Hydraulics & Hydrology Hydraulics & Hydrology ReviewReview

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Lecture3Dr. Jawad Al-rifai

WATER PRESSUREWATER PRESSURE

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Mass per unit volume is referred to as the densityof a fluid. The density of water at temp. 15C & pressure of 1

atmosphere is 999kg/m3

The force exerted by gravity on 1.0 cu ft (1.0 m3)of water is 62.4 lb (9.80 kN);

eq density X gravity (32.2ft/s2; 9.81m/s2 )Pressure is the force exerted per unit areaWater pressure is exerted equally in all directions,

and increases linearly with depth◦ Pressure in psi is equal to 0.433 times the depth in feet◦ Pressure in KPa is equal to 9.8 times the depth in meter

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A piezometer consists of a small tube rising from a container of water under

pressure

The height of the water in the tube denotes

the pressure of the confined water

Water pressure is commonly

measured by a Bourdon gauge

A mercury column can beused to measure relatively

high pressure values

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If the pressure measured is greater than atmospheric, this value is sometimes called gauge pressure

If the pressure measured is less than atmospheric,it is referred to as a vacuum

Absolute pressure is the term used for a pressure reading that includes atmospheric pressure◦The pressure relative to absolute zero

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4–2 PRESSURE-VELOCITY-HEAD 4–2 PRESSURE-VELOCITY-HEAD RELATIONSHIPSRELATIONSHIPS

The association between quantity of water flow,

average velocity, and cross-sectional areaof flow is given by the equation

This formula is known as the continuity equation

For an incompressible fluid such as water:For an incompressible fluid such as water:

If cross-sectional area decreases,If cross-sectional area decreases,velocity of flow must increasevelocity of flow must increase

If the area increases, the velocity decreasesIf the area increases, the velocity decreases

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Total energy in a hydraulics system is equal to the sum of elevation head + pressure

head + velocity head

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Valves, fittings, and other appurtenances disturbthe flow of water, causing losses of head◦In addition to the friction loss in the pipe

Distribution system losses due to appurtenances are relatively insignificant compared to pipe friction losses

In pumping stations & treatment plants, minor losses in valves & fittings are a major part of the total losses

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Unit head losses may be expressed as beingequivalent to the loss through a certain

length of pipe or by the formula

4–2 PRESSURE-VELOCITY-HEAD RELATIONSHIPS4–2 PRESSURE-VELOCITY-HEAD RELATIONSHIPS