of the Curtis stage blades.
There is also a reversing blade assembly which is bolted to the nozzle ring. When the steam exits the first row of the Curtis blades, it goes to the reversing blade. The reversing blades reverse the direction of the steam towards the second stage of Curtis blades.
Diaphragms: The function of the diaphragm is to separate the inner stages. It consists of the interstage seals and interstage nozzles.
Auxiliary steam valves: This is a support system to achieve higher efficiency. These valves are fitted between the nozzle ring and the steam chest. The main function of this kind of valves is to control the flow of steam.
Turning gears: When the turbine starts, before rotating at a higher speed the shaft needs steady speed up to maintain a uniform temperature. Turning gears are used to rotate the shaft slowly during the primitive stage, just after the turbine starts. Turning gears also functions in the same way when the turbine is turned off.
When we use gas, coal or solar energy sources to heat the water, steam is generated. This steam flows and passes through the blades of a turbine. The steam gets expanded and cools down. This helps the potential energy of the steam to convert into kinetic energy, which in turn rotates the turbine blades.
The turbine is connected to a generator, which produces electricity after that. Steam turbines are very popular in the power generating industry. Around 85% of the power production comes from the steam turbine power plants in the U.S. Steam turbines have a power output range of 90 to 1900 MW.