Pumps

Question 1

Name the two types of pumps

Answer 1

• Kinetic (centrifugal)

- Positive Displacement (reciprocating, rotary)

Question 2

Describe how the fluid is moved in a positive displacement pump

Answer 2

A pocket of fluid is trapped inside a set of solid boundaries. The pocket of fluid is moved in the direction of discharge by the movement of boundaries.

Question 3

What does the delivery rate of a positive displacement pump depend on?

Answer 3

The speed of the prime mover. It is independent of the delivery head.

Question 4

What happens to the flow rate if the discharge pressure of a positive displacement is changed?

Answer 4

The positive displacement pump will produce the same flowrate at any given speed.

Question 5

What is the requirement of the discharge valves of a positive displacement pump?

Answer 5

The pump must never operate against close valves on the discharge side of the pump. It has no shut off head. Pump operating against closed discharge valves will continue to produce flow until the pressure in the discharge line is increased and the pump or line bursts.

Question 6

What is dead-heading and how it can be prevented in positive displacement pumps?

Answer 6

The pump’s discharge will be closed either due to blockage or a valve being closed. There will be no shut off head in positive displacement pumps. The dead-heading can lead to spectacular failures. A relief valve should be installed.

Question 7

Give examples of positive displacement pumps

Answer 7

• reciprocating pump
• gear pump
• diaphragm pump
• screw pump

Question 8

Describe a reciprocating pump

Answer 8

The reciprocating pump compresses a material. It draws a liquid into the pump, the action of the pump increases and decreases the capacity volume. There needs to be a non-return valve otherwise there would just be an ebb and flow of the fluid. Piston would just be moving in and out. Sinusoidal pattern.

Question 9

Describe a gear pump

Answer 9

Uses the actions of rotating cogs and gears to transfer fluids. The rotating element develops a liquid seal with the outer casing and creates suction at the pump inlet. Fluid drawn into the pump, enclosed by the teeth of gears and then transferred to discharge.

Question 10

What is the displacement of a positive displacement pump?

Answer 10

It is the quantity of fluid moved per cycle

Question 11

Why is 100% theoretical displacement never achieved in positive displacement pumps?

Answer 11

Because there is leakage through clearances between the solid moving boundaries.

Question 12

What is the relationship between back leakage and the pressure difference across the pump?

Answer 12

As the pressure difference across the pump increases, the flowrate of the back leakage increases.

Question 13

What gives smoother flow - reciprocating or rotary pumps?

Answer 13

Rotary pumps

Question 14

Describe the action of a centrifugal pump

Answer 14

A centrifugal pump converts the input power to kinetic energy by accelerating the fluid in a rotating device.

Question 15

What is the most common centrifugal pump?

Answer 15

The volute pump

Question 16

Describe the volute pump

Answer 16

The fluid enters the pump through the eye of the impeller which rotates at high speeds. The fluid accelerates rapidly outwards from the pump casing and a vacuum is created at the eye of the impeller. Vacuum continuously draws fluid into the pump. The fluid gains kinetic energy.

Question 17

How is the impeller rotated in a volute pump?

Answer 17

It is rotated by an electric motor.

Question 18

In which direction does the impeller in a volute pump rotate?

Answer 18

It rotates in the same direction as the fluid flowing out. This minimises the energy lost due to heat.

Question 19

What diameter of impeller do we want in a volute pump?

Answer 19

We don’t want the maximum or the minimum impeller diameter because then we can’t upgrade.

Question 20

Does the centrifugal pump have a shut off head?

Answer 20

Yes. The centrifugal pump does have a shut off head. The pump will not fail if dead headed. It has no short term consequence. But may cause overheating.

Question 21

What is the net positive suction head?

Answer 21

It is the amount by which the head exceeds the vapour pressure of the fluid at the suction flange.

Question 22

When does cavitation occur?

Answer 22

When the pressure of the system is less than the vapour pressure of the fluid.
required NPSH > available NPSH

Question 23

Why is cavitation a problem?

Answer 23

Cavitation is the formation of vapour bubbles in the fluid. These can be a problem as when the bubbles move to regions of higher pressure, then the bubbles will implode. This can cause damage to the impeller blades of a pump.

Question 24

The effects of cavitation

Answer 24

Pitting of impeller blades, drop in efficiency, noise in the pump and vibration bearing. Can cause the shaft to break.

Question 25

How to avoid cavitation?

Answer 25

• Raise the height of the tank
• Select a pump with a lower requirement for the NPSH
• Increase the diameter of the pipe
• Remove the number of fittings in the suction flange.

Question 26

Least likely factor to contribute to inefficiencies in the pump

Answer 26

Material of the construction of the pump

Question 27

What could be changed if the flowrate is not at the desired intersection between the characteristic and system curve?

Answer 27

There could be an increase in the resistance to the flow (e.g. by adjusting a valve, installing a valve itself will cause an increase in the pressure drop in the system and a lower flowrate at intersection). Or the static heads could be changed.

Question 28

How to increase the efficiency of a pump?

Answer 28

Increase the impeller speed (use gearing)

Increase the impeller diameter

Question 29

What is a rating curve?

Answer 29

It is a plot of the head developed versus capacity.