A centrifugal pump requires priming when the pump casing and suction line are not completely filled with liquid before starting, particularly when:

1. The pump is installed above the liquid level, i.e. under a suction-lift arrangement.
2. The pump and/or suction pipe contains air or vapour after the pump has been stopped, drained, opened for maintenance, or has lost its prime.
3. The pump is started for the first time after installation.
4. Air has entered through the suction side due to leakage, a defective foot valve, or an improperly filled suction line.

Reason for priming

A centrifugal pump cannot normally pump air effectively. If the impeller rotates with air in the casing, it produces only a small pressure difference, which is generally insufficient to draw the liquid up through the suction pipe. Therefore, the pump casing and suction line must first be filled with liquid and the air removed.

Priming may be carried out by:

• Filling the pump casing and suction line manually.
• Using a foot valve to retain liquid in the suction line.
• Using an external priming device, such as a vacuum pump or ejector.

Once the casing and suction line are filled with liquid, the rotating impeller can produce the required pressure difference and the pump will operate normally.

The performance curves of a centrifugal pump show the relationship between the pump capacity and its operating characteristics. These curves are normally obtained by testing the pump with water at a constant rotational speed.

The horizontal axis represents the capacity or flow rate, (Q). Depending on the graph, the vertical axes represent head, efficiency and brake horsepower (power).

The main performance curves are as follows:

1. Head–Capacity Curve ((H-Q))

The head produced by the pump decreases as the flow rate increases.

• At zero flow, the pump develops its maximum or shut-off head.
• As the discharge or capacity increases, the head gradually decreases.
• At high flow rates, the head falls rapidly.

This is the characteristic downward-sloping pump head curve.

2. Efficiency–Capacity Curve ((\eta-Q))

The efficiency curve shows how effectively the pump converts the mechanical energy supplied to the shaft into useful hydraulic energy.

• At zero flow, the efficiency is zero.
• As the flow increases, the efficiency rises.
• It reaches a maximum value known as the Best Efficiency Point (BEP).
• Beyond the BEP, the efficiency decreases again as the flow increases further.

Thus, the efficiency curve is approximately bell-shaped or parabolic.

The pump should preferably be operated at or close to the BEP, as this gives maximum efficiency and generally results in lower vibration, noise and mechanical wear.

3. Brake Horsepower–Capacity Curve ((BHP-Q))

The brake horsepower curve shows the power required to drive the pump at different flow rates.

• The power requirement generally increases as the capacity increases.
• Therefore, the driving motor must be selected with sufficient capacity to meet the maximum expected power requirement.

Best Efficiency Point (BEP)

As shown in the graph, the BEP is the point at which the pump operates at maximum efficiency. It corresponds to a particular combination of flow rate, head and power requirement.

For satisfactory and economical operation, the pump should normally be selected so that its normal operating point is as close as practicable to the BEP.

System Operating or Duty Point

A centrifugal pump does not operate independently of the piping system. The actual operating condition depends on the system head, which consists of:

• Static head, and
• Frictional and other flow losses in the piping system.

When the system head curve is superimposed on the pump head-capacity curve, the point of intersection is called the:

• Operating Point, or
• Duty Point.

At this point, the head developed by the pump is exactly equal to the head required by the system.

Ideally, the pumping system should be designed so that the normal duty point lies at or near the pump's Best Efficiency Point (BEP).