Wk6 - Pumps & Pumping Systems

Question 1

What are pumps?

Answer 1

Machines used as a mechanical input to provide energy to a fluid.

Question 2

What are pumps used for? (2 reasons)

Answer 2

• To raise fluids from lower to higher elevations.
• To boost or maintain pressure and/or velocities within pipes.

Question 3

What are the two primary categories of pump?

Answer 3

• Positive displacement
• Rotodynamic

Question 4

What are Positive Displacement pumps used for?

Answer 4

Used in situations involving viscous liquids, or those containing suspended solids, or for groundwater control.

Question 5

What are Positive Displacement pumps further split into?

Answer 5

• Diaphragm pumps
• Piston pumps

Question 6

What are Rotodynamic pumps used for?

Answer 6

Permanent installations such as water treatment works.

Question 7

What are Rotodynamic pumps further split into?

Answer 7

• Axial flow pumps
• Radial flow pumps
• Mixed flow pumps

Question 8

How can cavitation be avoided?

Answer 8

By ensuring that the net positive suction head (NPSH) does not exceed the manufacturers limiting value.

Question 9

What is the formula for ‘Manometric Head’, ‘Hm’?

Answer 9

Hm = Hs + Hf + Hl

Hm = Manometric head
Hs = Static head
Hf = Frictional losses
Hl = Local losses

Question 10

How are local losses, ‘Hl’, calculated in pump network questions?

Answer 10

Using the formula provided in the question, typically:

Hl = number x V^2/2g

Question 11

How are frictional losses, ‘Hf’, calculated in pump network questions?

Answer 11

Using the Darcy Weisbach formula

Landa x L x V^2 / 2 x g x d

Question 12

What are the answers of ‘Hl’ and ‘Hf’ represented in initially?

Answer 12

Values of ‘V^2’

Question 13

How is ‘Q’ converted from ‘l/s’ to ‘m^3/s’

Answer 13

Divide by 1000

Question 14

How is a value for velocity, ‘V’ calculated?

Answer 14

V = Q/A

Using the discharge in m^3/s and the cross-sectional area of the pipe

Question 15

How is the manometric head calculated?

Answer 15

Hm = Hs + Hf + Hl

• Use the static head provided in the question
• Use the calculated value of velocity to satisfy the formulas for ‘Hf’ and ‘Hl’

Question 16

How is the ‘system curve’ plotted on a graph?

Answer 16

By plotting the corresponding values of ‘manometric head’ calculated against ‘discharge’.

Question 17

How is the ‘head-discharge curve’ plotted on a graph?

Answer 17

By plotting the discharge ‘Q’ against the total head ‘H’ provided in the question for a certain operating speed.

Question 18

How is the ‘efficiency curve’ plotted on a graph?

Answer 18

By plotting the discharge ‘Q’ provided against the efficiency ‘eta’

Question 19

How is the discharge calculated when the pump is operated at a particular speed?

Answer 19

The discharge value asked for in the question is the ‘Y-value’ corresponding to the intersection of the ‘head-discharge’ and ‘system curve’.

Question 20

What is the formula for power consumption of a pump?

What are the units involved?

Answer 20

P = (density x g x Q x Hm)/eta

P = power consumption (watts)
Density = 1000kg/m3
g = 9.81 m/s2
Q = discharge (m3/s)
Hm = manometric head (m)
eta = efficiency (decimal)

Question 21

What 2 assumptions must be made when considering a parallel pumping system?

Answer 21

• The head, ‘H’, remains constant
• The discharges, ‘Q’, of the individual pumps are added together

Question 22

How are the discharges of the parallel pumping system determined?

Answer 22

By superimposing a new head-discharge curve using the new values for discharge onto the original system curve and reading off the intersection point.

Question 23

What are the advantages of a parallel pump system?

Answer 23

• Redundancy within the system
• Flexibility in discharge

Question 24

What two assumptions must be made in series pumping systems?

Answer 24

• Head of each pump is added together
• Discharge is constant (remains the same as a single pump)

Question 25

How are the discharges of the series pumping system determined?

Answer 25

A new head-discharge curve using the new values for head is superimposed onto the original system curve. The discharge is then read off from the intersection point.

Question 26

What is the formula for the new discharge rate of a pump at a different operating speed?

What are the units involved?

Answer 26

Q2 = Q1 x (N2/N1)

Q1 = Original discharge (m3/s)
Q2 = New discharge (m3/s)
N1 = Original speed (revs/min)
N2 = New speed (revs/min)

Question 27

What is the formula for the new head of a pump at a different operating speed?

What are the units involved?

Answer 27

H2 = H1 (N2/N1)^2

H1 = Original head (m)
H2 = New head (m)
N1 = Original speed (revs/min)
N2 = New speed (revs/min)

Question 28

How do you predict the flow rate when pump speed increases?

Answer 28

Calculate new discharge (Q2) and head (H2) values, plot a line of new H-D and read off the intersection point against the original system curve.

Question 29

In a series pump system, how is the efficiency calculated from the head?

Answer 29

Efficiency = Head / No. of pumps

Question 30

In a parallel pump system, how is the efficiency calculated from the head?

Answer 30

Efficiency = Discharge / No. of pumps

Question 31

How is the efficiency calculated for use in the power consumption calculation?

Answer 31

The efficiency, or rate of flow per pump, is plotted on the discharge-efficiency graph and the efficiency is read off

Question 32

If the efficiency is smaller than the minimum value on the system curve, what happens?

Answer 32

There is no solution to the particular problem

Question 33

Briefly describe the two main types of positive displacement pumps and the situations in which they are mainly employed.

Answer 33

Diaphragm pumps and piston pumps.

Ideal for situations involving viscous liquids, or those containing suspended solids; in addition, they are largely employed for groundwater control or for de-watering on construction sites.

Question 34

How is the Hazen Williams equation arranged to get a value for ‘V^1.85’?

Answer 34

Hf

(6.78L / d^1.165 x c^1.85) x V^1.85