List out the characteristics of Centrifugal Pumps?

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Characteristics of Centrifugal Pumps

Centrifugal pumps are specified by four characteristics.

1. Capacity: This is defined as the quantity of liquid which is discharged from the pump in a given time. Capacity is expressed in ‘m3/hr’, ‘gal/min’, ..etc. The capacity of a pump is governed by the ‘Head’, the ‘Speed’ and the ‘Size’ of the pump.

2. Total Head: The total head of a pump is the difference between the pump suction and discharge pressures – expressed in terms of metres or feet head :

Suction Head : This is the vertical distance, in feet or metres, from the centreline of the pump to the level of liquid in the vessel from which the liquid is being pumped.

If  the liquid level is above the pumo centreline, the suction head is positive. If below the centreline, the suction head is negative.

Discharge Head: Is the discharge pressure of the pump, expressed in feet or metres of liquid.

Total Head: = Discharge head -Suction head ( See Figure : 22 )

3. Power: This is the energy used by the pump in a given time. Its unit is ‘Horsepower’ (HP). 1 HP is equivalent to 0.746 kilowatt. (kW).

4. Efficiency: This is a percentage measure of the pump’s effectiveness in transferring the power used into energy added to the pumped liquid.

The formula for calculation of efficiency is :

Efficiency = (Output power)/(Input power)X 100%

Pumps in industry, usually operate at 70% to 80% efficiency.

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Figure : 22 – Total Head

In Figure 22, the pump is taking suction from Tank ‘A’ and discharging to Tank ‘B’. The Head (or height) of water in ‘A’ to the centre-line of the pump is 23 feet. This is called the ‘Suction Head‘.

The discharge line inlet to ‘B’ is 50 feet above the pump centre-line. This is the ‘Discharge Head‘. The ‘Total Head‘ is the difference between the two figures.

This is 50 -23 = 27 feet.

(Note: If the suction vessel is below the pump centre line, the suction head will be a negative figure).

Using the formula for Static Head Pressure, we can find the suction and discharge pressures of the pump. (Both tanks are at atmospheric pressure).

Suction pressure = 23 x 0.433 = 10 Psig. Discharge pressure = 50 x 0.433 = 21.7 Psig

If a liquid other than water is used, the Specific Gravity of the liquid must be included in the above formula to obtain the pressures.

E.g. If we use an oil with S.G. of 0.88, the pressures would be: – Suction pressure = 23 x 0.433 x 0.88 = 8.8 Psig. Discharge pressure = 50 x 0.433 x 0.88 = 19.1 Psig

Net Positive Suction Head Required

The pump manufacturer’s specified margin of suction pressure above the boiling point of the liquid being pumped, is required to prevent cavitation. This pressure is called the ‘Net Positive Suction Head’ pressure (NPSH).

In order to ensure that a NPSH pressure is maintained, the Available NPSH should be higher than that required. The NPSH depends on the height and density of the liquid and the pressure above it.

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