Hydraulics

Pressure due to fluids

Pressure is a measure of the force acting over a specific area.

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

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

 

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

Head is the pressure exerted by the depth of water. One cubic metre of water contains 1000 litres. Each litre of water has a mass of one kilogram (kg). Therefore one cubic metre of water has a mass of 1000 kg.

Note: Each metre of head of water is equal to a pressure of 10 kPa.

The rule of thumb for allowing for height loss or gain is to: add 10 kPa for every metre the nozzle is higher than the pump, or subtract 10 kPa for every metre the nozzle is lower than the pump.

Lift Using Air Pressure

A head of one metre equates to 10kPa. Therefore an atmospheric pressure of 100 kPa should be able to create a head of water of about 10 metres in theory. 

For practical purposes, the maximum lift is approximately 7 to 8 metres.
A minus pressure of 30 kPa on the compound gauge would correspond to a lift of 3 metres because 1 metre of head / lift equals 10 kPa of pressure..

Open Water Supply Capacity

Firefighters often have to determine the capacity of a water supply to estimate how long that supply will last.

Consider a rectangular water tank, 6 m by 2½ m by 2 m

Capacity in litres< /p>= length x width x depth x 1000  (multiply by 1000 to convert cubic metres to litres)

= 6 x 2½ x 2 x 1000= 6 x 2½ x 2 x1000 = 30 x 1000= 30000 litres.

 

Consider a swimming pool

Capacity (in litres) = length x breadth x depth x 1000 (multiply by 1000 to convert cubic metres to litres)

If it has an uneven depth, use the average depth in the calculation.

For example, a rectangular swimming pool is 4 metres wide, 8 metres long and is 1 metre deep at one end and 2 metres deep at the other.

Capacity (in litres) = length x width x depth x 1000

= 8 x 4 x (1+2) ÷ 2 x 1000

      = 8 x 4 x 1.5 x 1000
      = 48 x 1000
      = 48 000 litres.

If you required, say, a total water supply of 2000 L/min to fight a particular fire, this pool would supply that rate of flow for up to 24 minutes.(48 000 L ÷ 2000 L / min = 24 min) .

Consider a cylindrical water tank

Capacity (in litres) = 0.8 x depth x (diameter)2 x 1000

For example, a cylindrical water tank (standing on end) is 3 metres deep and 4 metres in diameter.

Capacity (in litres) = (pi x d2 x h)/4            (pi / 4 approximates to 0.8), therefore

= 0.8 x 3 m x (4m x 4m) x 1000

= 0.8 x 3 m x 16 m2 x 1000

= 0.8 x 48 mx 1000

= 38.4 mx 1000

= 38 400 litres

Consider a flowing source

Firefighters often underestimate the water supply available from quite small streams. For example, consider a creek that has a depth of 0.5 metres, a width of 4 metres and flowing past at a speed of 5 metres per minute.

Rate of flow (litres per minute) = depth x width x speed of flow x 1000

= 0.5 x 4 x 5 x 1000

= 0.5 x 20 x 1000

= 10 x 1000

= 10 000 L / min.

Questions

A nozzle is discharging water at 200 litres per minute. How long will it take to empty a full tank of water measuring 3 metres x 2 metres x 1 metre? (30 minutes)