Service Delivery 3

Hydraulics

Aim

To ensure students can explain the principles of obtaining and delivering water.

Learning OutcomesAt the end of the session students will be able to:

• Understand the relationship between atmospheric pressure and suction lift

• Describe the principal characteristics of pressure

• Describe how friction causes pressure loss

• Explain the term ‘jet reaction’

• Carry out basic fireground calculations.

Calculating areas.

Area = wXh

Area = vertical hXw

a

b

Area = a+bXh

2

w

h h

w

Calculating volumes.

h

r

r hVolume =2

Calculating volumes.

Volume = hXwXdw

h

d

Calculating volumes.

Water

Volume = 2 x a x d

3

Pressure is perpendicular to any surface on which it acts.

Atmospheric Pressure

Pressure at any point in a fluid at rest is of the same intensity in all directions.

Pressure Gauges

Pressure applied from outside to a fluid contained in a vessel is transmitted equally in all directions.

PressurePressure gauges Pressure gauges

Downward pressure of a fluid in an open vessel is proportional to depth.

1m

2m

3m

The downward pressure of a fluid in an open vessel is proportional to the density of the fluid.

Mercury 10mm

Water 136mm

The downward pressure of a fluid on the bottom of a vessel is independent of the shape of the vessel.

Atmospheric pressure approx 1bar

Water

vacuum

Lifting water by atmospheric pressure.

Theoretical lift

Approximately 10 metres - assuming a perfect vacuum can be created and the water is cool and pure.

Pressure and head

• The height to which water is lifted or pumped is referred to as the head

• To raise a column of water 1 metre in height requires a pressure of 0.1bar. This applies to both ‘suction head’ and ‘delivery head’.

Factors limiting suction lift

• Lifting water from it’s existing level to the pump inlet

• Overcoming frictional resistance to the water

• Turbulence at the pump eye

• Creation of flow.

Practical lift

Because of these factors the maximum practical suction lift is generally regarded as 8 metres.

Lifting practically.

7m

3m

waterwater

2m

8m

water

Five principal laws govern the loss of pressure due to friction.

Friction loss varies directly with the length of the hose

1 2

Hose lengths

Friction loss

0.2b

0.4b

.

For the same velocity friction loss decreases with the increase in diameter

45mm 70mm 90mm.

Friction loss increases directly with the square of the velocity

2000 litres/min pressure loss 6bar

1000 litres/min pressure loss 1.5bar.

Friction loss increases with the interior roughness of the pipe.

For all practical purposes, friction loss is independent of pressure.

Calculating jet pressures

Pressure loss due to friction;

• 70mm non-percolating hose 0.2 bars per 25 metre length.

Pressure loss due to head

For every metre in height that water is pumped 0.1bar of pressure is lossed.

Question

What pressure will be required at the pump to supply 3 bars pressure at the branch.

6 lengths of hose are being used to supply a branch deployed at a height of 15m

Answer

Nozzle pressure required 6 bars

Pressure loss due to head 0.1b X 15 = 1.5b

Pressure loss due to friction 0.2b X 6 = 1.2b

Pump pressure required 8.7bars

Fireground flow rate calculation

• 45mm diameter hose = 300 litres/min

• 70mm diameter hose = 600 litres/min

Function of a branch and nozzle

To convert pressure energy into velocity energy and ensure that an effective jet of water leaves the nozzle.

Achieved by reducing the cross-sectional area that the water is flowing through

45mm diameter hoseline

20mm diameter nozzle.

Advantages of higher nozzle pressures

• A greater striking power is achieved

• A longer reach can be obtained

• A larger volume of water is applied.

Factors affecting the pressures adopted

• The type and size of nozzle used

• The type of jet required

• The intensity of the wind.

Jet Reaction

• The mass of water per second passing through the nozzle

• The squared diameter of the nozzle.

The magnitude of the reaction is dependant on;

Jet Reaction

Jet Reaction

= 1.57 x p x d

10

2

R = 1.57× 7 × (25x25)

10

= 687 Newtons.

1.57 x p x d

10

2

Using a 25mm nozzle at 7 bars pressure

R = 1.57× 7 × (12.5x12.5)

10

= 172 Newtons.

1.57 x p x d

10

2

Using a 12.5mm nozzle at 7 bars pressure

Confirmation Assessments will be based on this lesson and the corresponding study note

Learning Outcomes• Understand the relationship between

atmospheric pressure and suction lift

• Describe the principal characteristics of pressure

• Describe how friction causes pressure loss

• Explain the term ‘jet reaction’

• Carry out basic fireground calculations.

THE END