Which method is used to prime a centrifugal pump?

Applying a vacuum to the pump is the correct method for priming a centrifugal pump. Centrifugal pumps require the liquid being pumped to be present in the impeller in order to operate effectively. When a vacuum is applied to the pump, it reduces the pressure within the pump casing, allowing atmospheric pressure to push the liquid into the pump from the source. This creates a path for the liquid to flow into the impeller and facilitates the pumping process once the pump is started.

The other methods listed do not effectively initiate the flow of liquid into the pump. Increasing air pressure in the pump does not help prime a centrifugal pump since these pumps are designed to work with liquids, not air or pressurized air. Bleeding the suction line may help remove trapped air but does not actively prime the pump by drawing liquid into it. Opening the suction valve is necessary for the flow to occur, but it will not create the necessary conditions for the pump to fill with liquid, which is why applying a vacuum is the most effective method for priming a centrifugal pump.