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Note for Hydraulics and Hydraulic Machines - HHM By JNTU Heroes

  • Hydraulics and Hydraulic Machines - HHM
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Text from page-3

As the load on the turbine increases, the speed decreases in turn reducing the speed of the vertical bar of the governor. The fly balls of the centrifugal governor are brought to a lower level, thereby bringing the displacement lever downward. This through the fulcrum lifts the piston of the control valve and thereby opens the valve A and closes the valve B. Oil is pumped through valve A and into the servomotor, thereby pushing the piston of the servomotor backwards. This in turn increases the inlet area of the discharge into the turbine, thereby increasing the speed. Similarly, with decrease in load on the turbine, the fly balls move farther away from the vertical shaft of the governor, thereby lifting the displacement lever upwards. This through the fulcrum lowers the piston of the control valve and thereby opens the valve B and closes the valve A. Oil is pumped through valve B and into the servomotor, thereby pushing the piston of the servomotor forwards. This in turn decreases the inlet area of the discharge into the turbine, thereby decreasing the speed. In both the cases mentioned above, the process continues until the normal position is reached. smartworlD.asia 3

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HYDRAULIC TURBINES Introduction: The device which converts h ydraulic energy into mechanical energy or vice versa is known as Hydraulic Machines . The h ydraulic machines which convert h ydraulic energy into mechanical energy are known as Turbines and that convert mechanical energy into h ydraulic energy is known as Pumps . Fig . shows a general layout of a h ydroelectric plant . Headrace hL Hg Penstock Turbine H Animation as in the PPT Tailrace smartworlD.asia Head hL H Hg Tail Race It consists of the following: 1 . A Dam constructed across a river or a channel to store water. The reservoir is also 4

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known as Headrace. 2 . Pipes of large diameter called Penstocks which carry water under pressure from storage reservoir to the turbines . These pipes are usuall y made of steel or reinforced concrete. 3 . Turbines having different t ypes of vanes or buckets or blades mounted on a wheel called runner. 4 . Tailrace which is a channel carrying water away from the turbine after the water has worked on the turbines . The water surface in the tailrace is also referred to as tailrace . Important Terms: Gross Head (H g ): It is the vertical difference between headrace and tailrace. Net Head:(H): Net head or effective head is the actual head available at the inlet of the to work on the turbine . H=Hg -hL Where h L is the total head loss during the transit of water from the headrace to smartworlD.asia tailrace which is m ainl y head loss due to friction, and is given b y hf  4 f LV 2gd 2 Where f is the coefficient of friction of penstock depending on the type of material of penstock L is the total length of penstock V is the mean flow velocit y of water through the p enstock D is the diameter of penstock and g is the acceleration due to gravit y 5

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TYPES OF EFFICIENCIES Depending on the considerations of input and output, the efficiencies can be classified as (i) H ydraulic Efficiency (ii) Mechanical Efficiency Turbine Runner (iii) Overall efficienc y (i) H ydraulic Efficiency: ( h ) Shaft It is the ratio of the power developed b y the runner of a turbine to the power supplied at the inlet Inlet of turbine of a turbine. Since the power supplied is hydraulic, and the probable loss is between the striking jet and vane it is rightly called hydraulic efficiency. If R.P. is the Runner Power and W.P. is the Water Power  h 7. smartworlD.asia R.P.  W.P. (01) Mechanical Efficiency: (m) It is the ratio of the power available at the shaft to the power developed by the runner of a turbine. This depends on the slips and other mechanical problems that will create a loss of energy between the runner in the annular area between the nozzle and spear, the amount of water reduces as the spear is pushed forward and vice -versa . and shaft which is purel y mechanical and hence mechanical efficiency. If S . P . is the Shaft Power  S.P. m (iii) (02) R.P. Overall Efficiency: ( ) It is the ratio of the power available at the shaft to the power supplied at the inlet of a turbine . As this covers overall problems of losses in energy, it is known as overall efficienc y. This depends on both the h ydraulic losses and the slips and other mechanical problems 6

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