Centrifugal Pumps Questions and Answers

Answer: D

The device is the rotary pump.

Rotary pumps (gear, vane, lobe and screw pumps) are positive displacement machines like the reciprocating pump: they trap a fixed volume per revolution, are self-priming and deliver against high pressure. Like the centrifugal pump, however, they rotate continuously, so they are compact, have no valves and give a nearly steady flow without the pulsation of a piston pump. That combination is why textbooks describe them as having the advantages of both.

An air lift pump raises water by injecting compressed air into the rising pipe. A hydraulic ram uses the water hammer of a large flow to lift a small part of it, with no external power. A jet pump uses a high-velocity jet in an ejector, usually supplied by a centrifugal pump, to draw water from a deep well.

Answer: B

The WRONG statement is that the net positive suction head should be more than the theoretical atmospheric pressure.

Cavitation is avoided when the available NPSH exceeds the NPSH required by the pump, a value given by the manufacturer and usually a few metres. It has nothing to do with exceeding atmospheric pressure (about 10.3 m of water); with a suction lift the available NPSH is always less than atmospheric head.

The other statements are correct: the head at zero flow is the shutoff head; by the affinity laws head varies as speed squared; and turbine (deep-well) pumps are multistage centrifugal pumps, so they behave more like centrifugal than propeller pumps.

Answer: C

The jet pump is not a rotating machine.

A jet pump (ejector) has no moving parts: a high-velocity motive jet of fluid entrains the fluid to be pumped through momentum exchange in a mixing chamber and diffuser.

The others all rotate. A centrifugal pump uses a rotating impeller. A gear pump carries fluid round in the spaces between meshing rotating gears. A vane pump has a rotor with sliding vanes turning in an eccentric casing.

Answer: D

The power consumption becomes eight times.

The affinity laws for a centrifugal pump of fixed impeller diameter give

\[ \begin{aligned} Q &\propto N \\ H &\propto N^{2} \\ P &\propto N^{3} \end{aligned} \]

Doubling the speed N doubles the discharge, multiplies the head by four and the power by \(2^{3} = 8\).

'Doubled' is the change in discharge and 'Four times' the change in head, so both mix up the laws. Power cannot stay the same when the pump is doing more work at a higher speed.

Answer: C

An impeller with backward curved blades has a drooping head-discharge characteristic.

With backward curved blades (outlet angle less than 90°), the whirl velocity at outlet falls as the flow rises, so the theoretical head decreases steadily with discharge. This drooping characteristic is stable, and the power drawn reaches a maximum and then falls, so the motor cannot be overloaded. That is why nearly all centrifugal pumps use backward curved blades.

A rising head-discharge characteristic belongs to forward curved blades, and radial blades give a flat theoretical one. The options about reverse rotation and an unbalanced rotor describe nothing specific to the blade shape.