Pressure Loss Calculations

Question 1

What are the two main reasons for a reduction in pressure?

Answer 1

1) Fictional loss in the hose line

2) Effects of gravity when working above the water source or pump

Question 2

What are the factors affecting pressure loss?

Answer 2

1) smaller internal diameter of hose, greater amount of frictional loss
2) higher the required output, higher amount of frictional loss
3) working above water source=pressure loss working below = pressure gain
4) friction loss calculated per length of hose, will increase as length of hose lines increase

Question 3

What are the 3 laws governing the loss of pressure through friction?

Answer 3

1) longer the hose lines, greater the frictional loss
2) if flow rate through line hose is doubled, pressure loss due to friction will increase by a factor of 4
3) the smaller the hose diameter, the greater the frictional loss

Question 4

What is the frictional loss with 45mm hose lines?

Answer 4

a) 45mm hose has pressure loss of 1 bar per length

b) frictional loss is approximately 5 times more than that of 70mm hose when supplying at maximum flow rates

Question 5

What is the frictional loss with 70mm hose lines?

Answer 5

a)it has a pressure loss of 0.2 bar per length

Question 6

What is the effect of gravity on the pressure at the branch?

Answer 6

a) a column of water will exert a downward pressure of 1 bar for every 10 metres of height (known as head of water and equates to 0.1 bar for every 1 metre)
b) pressure at branch will reduce by 0.1 bar every metre branch is above level of pump
c) pressure at branch will increase by 0.1 bar for every metre branch is below the pump

Question 7

How do you calculate the pressure required at the pump?

Answer 7

a) working pressure of the branch
b) frictional loss rate for the hose
c) effects of gravity

Question 8

How is the pressure loss reduced?

Answer 8

Largest diameter hose available should be used to augment water supplies and hose should be twinned (amount of pressure lost to friction is quartered)

Question 9

What size hose should be connected to 45mm hose at the branch length to allow operator greater manoeuvrability?

Answer 9

70mm hose, allows the pump to work more efficiently at a lower engine speed

Question 10

If you can’t get the desired pressure you can…?

Answer 10

a) withdraw the branch
b) lower the flow setting
c) change to larger diameter hose