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Pressure, force and flow of water

SI units of pressure, force and flow

Explanations

Working out pressure

To work out pressure, we need to know two things:

the force or weight applied
the area over which the force or weight works

Example -

A force of 20 N acted over an area of 2 m2 (two square metres).

force ÷ area = pressure

20 ÷ 2 = 10 N/m2

Pascals and Newtons are measurements of pressure.

Notice that the unit of pressure here is N/m2 (newtons per square metre). Sometimes you will see another unit being used. This is called the pascal, Pa.

1 Pascal = 1 N/m2, so in the example above the pressure is 10 Pa.

Definitions

Velocity

The rate at which an object changes its position, taking into account speed and direction (m/s).

Acceleration

The rate at which an object increases its velocity.

Acceleration due to gravity

Gravitational pull of 9.81m/s (gravity is 9.81KN/m2).

Flow rate

The amount of fluid that flows through a pipe at a given time.

Force

An influence on an object that may cause it to move (N)+.

Head height x 9.81(gravity)= Intensity of Pressure N/m2

Kilopascals/kPa Bar Meters head

10 0.1 1

Static pressure is worked out when the water is stationary and dynamic pressure is worked out when the water is flowing.

Head refers to the height where the water comes from/stored. Pressure: the greater the head, the greater the pressure

Siphon

Siphonic action happens due to atmospheric pressure.

Both cisterns have atmospheric pressure acting on them.

The lower beaker has greater pressure because it is lower.

Pressure and flow

Force

If you reduce the pipe diameter, like on this garden hose, the speed increases and the water shoots out further, but the pressure and flow rate have reduced.

1 litre of water = 1kg

If the cistern on the left holds 40 litres of water, you can work out the force of water leaving the tap.

40kg x 9.81m/s gravity = 392N

What is flow rate?

The amount of fluid that flows through a pipe at a given time.

Flow rate can be affected by many factors:

- Changes in direction
- Pipe size
- Pressure
- Length of pipe
- Frictional resistance
- Constrictions.

Changes in pipework

If an appliance requires an increase in the flow rate, the elbows should be removed and replaced by machined bends, as this will ease the restriction.

The pipe diameter can be increased, to increase the velocity of the water, which is commonly the case for a bath feed compared to a basin feed. Resistance in pipes can be caused as you the plumber installs the system by the fittings you install. For every elbow, tee etc...will add resistance to the water. Notice the figures below highlights that the pulled bend adds less resistance than fittings, therefore whenever possible use a bend rather than a fitting.

Elbow 0.5m

Tee 0.6m

Stop valve 4.0m

Check valve 2.5m

Pulled bend 0.3m

Check your knowledge

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