Hydraulic Machine

Suman kumar

• TURBINES

• Turbines are defined as the hydraulic machines which convert hydraulic energy in to mechanical energy.

• This mechanical energy is used in running an electric generator which is directly coupled to the shaft of the turbine.

• Thus the mechanical energy is converted in to electrical energy

Hydro-electric power plant

• Hydraulic turbines are the machines which use the energy of water and convert it to mechanical energy.

• The mechanical energy developed by a turbine is used in running an electric generator which is directly coupled to the shaft of the turbine.

• The electric generator thus develops electric power, which is known as hydro-electric power

General Layout of a Hydraulic Power Plant

• A dam constructed across a river to store water• Pipes of large diameters called penstocks

• Turbines having different types vanes fitted to the wheel

• Tail race, which is a channel which carries water away from the water

DEFINITIONS OF HEADS

• Gross Head• : The difference between the head race level and tail race

level when no water is flowing is known as Gross head. It is denoted by Hg

• Net Head• : It is also called effective head and is defined as the head

available at the inlet of the turbine. It is denoted by H Net Head H = Hg –hf

• Where• Hg = Gross head, hf = Head loss due to friction

• CLASSIFICATION OF HYDRAULICTURBINES•

• The hydraulic turbines classified are

• According to the type of energy at inlet

Impulse turbine and Reaction turbine

• According to the direction of flow through runner

Tangential flow turbine ,Radial flow turbine Axial flow turbine and Mixed flow turbine

• According to the head at the inlet of turbine

High head turbine, Medium head turbine and Low head turbine

• According to the specific speed of the turbine• Low specific speed turbine• Medium specific speed turbine and High specific speed turbine

• IMPULSE TURBINE• If at the inlet of the turbine, the energy available is only

kinetic energy, the turbine is known impulse turbine• Example:• Pelton wheel turbine• REACTION TURBINE• If at the inlet of the turbine, the water possesses kinetic

energy as well as pressure energy, the turbine is known as reaction turbine.

• • Example:• • Francis turbine, Kaplan turbine

• RADIAL FLOW TURBINE

• If the water flow in the radial direction through the runner, the turbine is called radial flow turbine

• INWARD RADIAL FLOW TURBINE:• If the water flows from outward to inward, radially the

turbine is known as inward radial flow turbine.

• OUTWARD RADIAL FLOW TURBINE :If the water flow radially from inward to outwards, the turbine

is known as outward radial flow turbine.

• AXIAL FLOW TURBINE:•  • If the water flow through the runner along the

direction parallel to the axis of rotation of the runner, the turbine is called axial flow turbine.

 • MIXED FLOW TURBINE:

• If the water flows through the runner in the radial direction but leaves in the direction parallel to axis of rotation of the runner, the turbine is called mixed flow turbine.

•  TANGENTIAL FLOW TURBINE: • If the water flows along the tangent of the runner, the turbine

is known as tangential flow turbine.

IMPULSE TURBINE

• If at the inlet of the turbine, the energy available is only kinetic energy, the turbine is known impulse turbine

• Tangential flow turbine• In tangential flow turbine, water flows along the

tangent to the path of the runner• Example:• Pelton wheel turbine

Impulse turbine (PELTON WHEEL)

• Converts kinetic energy alone

The main parts of the pelton turbine are

• Nozzle and flow regulating arrangement

• Runner and buckets

• Casing and• Breaking jet

Nozzle and flow Regulating Arrangement

• The amount of water striking the buckets (vanes) of the runner is controlled by providing a spear in the nozzle as shown in fig. The spear is a conical needle which is operated either by a hand wheel or automatically in an axial direction depending upon the size of the unit. When the spear is pushed forward into the nozzle the amount of water striking the runner is reduced. On the other hand, if the spear is pushed back, the amount of water striking the runner increases.

• Runner with Buckets• Fig shows the runner of a pelton wheel. It consists of a circular disc on the

periphery of which a number of buckets evenly spaced are fixed. The shape of the buckets is of a double hemispherical cup or bowl. Each bucket is divided into two symmetrical parts by a dividing wall which is known as splitter. The jet of water strikes on the splitter. The splitter divides the jet into two equal parts and the comes out at the outer edge of the bucket. The buckets are shaped in such a way that the jet gets deflected through 160ᵒ or 170ᵒ. The buckets are made of cast iron, cast steel bronze or stainless steel depending upon the head at the inlet of the turbine.

•  Casing• The function of the casing is to prevent the splashing of the water and to discharge

water to tail race. It also acts as safeguard against accidents. It is made of cast iron or fabricated steel plates. The casing of the pelton wheel does not perform any hydraulic function.

• Breaking jet• When the nozzle is completely closed by moving the spear in the forward direction

the amount of water striking the runner reduces to zero. But the runner due to inertia goes on revolving for a long time. To stop the runner in a short time, a small nozzle is provided which directs the jet of water on the back of the vanes. This jet of water is called breaking jet.

VELOCITY TRIANGLES AND WORKDONE FOR PELTON WHEEL

•

• Fig shows the shape of the buckets of the pelton wheel. The set of water from the nozzle strikes the bucket at the splitter which splits up the set into two parts. These part of the set, glides over the inner surfaces and comes out at the outer edge.

• The inlet velocity triangle is drawn at the splitter and outlet velocity triangle is drawn at the outer edge of the bucket.

Example

• A Pelton wheel with nozzle, for which the coefficient of velocity is 0.97 is 400m below the water surface of lake. The jet diameter is 80 mm, the pipe diameter is 0.6 m, its length is 4 km and the coefficient of friction(f) is 0.032. the bucket deflects the jet through 165 deg and they run at 0.48 jet speed, bucket friction reduces the relative velocity at outlet by 15% of the relative velocity at inlet. If mechanical efficiency of the turbine is 90%, determine the flow rate and shaft power developed by the turbine.

RADIAL FLOW REACTION TURBINES

• Reaction turbine means that the water at the inlet of the turbine possesses kinetic energy as well as pressure energy. As the water flows through the runner, a part of pressure energy goes on changing into kinetic energy.

• Thus the water through the runner is under pressure. The runner is completely enclosed in an air

• tight casing and casing and the runner is always full of water.• Radial flow turbine are those turbines in which the water

flows in the radial direction.• The water may flow radially from outwards to inwards or

from inwards to outwards• Examples• Francis Turbine Kaplan Turbine ,Propeller Turbine

Main parts of radial flow Reaction Turbine• 1.Casing 2.Guide mechanism 3.Runner, and

4.Draft tube

INWARD RADIAL FLOW TURBINE

• If the water flows from outwards to inwards

through the runner, the turbine is known as inward radial flow turbine 

• The water flows over the moving vanes in the inward radial direction and is discharged at the runner diameter of the runner. The outer diameter of the runner is the inlet and the inner diameter is the outlet.

Velocity Triangles

INWARD RADIAL FLOW TURBINE

Inlet Triangle

Outlet Triangle

Outward radial Flow reaction Turbine

• The outward radial flow reaction turbine in which the water from casing enters the stationary guide wheel.

• The guide wheel consists of guide vanes which direct water to enter the runner which is around the stationary guide wheel.

• The water flows through the vanes of the runner in the outward radial direction and is discharge at the outer diameter of the runner.

• The inner diameter of the runner is inlet and outer diameter is the outlet.

Outward radial Flow reaction Turbine

• The velocity triangles at inlet and outlet will be drawn by the same procedure as inward flow turbine.

Francis turbines 

• This is the most common turbine type in

hydroelectric stations.• The Francis turbine is a radial-flow turbine with

water flowing in a radial direction inward over the curved runner blades toward the centre of the turbine

• Francis turbines are suitable for hydroelectric systems with water heads between 2 meters to 200 meters, and the efficiency can be over 90%.

• A Francis turbine comprises mainly the four components• 1.spiral casing,2. guide on stay vanes,3. runner blades and4. draft-tube

• Spiral Casing : • The fluid enters from the penstock (pipeline leading

to the turbine from the reservoir at high altitude) to a spiral casing which completely surrounds the runner. This casing is known as scroll casing or volute.

• The cross-sectional area of this casing decreases uniformly along the circumference to keep the fluid velocity constant in magnitude along its path towards the guide vane.

Flow Distribution Analysis of Casing

Static Pressure Distribution in Casing

• Guide or Stay vane:• The basic purpose of the guide vanes or stay vanes is to convert a

part of pressure energy of the fluid at its entrance to the kinetic energy and then to direct the fluid on to the runner blades at the angle appropriate to the design.

• Moreover, the guide vanes are pivoted and can be turned by a suitable governing mechanism to regulate the flow while the load changes.

• The guide vanes are also known as wicket gates. • The flow in the runner of a Francis turbine is not purely radial but

a combination of radial and tangential. • The flow is inward, i.e. from the periphery towards the centre. • The height of the runner depends upon the specific speed. The

height increases with the increase in the specific speed. • The main direction of flow change as water passes through the

runner and is finally turned into the axial direction while entering the draft tube

R a d i a l v i e wrunner guide vanes and stay vanes

Water from spiral casing

Water inlet

• Draft tube: • The draft tube is a conduit which connects the runner

exit to the tail race where the water is being finally discharged from the turbine.

• The primary function of the draft tube is to reduce the velocity of the discharged water to minimize the loss of kinetic energy at the outlet.

• This permits the turbine to be set above the tail water without any appreciable drop of available head.

Francis turbine

Velocity Triangles of  Francis turbines

• The velocity triangle at inlet and outlet of the Francis turbines are drawn in the same way as incase of inward flow reaction turbine.

• As in case of Francis turbine, the discharge is radial at outlet, the velocity of whirl at outlet Vw2 will be zero.

Example

• The internal and external diameters of an inward turbine are 40 cm and 80 cm. the width of the runner at inlet is 20 cm. The velocity of flow through the runner is 2m/s and it is constant from inlet to exit. The guide blade angle at inlet is 10ᵒ to the tangent of the runner. Assuming the discharge through the runner is radial, draw the inlet and outlet velocity triangles and find out the following if the runner is running at 200 rpm.

• (i) V1,Vr1 and Vw1 (ii) runner blade angles• (iii) Width of the runner at outlet• (iv) Flow quantity through the turbine and head at inlet of

the turbine.• (v) power developed and hydraulic efficiency.

Example 2

• A Francis turbine running at 400 rpm when the head available is 60 m. The constant velocity of the flow through the runner is 10 m/s and hydraulic efficiency is 80%. Determine angles of the rotating blades.

AXIAL FLOW REACTION TURBINE

• If the water parallel to the axis of the rotation of the shaft, the turbine is known as axial flow turbine.

• For the axial flow reaction turbine the shaft of the turbine is vertical. The lower end of the shaft is made larger which is known as ‘hub” or boss.

• The vanes are fixed on the hub and hence acts as a runner for axial flow reaction turbine.

 • The following are the important type of axial flow reaction

turbines Propeller Turbine and Kaplan Turbine

• Propeller Turbine

• The vanes are fixed to the hub and they are not adjustable, the runner is known as propeller turbine.

• Kaplan Turbine

• The vanes on the hub are adjustable the turbine is known as a Kaplan turbine. This turbine is suitable where a large quantity of water at low heads is available.

Kaplan Turbine

• Kaplan turbine, which consists of a hub fixed to the shaft. On the hub, the adjustable vanes are fixed as shown in fig.

• The main parts of Kaplan turbine are

• Scroll casing Guide vanes mechanism ,Hub with vanes or runner of the turbine , and Draft tube

Axial flow (Kaplan) Turbine

• Higher specific speed corresponds to a lower head. This requires that the runner should admit a comparatively large quantity of water.

• For a runner of given diameter, the maximum flow rate is achieved when the flow is parallel to the axis. Such a machine is known as axial flow reaction turbine.

• An Australian engineer, Vikton Kaplan first designed such a machine. The machines in this family are called Kaplan Turbines

• Figure shows a schematic diagram of propeller or Kaplan turbine. The function of the guide vane is same as in case of Francis turbine.

• Between the guide vanes and the runner, the fluid in a propeller turbine turns through a right-angle into the axial direction and then passes through the runner.

• The runner usually has four or six blades and closely resembles a ship's propeller.

• Neglecting the frictional effects, the flow approaching the runner blades can be considered to be a free vortex with whirl velocity being inversely proportional to radius.

• While on the other hand, the blade velocity is directly proportional to the radius.

• To take care of this different relationship of the fluid velocity and the blade velocity with the changes in radius, the blades are twisted. The angle with axis is greater at the tip that at the root.

Propeller Turbine

• A propeller turbine generally has a runner with three to six blades in which the water contacts all of the blades constantly.

• Picture a boat propeller running in a pipe. Through the pipe, the pressure is constant; if it isn't, the runner would be out of balance.

• The pitch of the blades may be fixed or adjustable.• The major components besides the runner are a scroll case,

wicket gates, and a draft tube.

Kapaln and Propeller turbine

GUIDE VANES/WICKET GATES

ROTOR

DRAFT TUBE

Cavitation in reaction turbines• If the pressure of a liquid in course of its flow

becomes equal to its vapour pressure at the existing temperature, then the liquid starts boiling and the pockets of vapour are formed which create vapour locks to the flow and the flow is stopped.

• The phenomenon is known as cavitation. • To avoid cavitation, the minimum pressure in the

passage of a liquid flow, should always be more than the vapour pressure of the liquid at the working temperature.

• In a reaction turbine, the point of minimum pressure is usually at the outlet end of the runner blades, i.e at the inlet to the draft tube.

• For the flow between such a point and the final discharge into the trail race (where the pressure is atmospheric), the Bernoulli's equation can be written, in consideration of the velocity at the discharge from draft tube to be negligibly small, as

• where, Pe and Ve represent the static pressure and velocity of the liquid at the outlet of the runner (or at the inlet to the draft tube).

• The larger the value of Ve, the smaller is the value of Pe and the cavitation is more likely to occur.

• The term hf in Eq. represents the loss of head due to friction in the draft tube and z is the height of the turbine runner above the tail water surface. For cavitation not to occur  Pe >Ve where  is the vapour pressure of the liquid at the working temperature.

Governing of Reaction Turbines

Governing of Reaction Turbines• Governing of reaction turbines is usually done by

altering the position of the guide vanes and thus controlling the flow rate by changing the gate openings to the runner.

• The guide blades of a reaction turbine are pivoted and connected by levers and links to the regulating ring.

• Two long regulating rods, being attached to the regulating ring at their one ends, are connected to a regulating lever at their other ends.

• The regulating lever is keyed to a regulating shaft which is turned by a servomotor piston of the oil.

Runway Speed

• Runaway speed is the speed at which the turbine exceeds its

designed maximum rotational speed. • When this occurs it is possible for the turbine to disintegrate

due to massive centrifugal forces.• For safe design the various rotating component are designed

for the runway speed.• For Pelton turbine runway speed ranges from 1.8 to 1.9

times of its normal speed.• For Francis turbine runway speed ranges from 2 to 2.2 times

of its normal speed.• For Kaplan turbine runway speed ranges from 2.5 to 3 times

of its normal speed.

Surge Tank