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Cavitation means bubbles are forming in the liquid.
To avoid Cavitation, we have to increase the Pump size to One or Two Inch;
To increase the pressure of the Suction Head, or
Decrease the Pump Speed.
Centrifugal pump. Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.
The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.
One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.
Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate.
It is given in the following formulae:
Centrifugal force F= (M*V2)/R.
The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.
In a radialflow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades.
Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
Labyrinth mated with shaft serration’s or shaft seal strips.
Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.
Radial – clearance at the tips of the rotor and casing.
Axial – the foreandaft
clearance, at the sides of the rotor and the casing.
Thrust bearings keep the rotor in its correct axial position.
In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a "stage." A single Curtis stage may consist of two or more rows of moving blades.
Partitions between pressure stages in a turbine’s casing are called diaphragms. They hold the vaneshaped nozzles and seals between the stages. Usually labyrinthtype seals are used. Onehalf of the diaphragm is fitted into the top of the casing, the other half into the bottom.
Topping and superposed turbines arc highpressure, noncondensing units that can be added to an older, moderatepressure plant. Topping turbines receive highpressure steam from new highpressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.
This unit has the ends of the Babbitt bearing extended radically over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.
Cavitation is the formation of vapor cavities (Air bubbles) in a liquid.
Positive displacement pumps are those that operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump.
The kinematic viscosity is the ratio of absolute viscosity to the density of the liquid.
Uniform Flow: The flow is defined as uniform flow when in the flow field the velocity and other hydrodynamic parameters do not change from point to point at any instant of time.
Non-Uniform Flow: When the velocity and other hydrodynamic parameters changes from one point to another the flow is de??ned as non-uniform flow.
Unsteady uniform flow - changes with time.
Steady uniform flow - does not change with time.
Coefficient of contraction is the ratio of area of jet at vena contracta to the area of orifice.
The typical value may be taken as 0.64 for a sharp orifice (concentric with the flow channel). The smaller the value, the more effect the vena contracta has.