1 Hydraulic Machines and Dams

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HYDRAULIC MACHINESThese are machines and tools that useliquidfluid power to do simple work. For example

converting hydraulic energy into mechanical energy and vice versa.

Hydraulic fluid is transmitted throughout the machine to varioushydraulic motors andhydraulic

cylinders and which becomes pressurized according to the resistance present. The fluid iscontrolled directly or automatically bycontrol valves and distributed throughhoses andtubes.

A very large amount of power can be transferred through small tubes and flexible hoses, and the

high power density and wide array of actuators can make use of this power.

Hydraulic machinery is operated by the use of hydraulics, where a liquid is the powering

medium.

Examples of energy conversions:

Pumps: Convert mechanical energy (often developed from electrical source) into hydraulic

energy (position, pressure and kinetic energy).Water turbines: Convert hydraulic energy into mechanical energy and mechanical energy isused to drive generators that develop electricity.

Water turbines are generally designed and manufactured to each power station s own conditions

of water head, discharge, and water and power demands.

Electrical energy (Input) MOTORSMechanical Energy (Shaft power) PUMPS

Hydraulic Energy (Output)

Mechanical Energy (Water, Input) TURBINES Mechanical Energy (shaft power)

GENERATORS Electrical Energy (Output)

Small electric pump Gear pump
http://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Fluid_powerhttp://en.wikipedia.org/wiki/Hydraulic_fluidhttp://en.wikipedia.org/wiki/Hydraulic_motorhttp://en.wikipedia.org/wiki/Hydraulic_cylinderhttp://en.wikipedia.org/wiki/Hydraulic_cylinderhttp://en.wikipedia.org/wiki/Control_valveshttp://en.wikipedia.org/wiki/Hydraulic_machinery#Hose.2C_tubes_and_pipeshttp://en.wikipedia.org/wiki/Hydraulic_machinery#Hose.2C_tubes_and_pipeshttp://en.wikipedia.org/wiki/Hydraulic_machinery#Hose.2C_tubes_and_pipeshttp://en.wikipedia.org/wiki/Hydraulic_machinery#Hose.2C_tubes_and_pipeshttp://en.wikipedia.org/wiki/Control_valveshttp://en.wikipedia.org/wiki/Hydraulic_cylinderhttp://en.wikipedia.org/wiki/Hydraulic_cylinderhttp://en.wikipedia.org/wiki/Hydraulic_motorhttp://en.wikipedia.org/wiki/Hydraulic_fluidhttp://en.wikipedia.org/wiki/Fluid_powerhttp://en.wikipedia.org/wiki/Liquid


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Pump Efficiency

e= output/input = water power /shaft power

Pump Efficiency = QH/TN

Where Q = flow rate capacity

H = total head (static head + losses)

T = torque (rotation)ofshaft

N = speed (rpm,rad/sec)

Pump performance characteristics

The actual head rise is always less than the ideal head rise by an amount equal to the head loss.

Head Term in Pumping

Static suction lift:The vertical distance from the water level in the source tank to the

centerline of the pump. If the pump is located at a higher level than the source tank, the

static suction lift is negative

Static discharge head: The vertical distance from the centerline of the pump to the water

level in the discharge tank.

Total static head:The sum of static suction lift and the static discharge head, which is equal

to the distance between the water levels of discharge and source tanks.

System head curve

Total static head is used for selection of a pump and it represents the behavior of the piping

system. For any piping system there are losses. Head in pump, Hp is used to calculate the power

requirements for the pump.

Hp= Hs+ hf+ hm

Hs= static head

hf= friction loss

hm= minor loses

Power, P= QHp/e

Note: H s is the total suction head on the sketch and not static suction l if t (see the fi gure).


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For any pumping system, the friction loss hf, by darcys equation is given by;D2g

fLvh

2

f

And for minor losses, hm is given by; 2g

vkh

2

f

Head in Pump for a system head curve can be expressed in terms of Q and D as:

4

2

5

2

pD

KQ

D

fLQ

g

0.81HsH

The plot of the above equation Hp and Q gives the system head curve. This curve represents the

behavior of the piping system and is important in pump selection.

Example 1:What will be the discharge in this water system if the pump has the characteristicsshown below?


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Q (m /s) 0 0.05 0.10 0.15 0.2 0.25 0.3 0.35

Hp (m): 55 57 55 52 48 40 25 10

Assume that Ke = 0.5, Kb = 0.35 and Ke = 1.0

Solution,

Determining Hp for system head curve, first determine the local losses and friction losses.

For Q = 0, Hs = 230200 = 30m. Giving Hp as 30m.

Plot a graph of Hp against Q for both pumping and the system head curve. The point of

intersection is the discharge required.


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First determine Hp for system head curve the plot all curves.

Power = 65*32/68 = 30KW.


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HYDRAULIC STRUCTURES

DAM ENGINEERING

A dam is a hydraulic structure of fairly impervious material built across a river to create a reservoir

on its upstream side for impounding water for various purposes.

A dam and a reservoir are complements of each other.

Dams are generally constructed in the mountainous reach of the river where the valley is narrow

and the foundation is good.

Generally, a hydropower station is also constructed at or near the dam site to develop hydropower.

Dams are probably the most important hydraulic structure built on the rivers. These are very huge

structure and require huge money, manpower and time to construct.

CLASSIFICATION OF DAMS

Based on function served

Storage dams

Detention dams

Diversion dams

Debris dams

Coffer dams - a temp dam constructed for facilitating construction. It is an enclosure constructed

around a site to exclude water so that the construction can be done in dry.

Based on Hydraulic Design

Overflow dams

Non-overflow dams

Based on Materials of Construction

Masonry dam

Concrete dam

Earth dam

Rock fill dam

Timber dam


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Steel dam

Combined concrete-cum-earth dam

Composite dam.

*****And others******

Based on Rigidity

Based on structural action

SELECTION OF TYPE OF DAM

Selection of the most suitable type of dam for a particular site requires a lot of judgment and

experience.

It is only in exceptional cases that the most suitable type is obvious.

Preliminary designs and estimates are usually required for several types of dams before making the

final selection on economic basis.

Various factors govern the selection of type of dam including:

Topography and valley shape

Geology and foundation conditions

Availability of construction materials

Overall cost

Spillway size and location

Earthquake hazards

Climatic conditions

Diversion problems

Environmental considerations

Roadway

Length and height of damLife of dam

Miscellaneous considerations

SITE SELECTION FOR A DAM

A dam is a huge structure requiring a lot of funds.

Extreme care shall be taken while selecting the site of a dam.

A wrong decision may lead to excessive cost and difficulties in construction and maintenance.

Various factors should be considered when selecting the site of a dam including:

TopographySuitable Foundation

Good Site for reservoir(i) Large storage capacity (ii) Shape of reservoir basin (iii) Water

tightness of the reservoir (iv) Good hydrological conditions (v) Deep reservoir (vi) Small

submerged area (vii) Low silt inflow (viii) No objectionable minerals

Spillway site

Availability of materials

Accessibility


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Healthy surroundings

Minimum overall cost

o Other considerations

EMBANKMENT DAM

Is a massive artificial dam. It is typically created by the placement andcompaction of a complex semi-

plastic mound of various compositions of soil, sand, clay and/or rock. It has a semi-permanent

waterproof natural covering for its surface and a dense, waterproof core. This makes such a dam

impervious to surface or seepageerosion.The force of the impoundment creates a downward thrust

upon the mass of the dam, greatly increasing the weight of the dam on its foundation. This added force

effectively seals and makes waterproof the underlying foundation of the dam, at the interface between

the dam and itsstream bed.Such a dam is composed of fragmented independent material particles. The

friction and interaction of particles binds the particles together into a stable mass rather than by the use

of a cementing substance.

Types

Embankment dams come in two types: the earth-filled dam(also called an earthen dam or terrain dam)

made of compacted earth, and therock-filled dam. A cross-section of an embankment dam shows a

shape like a bank, or hill. Most have a central section or core composed of an impermeable material to

stop water from seeping through the dam. The core can be of clay, concrete, orasphalt concrete.This

dam type is a good choice for sites with wide valleys. Since they exert little pressure on their

foundations, they can be built on hard rock or softer soils. For a rock-fill dam, rock-fill is blasted using

explosives to break the rock. Additionally, the rock pieces may need to be crushed into smaller grades to

get the right range of size for use in an embankment

dam.

CONSIDERATIONS DURING THE INVESTIGATION & EVALUATION OF BOTH PROPOSED AND EXISTING

DAMS
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The embankment must be safe against excessive overtopping by wave action especially during pre-

inflow design flood conditions.

The slopes must be stable during all conditions of reservoir operations, including rapid drawdown,

if applicable.

Seepage flow through the embankment, foundation, and abutments must be controlled so that no

internal erosion (piping) takes place and there is no sloughing in areas where seepage emerges.

The embankment must not overstress the foundation.

Embankment slopes must be acceptably protected against erosion by wave action and from

gullying and scour against surface runoff..Hydraulics

The embankment, foundation, abutments and reservoir rim must be stable and must not develop

unacceptable deformations under earth quake conditions.

Earth dams

An earth dam is made of earth (or soil) and resists the forces exerted upon it mainly due to shearstrength of the soil. These are the common dams constructed. They are usually built in wide valleys

having flat slopes at flanks (abutments). Can be homogeneous when the height of the dam is not great

and are of zoned sections, with an impervious zone (called core) in the middle and relatively pervious

zones (called shells or shoulders) enclosing the impervious zone on both sides.

N.B: Read about Advantages and disadvantages of earth dams and concrete dams

DESIGN OF DAMS

A survey is done to present on paper a contour map of the reservoir up to and exceeding the maximum

flood level, and to provide details for the location of the embankment, spillway and outlet works.

From the contour map, the capacity of the reservoir can be assessed for varying dam heights. A depth-

capacity curve can then be drawn up to provide a quick and easy method for the dam designer to

choose the optimum full supply level. A simplified example of a depth-capacity curve is shown in Figure

below.


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Catchment Yield, Y

Is based on the expected annual runoff from a catchment and is used in assessing the feasibility of a

dam and in determining the required height of the embankment. The latter is important to allow the

dam designer to size the dam to suit expected inflow and estimate the area that can be irrigated. It is

estimated as follows:

Where the average percentage of runoff is not known, use as a guide, a figure of 10 percent of the

mean annual rainfall for the catchment area. If more information is known, take the rainfall on a

return period of 1 in 10 years as a guideline.

Calculate the annual runoff for the catchment, in mm, based on the percentage determined above.

This is Rr.

Measure the catchment area A in km2, upstream of the proposed embankment. Ignore any

upstream dams (as these may already be full at the time of a flood event often at the end of a

rainy season and thus offer no retardation of any flood moving downstream) and calculate the

area of the whole catchment.

The annual runoff for the catchment (the catchment yield in an average year), Y, in m3, is given by:

Y = Rr x A x 1000

Determining the capacity of the reservoir without knowing the area of reservoir

This is obtained from6

LTHQ


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Where:

Q is the storage capacity in m3and should not exceed Y above.

L is the length of the dam wall at full supply level (FSL) in m.

T is the throwback, in m and approximately in a straight line from the wall.

H is the maximum height of the dam, in m, at FSL.

Determining the capacity of the reservoir knowing the area of reservoir

This is obtained from;3

HAQ

Where;

Q is the reservoir capacity in m3

A is the reservoir area in m2

H is the maximum depth of the reservoir in m

For example:

Give the maximum depth if 3.25m and areas if 327,000m2, the reservoir capacity becomes 354,250 m

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1 Hydraulic Machines and Dams