The axial piston pump rotor consists of a round cylinder block with multiple cylinders, parallel to the
cylinder block axis. The cylinder block rotates at an angle to the axis of the drive shaft, and the fluid is
pumped by reciprocating action of the pistons in the cylinder block.
2.2 Hydraulic Turbines
Hydraulic turbines are classified as either reaction turbines or impulse turbines referring to the hydraulic
action by which the pressure or potential energy is converted to rotating or kinetic energy. The reaction
turbines include the Francis and the propeller types, while the impulse turbines are represented by the
Pelton type turbine.
Impulse turbines convert all available head into kinetic or velocity energy through the use of contracting
nozzles. The jets of water from the nozzles act on the runner buckets to exert a force in the direction of
flow. This force, or impulse as it is referred to, turns the turbine. Impulse turbines are primarily used for
heads of 800 feet or more although they are also used in some low now, low head applications.
Water flow to an impulse turbine is controlled by a needle valve. The position of the needle valve is
controlled by a governor to change speed or load. A moveable deflector plate, controlled by the governor,
is positioned in front of the nozzle to rapidly deflect some of the water away from the turbine during a load
The head pressure in a reaction turbine is only partially converted to velocity. While the reaction turbine
obtains some power from the impulse force from the velocity of the water, most of its power is a result of
difference in pressure between the top and bottom of the runner buckets.
Water entering the spiral or scroll case is directed to the turbine runner by the guide vanes and the wicket
gates. The wicket gates, controlled by the governor through hydraulic servomotors, control water flow to
A propeller turbine is similar in appearance to a boat propeller. Water is directed and controlled in much
the same manner as with the Francis turbine. A variation of the propeller turbine is the Kaplan turbine
which features adjustable blades that are pivoted to obtain the highest efficiency possible at any load
2.3 Cavitation Erosion, Abrasive Erosion, and Corrosion
Pump impellers, turbine runners, and their related components may be damaged by a number of different
actions, the most common being cavitation erosion, abrasive erosion, and corrosion. The appropriate repair
procedure will depend on the cause of the damage.
Cavitation is the formation of vapor bubbles or cavities in a flowing liquid subjected to an absolute
pressure equal to, or less than, the vapor pressure of the liquid. These bubbles collapse violently as they
move to a region of higher pressure causing shock pressures which can be greater than 100,000 pounds per
square inch. When audible, cavitation makes a steady crackling sound similar to rocks passing through the
pump or turbine. Cavitation erosion or pitting occurs when the bubbles collapse against the metal surface
of the impeller or turbine runner and occurs most frequently on the low pressure side of the impeller inlet