Hydrodynamic Pumps

These pumps transfer the water through a rapidly rotating impeller; they can operate for short periods with the discharge closed, but generally require priming prior to start-up; i.e. the pump and the suction line must be filled with water. These centrifugal pumps require a motor to drive the impeller at high speed inside a specially shaped casing.

The water inlet is situated in the centre of the pump housing and, as the impeller inside spins rapidly, water is thrown to the outside and is pushed through the outlet. The pump performance can be improved greatly by increasing the number of impellers (multi-stage) which boosts the flow and the pressure of the water.

Long, narrow multi-stage centrifugal pumps can be lowered to the bottom of bores and can deliver large volumes of water under greater pressure than the reciprocating pumps.

These pumps are also characterised by the fact that when the pump pressure is changed, so is their discharge.

Centrifugal Pumps

Centrifugal pumps have few moving and wearing parts, which keeps maintenance time and costs low and thus they can be used in a wide range of applications.

Normally driven by electric motors, they can however be driven by a combustion engine. These pumps give a constant discharge and are quiet in operation.

Centrifugal pumps cannot pump air so they need the suction line to be filled with water before they can pump correctly. This can be done through a "priming tank" or water stored in a rising main. The suction line requires a footvalve at the inlet so that water is contained in the line.

The air can be mechanically exhausted from the suction line with a small hand operated diaphragm pump. There are a variety of priming devices that have been developed to assist in the starting of these types of pumps.

Self priming centrifugal pumps have a chamber which surrounds the impeller, and this chamber must be filled with water initially before operating for the first time. This chamber remains filled with water when the pump stops, so the pump is primed ready for the next operation.

Maintenance

Maintenance of centrifugal pumps requires regular inspection of glands and seals and impellers.

Inspect the drive shaft for any leakage of water from the bearing. If the pump is allowed to run dry for a long period of time the gland overheats and allows water to leak along the shaft.

When pumping dirty water the impeller is likely to block and wear.

Types of centrifugal pumps

Singlestage pump

This type of pump has one impeller connected to the drive shaft. As the drive shaft rotates water is drawn into the inlet chamber and the impeller forces the water into the outlet of the pump.

Multistage pump

To increase the lift of a pump a number of impellers are fitted to the shaft. These extra impellers increase the pressure of the pump. Multistage pumps are common in bore applications.

Slurry and gravel pumps

These types of pumps are not used widely for irrigation purposes, but may be used to pump out silt traps etc. Pumping dirty water creates problems with wear and blockages of the impeller in normal pumps. Slurry pumps have the impellers designed with fewer vanes which are more open to allow the gravel and slurry to pass through. The impeller vanes can also be designed to cut straw and similar material.

These types of pumps are constructed of abrasion resistant materials.