Submersible Pump Construction
Wet motors are those in which the well water has access to the inside of the motor, with the rotor and bearings actually operating in water. In this type of motor, windings of the stators are completely sealed off from the rotor by means of a thin stainless steel inner liner. A filter around the shaft is required to prevent the entrance of abrasive material into the motor. The wet type motor should be filled with water during installation so that the bearings will have sufficient lubrication when the motor is first started.
The stator windings are continuous for the whole length of the motor. The rotors are made in sections on a continuous shaft, with bearings between them to guide the shaft and maintain correct alignment. The electric cables, leading from the motor to the starting box on the ground surface, are water proofed and are placed outside the discharge pipe.
Propeller pumps are often referred to as axial flow pumps. The principle of operation is similar to that of a boat propeller, except that the impeller is enclosed in a housing. This type of pump is particularly adapted to handling large volumes of water at comparatively low heads. Propeller pumps are not easily clogged by foreign materials and suspended sediments in water. They are usually limited to pumping heads of around 1 to 2.5 metres and are available in sizes ranging from 20 to 120 cm in diameter.
The impeller, also known as the propeller, operates in a cylindrical casing which is an extension of the pump discharge column. A flared entrance below the propeller is used to cut down the entrance losses, and guide vanes above smooth out the disturbances caused by the propeller.
Water is moved up by the lift of the propeller blades. Each blade of the propeller helps to impart a velocity in the direction of the shaft. The number of blades is usually 3 to 5. They are set on the shaft at angles determined according to the head and speed. The propellers are usually made of bronze and are cast in one piece. Sometimes, however, the blades are cast separately and may be threaded on their hub ends to receive a nut for the purpose of attaching them to the hub. With this arrangement the blade angle may be adjusted to suit the operating conditions. The propeller is balanced hydraulically and statically. The blades are carefully cast and scraped to reduce skin friction. They are keyed to the drive shaft and are accurately positioned by a locking collar and nut. A cone-shaped cover is usually installed over the locking nut to eliminate eddies and to prevent the entrance of sand or grit into the lower pump bearings. Diffusion casings are usually provided on propeller and mixed flow pumps to convert into pressure the tangential component of the velocity of discharge from the impeller. The discharge column, discharge head and pump drive of propeller pumps are, in general, similar to the vertical turbine pump. Propeller pumps are commonly of the oil-lubricated type.