Pool pumps

 Swimming pools use only one type of pump for circulation purposes: a centrifugal pump. A centrifugal pump has an impeller that rotates on an axis creating a centrifugal force and displacement of the water.
There are two types of centrifugal pumps used. The most common is the self-priming pump. This type of pump can re-prime itself using only the water in the pump housing. The self-priming pump will not lose prime even with large amounts of air in the system. Pool pumps that are located above the pool water level must be of this type.
A flooded suction pump will lose prime when air is in the system. A flooded suction pump generally runs at lower revolutions per minute and therefore has longer life expectancy. Flooded suction pumps are used in systems where the pump is located below the pool water level.
Every pump has the same basic
components:
– The volute or pump housing
– The shaft, which is an extension of the   
    pump motor
– Mechanical shaft seal
– A motor adaptor and seal plate
– And the most important part: the impeller
The volute is sometimes called the diffuser. It gives the self-priming pump the ability to handle air and re-prime itself. The shaft is an extension of the pump motor and provides the mechanical motion to the impeller.
Older pumps may still use lubricated fiber rings to seal the shaft. Most pumps today use a mechanical shaft seal that operates without external lubrication and does not require adjustment. Some pumps have their own shaft that must be couples to the motor shaft. These are called frame mounted pumps. The close-coupled pump uses the motor shaft and relies on the bearing support of the motor.
Impeller
The impeller causes the water to flow. As it rotates, the center or eye of the impeller draws water in from the suction or influent piping from the pool. The water is then thrown out as with a flinging motion by the centrifugal movement of the impeller.
The volute or housing collects the water and as the velocity of flow increases, pressure is created. The water exits the pump through the effluent or out-going piping. It is at this point that the necessary for is developed to push water through the circulation system and back to the pool.
Centrifugal pumps are not positive displacement pumps. For each revolution of the impeller, a different amount of water will be discharged depending on conditions. The most important factors are exit pressure, air in the system and restricted influent line. A blocked suction line and air in the system can cause cavitation and damage the pump.
Cavitation
Cavitation occurs when the impeller is starved for water and cannot maintain the discharge requirements. The sound of the pump changes when it is cavitating. In severe cases the pump and motor may vibrate. The pool operator must check the obvious reasons for cavitation:
– Debris in the skimmer basket
–    Debris in the hair and lint basket
–   A dirty vacuum filter
–    A partially closed or restricted suction line
–    A throttling valve on he effluent line not
       properly restricted
–    A leak in the plumbing on he vacuum side   
       of the circulation system
Cavitation is a symptom of a problem and immediate corrective action must be made. If allowed to continue, cavitation may result in serious damage to the pump, motor or both.