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A Kaplan turbine operates based on the principle of axial flow reaction. The axial flow of water recapitulates the pressure and kinetic energy into power.
A Kaplan turbine converts water into power by converting the hydrostatic head and the kinetic energy of the water. Its only disadvantage is that it can be damaged by cavitation. This is a major reason why it is important to monitor the vibration signals of the runner and draft tube.
Optimal CC for a given head can be determined from scaled model tests and verified in the field through index testing. The optimal runner blade and wicket gate position for a specific head is defined by the so-called combination curve or CAM curve.
Cavitation usually causes a decrease in efficiency and vibrations of a Kaplan turbine, and effectively identifying the cavitation state of the turbine is an important problem for the maintenance of these units. This paper presents a novel method for monitoring the occurrence and development of cavitation in Kaplan turbines based on multifractal detrended fluctuation analysis (MF-DFA) of the vibration signals of the runner and the draft tube.
Intake & Draft Tubes
The runner is an essential component in the Kaplan turbine which helps to generate electricity. The axial water flow on the runner blades can cause rotation & hence help to turn the shaft of the turbine. The runner is connected to the draft tube which is used for slowing down the water velocity & increasing the pressure.
The draft tube is used to slowly enhance the area & improve the performance of the turbine. It converts the kinetic energy of the water into pressure energy & decreases the speed of the water before it is discharged to the tailrace.
The draft tube can be of two types – simple elbow & moody. The simple elbow draft tubes are of circular shape with curricular inlets & outlets. They are mainly used at low-head positions like this. The moody draft tubes have a central core component that divides the outlet into two parts with one inlet & two exits which reduces the swirling motion of water.
Kaplan turbine is a reaction type turbine with adjustable runner blades to avoid cavitations and shocks. This machine works on the principle of axial flow reaction and can be used to generate electricity for hydro power plants. It uses a spiral type of casing with a constant reduction in cross-section area to prevent water from losing pressure as it moves through the turbine.
Water enters the regulating gate from the penstock and moves through the guide vanes where it is directed towards the runner blades. The runner blades are adjustable and they rotate as water strikes them due to the reaction force.
The rotational motion of the runner blades is transferred to the generator through a draft tube. The draft tube is shaped like an elbow and it has curricular inlets and outlets. This draft tube has a moderate efficiency.
The axial flow turbine uses the concept of converting pressure energy into kinetic energy and then recapitulating it as electrical power. It is the main type of turbine for electricity generation in industrial and electricity sectors. This type of turbine can function on low specific speed and high water heads, which is not possible with other types of turbines.
The scroll casing of Kaplan has a continuously decreasing area, which helps to direct the water towards the guide vanes. The guide vanes help in reducing the velocity of the water at the runner outlet and increase the efficiency of the turbine. The water which has a high head and low flow rate is directed to the turbine by the draft tube which has a conical shape with curricular inlet and outlet.
This draft tube is made of stainless steel and helps in minimizing cavitation. The runner blades of Kaplan can be easily adjusted to the ideal angle for maximum power production. A governor servo mechanism is used to automatically adjust the blades.