HYDRAULIC TURBINES

HYDRAULIC TURBINES

In 1822 French mining Engineer Claude Burdin first used the word Turbine from the Latin word Turbo. A turbine can be defined as the machine which contains rotating parts that is shafts or drums having vanes or blades on its periphery.

There are different kinds of turbine for different purposes. The different kinds of turbines are like gas turbine, steam turbine and water turbines. While talking about the generation of electricity through water, then water turbines are used in Hydroelectricity power plants. Hydropower also falls on the major category of Civil engineering works and in the country like Nepal, it has a vast scope. The country is small but Nepal is renowned as one the second largest countries in terms of water potentialities in the entire world.

We will talk about the hydraulic turbines only and briefly about the water wheels.

WHAT IS A WATER WHEEL?

A water wheel is a machine that consists of wheels with vanes or blades or buckets on its periphery which converts the kinetic or potential energy of flowing water through it into some useful forms of power.

Vitruvius, an ancient engineer during 14 A.D. is supposed to be the one who brought the idea of water wheels and their creation. These water wheels were used in the past up until 20^th century for grinding of woods so as to make the paper, for hammering wrought irons etc. Water wheels are now replaced by modern turbines.

TYPES OF WATER WHEELS

1)    A HORIZONTAL WHEEL WITH A VERTICAL AXLE

2)    A VERTICAL WHEEL WITH A HORIZONTAL AXLE

Within the vertical wheel with a horizontal axle, it can be divided into three categories which are as:

1)    The undershot water wheel: in this water wheel, water jet flowing, strikes the vanes of wheel at the bottom of the wheel.

2)    The breast water wheel: in this water wheel, water jet flowing, strikes the vanes of wheel near the middle of the wheel.

3)    The overshot water wheel: in this water wheel, water jet flowing strikes the vanes of the wheel at the tip of the wheel.

While talking about the efficiency of the water wheels, among all the overshot water wheels are the most efficient type.  But as the water wheels are mostly dependent on the flowing water and its speed, there are certain limitations as where they can be placed.

HDYRO ELECTRIC PLANT

In general, a hydro electric plant is a place where electricity is generated and distributed.  A hydro power plant has three types of facilities that it uses: impoundment, diversion or pumped. Mostly, a hydro power uses the impoundment storage facility. It can be achieved by building of the dams or reservoirs.

Furthermore, a hydro power or electric plant can be large, small or micro hydro power depending on the size of water bodies, budget available, capability, required energy etc. In Nepal, there are various hydro power plants. Names of some of them are: Kaligandaki hydro power plant, Marsyangdi power plant, Kulekhani power plant, Trishuli hydro power plant etc.

A hydro power plant has following units:

1)    A dam or reservoir

2)    Pipes or tunnels for conveying the water

3)    Penstock pipes in case of very steep slopes

4)     A surge tank to protect the pipes from bursting due to changes in discharge rates

5)    A generator

In power house the generator and turbines are kept. Generator can be taken as the external load to the turbines which are directly attached to the shaft of turbines.

WHAT IS HYDRAULIC TURBINE??

Hydraulic turbine is a mechanical device which consists of rotating parts called a rotor assembly and consists of vanes or blades. A hydraulic turbine consists of wheels called runners and a guiding apparatus at inlet that guides the water in turbines in correct direction. These hydraulic turbines are used in the Hydro power plant for the generation of electricity. Turbines converts the potential or kinetic energy of water into mechanical energy and this mechanical energy of the turbine is converted into electrical energy by generators which are coupled directly with the shaft of turbines.

Turbines can be classified as two types

1)    According to the nature of energy head of water at inlet: It has further two categories which are as

i)                   Reaction or pressure turbines:  A reaction turbine is a turbine in which the water entering the wheel possesses pressure and kinetic energy both. 

Sir Charles Parsons invented the reaction turbine who was an Anglo- Irish engineer.

In reaction turbines, due to the change in pressure within the water as it moves through the turbine are responsible for the generation of energy. They must be either encased or fully submerged in the water. These turbines are large in size and have low speed and even the component parts are difficult to repair. They are suitable for low heads and only a fraction of total available head is converted into kinetic head after water enters the wheel. Examples are Francis turbine, Kaplan turbines, Thomson turbine, Scotch turbines etc.

ii)                 Impulse turbine or velocity turbines: A velocity turbine is a turbine in which the water entering the runner possesses only the kinetic energy. They are small in size and have higher speed than reaction turbines. The component parts are easy to repair as they are accessible easily. They are suitable for high heads and all the total head available is converted is converted into kinetic head before water strikes wheel. Examples are Pelton wheels, Turgo wheels etc.

2)    According to the  direction of flow of water:

There are two types:

i)                   Radial flow turbines:

The water moves along the vanes towards or away from the axis of rotation of turbines. There are two types of radial flow turbines:

a)     Inward flow turbine: Francis turbines, Thompson turbines are inward flow turbines.

b)    Outward flow turbine: Fourneyron turbine is an example of outward flow turbine.

ii)                 Parallel flow turbines:

The water moves parallel to the axis of rotation. Pelton wheel, Turgo wheel at can be taken as the parallel flow turbines.

iii)               Mixed turbines

In mixed turbines, water flow radially when entering the turbines at inlet and when water leaves at outlet, the flow direction is parallel to the axis of the rotation of turbine. Francis turbine can be taken as the mixed turbine.

Any of these turbines can be used in Hydro power plant but the most suitable type of turbines is selected for the particular plant according to the site and head available. Specific speed requirements are found out for selecting or preferring the types of turbines that is to be used. Some of the turbines are built for the pumped storage hydroelectricity which can act as pump as well and can reverse the flow.

CONTROL SYSTEM TO CONTROL THE SPEED OF TURBINES

Water turbine speed is to be controlled so as to protect the turbine itself. Governors were used to control the speed of turbine.  During the first generation governors, fly balls were used to control the speed of turbines. Then afterwards mechanical governors were used on early twentieth century. During the late twentieth, electronic governors also known as second generation governors were brought into use. It consists of rotational speed sensor instead of fly ball. Finally in the modern times now, algorithms are used through the means of computerized system. So the electronic governors were replaced by digital governors which are also known as the third generation governors.   

RUNAWAY SPEED

RUNWAY SPEED can be defined as the maximum speed reached by the turbine after the removal of external load. Here, the external load refers to the generators. When the turbine is working and then the working generators are removed or disconnected from the turbine, then also turbine continues to work and its speed continues to increase and reaches the maximum speed which is RUNAWAY SPEED.

WHAT CAN BE THE ENVIRONMENTAL IMPACTS DUE TO THE USE OF HYDRAULIC TURBINES?

While the use of any kind of hydraulic turbines do not create any kind of negative impacts on the environment and are generally safe but the Hydro Electric Plant can adversely affect the environment and the living things under the water.

Dams can create negative impacts on the fishes, their migration and killing them. Silts deposition can be the next problem that can change the ecology of rivers.

But on the other hand, dams created for the Hydro Electric Plant can also significantly be helpful for the irrigation purpose.

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