Pumps

Question 1

List the components of a pump

Answer 1

• Casing (keeps liquid inside and protects internal parts)
• Inlet/suction (where the liquid enters the pump)
• Outlet/discharge (where liquid exits the pump)
• Component that physically moves the liquid through the pump (different for each type)
• Valves (installed in suction or discharge lines so the pump can be isolated/removed for maintenance)
• Driver or motor

Question 2

What process conditions affect pump selection

Answer 2

• The rate at which the fluid needs to be pumped
• The viscosity
• Possible non-Newtonian characteristics
• How clear it is
• If it contains suspended particles
• Corrosiveness

Question 3

What are the two main categories of pumps

Answer 3

• Positive displacement

- Centrifugal

Question 4

How pumps are named

Answer 4

• By the liquid that it pumps
• By its specific function on the plant
• By its location

Question 5

How does a positive displacement pump work

Answer 5

The volume of the liquid delivered is directly related to the displacement if the reciprocating pump. The flow rate of the liquid will be proportional to the pump speed and is not appreciably influenced by pressure

Question 6

How does a centrifugal pump work

Answer 6

It depends on giving the liquid a high kinetic energy, which is partially converted into pressure energy. It doesn’t require precision engineering since there is a gap between the impeller and the casing. This means it can pump slurries

Question 7

List some reciprocating pump types

Answer 7

• Piston pump
• Plunger or ram pump
• Diaphragm pump
• Metering pump

Question 8

List some rotary pump types

Answer 8

• Gear pump
• Flow inducer or peristaltic pump
• Mono pump
• Screw pump

Question 9

Describe the piston pump

Answer 9

The piston pump consists of a cylinder with a reciprocating piston connected to a rod which passes through a gland at the end of the cylinder. The liquid enters the suction line through a suction valve and is discharge through a delivery valve into the delivery line

Question 10

The total pressure at the delivery of the piston pump is the sum of what components?

Answer 10

• The static pressure at the delivery point
• The pressure required to overcome frictional losses in the delivery pipe
• The pressure required for the acceleration of the fluid

Question 11

Describe the plunger or ram pump

Answer 11

This pump is very similar to the piston type but it differs in that the gland is at the end of the cylinder rather than the middle, thus making replacement easier.

Question 12

Describe a diaphragm pump

Answer 12

In one section a plunger or piston operates in a cylinder in which a non-corrosive fluid is displaced. The movement of the fluid is transmitted by means of a flexible diaphragm to the liquid to be pumped.

Question 13

Why was the diaphragm pump developed

Answer 13

For handling corrosive liquids or liquids containing suspensions of abrasive solids

Question 14

What are the two main types of diaphragm pumps

Answer 14

• Induced flow flexible discs

- Edge retained diaphragm

Question 15

Describe a metering pump and when it is used

Answer 15

Positive displacement pumps driven by constant speed electric motors. They are used where a constant and accurately controlled rate of delivery of a liquid is required

Question 16

Describe a gear pump

Answer 16

Two gear wheels operate inside a casing - one of the gears is driven and the other operates in mesh with it. The liquid is carried around in the space between the consecutive teeth and the casing and is then ejected as the teeth become meshed

Question 17

What are the advantages of a gear pump

Answer 17

• Pump is self-priming
• There is uniform discharge with little pulsation
• The rotation is reversible
• Can handle gaseous fluids
• Due to small clearances there are only small flow variations with viscosity variations and pressure fluctuations

Question 18

What are the disadvantages of a gear pump

Answer 18

• Can only pump clean fluids
• Cannot run dry
• They need close tolerances to operate, so limits and alignment is critical
• Needs pressure relief as with a piston pump

Question 19

Describe the flow inducer or peristaltic pump

Answer 19

An elastic tube is compressed in stages by a rotor. The tubing is fitted to a curved track mounted around a rotor carrying three rollers. As the rollers rotate they flatten the tube against the track at the points of contact. The fluid is then squeezed forward by positive displacement.

Question 20

What are the advantages of a peristaltic pump

Answer 20

• Self-priming
• It can run dry
• It is reversible
• Will pump gaseous fluids
• Can pump abrasive materials
• Has only one contact part
• There are no glands or mechanical seals
• Clinically clean

Question 21

What are the disadvantages of a peristaltic pump

Answer 21

• Tube failure means loss of product and contamination
• Cannot run against a closed valve
• Pressure is limited to the tube strength
• At low rotational speeds pulsation can be a problem
• Relatively high cost or replacement tubes

Question 22

Describe the mono pump

Answer 22

A specially shaped helical metal rotor revolves eccentrically within a double helix. A continuous forming cavity is created as the rotor turns. The cavity progresses towards the discharge

Question 23

What are the advantages of a mono pump

Answer 23

• Gives a uniform flow
• Quiet in operation
• Will pump against high pressures
• Can handle corrosive and gritty liquids

Question 24

Describe the screw pump

Answer 24

Fluid is captured in a channel between a screw and the cylinder wall of the pump casing and moved toward the discharge as the screw turns

Question 25

Describe the centrifugal pump

Answer 25

The fluid is fed to the center of a rotating impeller and is thrown outward by centrifugal actio. Because of the high speed of rotation the liquid gains high kinetic energy and the conversion of kinetic energy into pressure energy gives increased pressure at the pump discharge

Question 26

Explain how centrifugal pumps can be operated efficiently

Answer 26

The optimum conditions for operation is at the duty point, where a vertical line from the point of maximum efficiency crossed the head curve (graph on pg 151)

Question 27

What is cavitation and how does it happen

Answer 27

Centrifugal pumps add pressure to a liquid, therefore the minimum pressure occurs on the suction side. If this pressure is less than the vapour pressure at the liquid temperature, boiling will occur. If there is sufficient vapour generated, the pump flow may stop. The pressure inside the pump causes the vapour bubbles to collapse. This is cavitation.

Question 28

What can be done to prevent cavitation

Answer 28

Pumps should be arranged so that the inlet is under a suction head or the pump may be fed from a tank. You can also increase the suction head or suction line diameter, by decreasing the fluid temperature, by reducing the pump speed or by throttling the flow in the delivery line.

Question 29

What are the main advantages of a centrifugal pump

Answer 29

- Simple in construction and can be made from a wide range of materials - Complete absence of valves - Operates at high speed and can be coupled directly to an electric motor - Gives a steady delivery - Maintenance costs are lower - No damage is done if the delivery line gets blocked - Much smaller - Liquids containing high proportions of suspended solids are readily handled

Question 30

What are the main disadvantages of a centrifugal pump

Answer 30

- Will not develop a high pressure - Operates at high efficiency over only a limited range of conditions - Not self-priming - If a non-return valve is not incorporated in the delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops - Very viscous liquids cannot be handled efficiently