About pump

Pump is a device which moves liquid or slurry through pressure differential.
Pumps are usually classified in following ways

- Classified by structural, e.g. positive displacement pumps, centrifugal pumps, jet pumps, etc.

- Classified by liquid pumped e.g. water pumps, chemical pumps, sea water pump, sewage pumps, etc.

- Classified by material of constructions, e.g. metallic pumps, plastic pumps, FRP pumps, etc.
The pump deliver fix amount of fluid in each operating cycle.
Positive displacement are usually classfied as reciprocating and rotary positive displacement pump
Diaphragm pump, piston pump and plunger pump
Gear pump, lobe pump, vane pump, flexible vane pump, screw pump, peristaltic pump and rotary piston pump
Centrifugal pumps provide constant head (regardless fluid density) under same flow rate and same rotational speed. So, centrifugal pumps require head rather than pressure in pump selections.
Pressure (Pa) = Density (kg/m³) × gravitational acceleration (9.81m/s²) × Head (m)
Centrifugal pumps are structually classified from various perspective

- Liquid flow on impeller - radial flow, mix flow or axial flow

- Enclosure of impeller - Closed, semi-open or open

- Number of impeller in serial - Single stage or multi-stage

- Flow control method - Single volute, double volute or diffuser

- Number of suction on impeller - single or double

- Operating method - self-priming, non-self-priming, sump or submersible
Cavitation usually occurs at the impeller eye, or where the liquid has high flow velocity, such as impeller edge or near the cut water. So, damage of cavitation are usually found behind impeller blades, near the eye of impeller or outer diameter tip of impeller.
When liquid pressure at pump inlet is lower than vapour pressure of liquid being pumped, vapour bubbles will be produced. When liquid and bubbles are being pumped through impeller, pressure increases also. Then, vapour bubbles will be transformed back to liquid again by increased pressure. When vapour bubbles burst, surrounding liquid becomes jet flow and damaged pump parts. This phenomenon is "cavitation."
1. Noise or resonance.

2. Wetted parts damage.

3. Drop of pump performance.
NPSH = Net Positive Suction Head

This indicate the level of vacuum at pump inlet. There are 2 NPSH values, NPSHa and NPSHr.

These 2 values are used for evaluation if cavitation will occur.
NPSHa = NPSH available

It is controlled by suction piping, suction flow and liquid temperature. NPSHa is not related to pump structure.
NPSHr = NPSH required.

It is the NPSH value that cavitation will occur. This is tested and recorded by pump manufacturers under controlled conditions. So, NPSHr values are supplied by pump manufacturers