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Tuesday, 20 August 2019

August 20, 2019

EXPLAIN GOVERNING OF PELTON TURBINE

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine.  


Today we will understand here the governing of Pelton turbine or Impulse turbine with the help of this post. 

We have already understood that hydraulic turbines are basically defined as the hydraulic machines which convert hydraulic energy in to mechanical energy and this mechanical energy will be given to a generator to produce the electric energy. Generator will be directly coupled with the hydraulic turbine. 

In order to maintain the constant frequency of electric power output, the rotor of the turbine has to rotate with a constant speed and therefore it is needed to maintain the constant rotational speed of the turbine rotor. 

The process by which the speed of rotation of turbine rotor is kept constant will be termed as governing of a turbine and it is well discussed in our previous post. So let us directly come to the point to find out the governing of Pelton turbine or impulse turbine. 

Governing of Pelton turbine 

Oil pressure governor is used for governing of Pelton turbine. Oil pressure governor, as displayed here in following figure, will have following components.
  1. Oil sump
  2. Oil pump
  3. Servomotor or relay cylinder
  4. Control valve or distribution valve or relay valve
  5. Centrifugal governor or pendulum
  6. Piping arrangements
  7. Spear rod or needle

Gear pump is used here as oil pump in the oil pressure governor. Gear pump will be driven by the power obtained from the turbine shaft.
Centrifugal governor or pendulum will be connected with the turbine main shaft with the help of belt or gear. 

Piping arrangements will connect the oil sump with the control valve and control valve to servomotor or relay cylinder. 

Following figure displayed here indicates the position of the piston in the relay cylinder, position of control valve or relay valve and fly balls of centrifugal governor, when the turbine is running at the normal speed. 
Let us discuss here the case when electrical load decreases or increases due to change in demand. We will see here how the governor will work to maintain the rotational speed of the rotor constant. 

Case 1: Electrical Load decreases 

When the electrical load decreases, resisting torque will also be reduced. Therefore for a given driving torque, rotational speed of the rotor of turbine will be increased. As centrifugal governor will be connected with the turbine main shaft with the help of fan or gear, rotational speed of governor will also be increased. 

Due to the increase in rotational speed of the centrifugal governor, centrifugal force acting on the fly-balls will be increased and fly-balls will move in upward direction. 

Sleeve will also move in upward direction due to the movement of fly-balls in upward direction.
As we can see here in figure, there is a horizontal lever which is supported over a fulcrum and connects the sleeve and piston rod of control valve. 

Once sleeve will move in upward direction, horizontal lever will turn about the fulcrum and hence piston rod of control valve will move in downward direction. Due to the movement of piston rod of control valve towards downward, V1 valve will be closed and V2 valve will be opened. 

Gear pump will suck the oil from oil sump and discharge oil under pressure to the control valve. Oil under pressure will flow through the valve V2 to servomotor or relay cylinder and will exert the pressure force at face A of piston of relay cylinder. Therefore, piston along with piston rod and spear will move towards right. 

Due to the movement of piston along with piston rod and spear towards right, area of flow of water at the outlet of nozzle will be reduced and hence flow rate of water to the turbine will also be reduced.
Speed of rotation of rotor of turbine will be reduced due to the reduction in flow rate of water to the turbine. 

The fly-balls, sleeve, lever and piston rod of control valve will come to its original position when the speed of rotation of rotor of turbine becomes normal. 

Case 2: Electrical Load decreases 

When the electrical load increases, resisting torque will also be increased. Therefore for a given driving torque, rotational speed of the rotor of turbine will be decreased. As centrifugal governor will be connected with the turbine main shaft with the help of fan or gear, rotational speed of governor will also be decreased. 

Due to the decrease in rotational speed of the centrifugal governor, centrifugal force acting on the fly-balls will be decreased and fly-balls will move in downward direction.
Sleeve will also move in downward direction due to the movement of fly-balls in downward direction. 

Once sleeve will move in downward direction, horizontal lever will turn about the fulcrum and hence piston rod of control valve will move in upward direction. Due to the movement of piston rod of control valve towards upward direction, V1 valve will be opened and V2 valve will be closed. 

Gear pump will suck the oil from oil sump and discharge oil under pressure to the control valve. Oil under pressure will flow through the valve V1 to servomotor or relay cylinder and will exert the pressure force at face B of piston of relay cylinder. Therefore, piston along with piston rod and spear will move towards left. 

Due to the movement of piston along with piston rod and spear towards left, area of flow of water at the outlet of nozzle will be increased and hence flow rate of water to the turbine will also be increased. 

Speed of rotation of rotor of turbine will be increased due to increase in flow rate of water to the turbine. 

The fly-balls, sleeve, lever and piston rod of control valve will come to its original position when the speed of rotation of rotor of turbine becomes normal. 

As we have already studied that in order to maintain the constant frequency of electric power output, the rotor of the turbine has to rotate with a constant speed and therefore it is needed to maintain the constant rotational speed of the turbine rotor. 

This is the mechanism of oil pressure governor that control the flow rate of water to the turbine according to the electrical load in order to maintain the constant rotational speed of the turbine rotor. 

Do you have any suggestions? Please write in comment box. 

Further we will find out, in our next post, governing of Reaction turbine. 

Reference: 

Fluid mechanics, By R. K. Bansal 

Image courtesy: Google  

Also read 

Monday, 19 August 2019

August 19, 2019

WHAT IS GOVERNING OF TURBINE?

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine.  


Today we will understand here the basics of governing of a turbine with the help of this post. Once we will come to know the meaning of governing of a turbine, we will be able to easily understand the concept of governing of Pelton wheel and governing of Francis turbine. 

Governing of turbines 

We have already understood that hydraulic turbines are basically defined as the hydraulic machines which convert hydraulic energy in to mechanical energy and this mechanical energy will be given to a generator to produce the electric energy. Generator will be directly coupled with the hydraulic turbine. 

In order to maintain the constant frequency of electric power output, the rotor of the turbine has to rotate with a constant speed and therefore it is needed to maintain the constant rotational speed of the turbine rotor. 

Now we must understand here that how to maintain the constant rotational speed of the turbine rotor. 

Rotational speed of rotor of a turbine will be dependent over the driving torque and resisting torque.
Driving torque will be provided by fluid flowing through the blade passages by its change of angular momentum and resisting torque will come from the electrical load. 

Balance between these two types of torque i.e. driving torque and resisting torque will enable the rotator of turbine to rotate at constant angular speed. 

When electrical load will be changed, electrical load might be increased or decreased depending on demand, speed of rotor of turbine will be changed if there is no provision to change the driving torque. Hence, frequency of electric power output will be changed due to change in rotational speed of rotor of turbine and it is not a desirable result. 

Let us take one case where electrical load is increased. Let us think what will happen due to increase in electrical load. Resisting torque will be increased and therefore for a given driving torque, speed of rotation of rotor of turbine will be decreased. In this case it will be required to increase the driving torque to boost up the rotational speed of rotor up to its original speed. 

Suppose if electrical load is decreased then will happen. Resisting torque will be decreased and therefore for a given driving torque, speed of rotation of rotor of turbine will be increased. In this case it will be required to decrease the driving torque to reduce the rotational speed of rotator up to its original speed. 

Therefore in order to restore the initial speed of rotation of rotor of turbine, we need to change the driving torque up to the desired value of resisting torque which is changed due to change in electrical load. 

As we know that energy given to the rotor of the turbine will be directionally proportional to the fluid flow rate, therefore change in driving torque will be done by change in fluid flow rate. 

If electrical load is increased, it will be required to increase the driving torque to boost up the rotational speed of rotor up to its original speed and driving torque will be increased by increasing the fluid flow rate. 

If electrical load is decreased, it will be required to decrease the driving torque to reduce the rotational speed of rotator up to its original speed and driving torque will be decreased by decreasing the fluid flow rate. 

Therefore, by controlling the fluid flow rate, driving torque could be changed to meet with the resisting torque to maintain the constant speed of rotation of rotor of turbine in order to produce the constant frequency of electrical power output. 

This is the basic principle behind the governing of all type of turbine. This process by which the speed of rotation of turbine rotor is kept constant will be termed as governing of a turbine. 

Governing of a turbine is very necessary as a turbine is directly coupled with electrical generator which is required to run at constant rotational speed in order to produce the constant frequency of electrical power output. 

Do you have any suggestions? Please write in comment box.  

Further we will find out, in our next post, governing of Pelton turbine

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google  

Also read  

Friday, 16 August 2019

August 16, 2019

SPECIFIC SPEED OF TURBINE AND ITS SIGNIFICANCE

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine


Now we will focus here to understand the term specific speed in turbines with the help of this post. We will also find out here the significance of specific speed in turbine. 

Specific speed

Specific speed is basically defined as the speed of a turbine which is identical in shape, geometrical dimensions, blade angles, gate opening etc., with the actual turbine but of such a size that it will produce unit power when working under unit head. 

Specific speed will be denoted by symbol Ns. 

Different types of turbines are compared by using the value of specific speed as every type of turbine will have different specific speed. 

Where,
N = Speed of actual turbine
Ns = Specific speed of turbine
P = Power developed or shaft power
H = Head under which the turbine is working 

Significance of specific speed 

There is very important role that specific speed plays during the selection of type of turbine. Specific speed will predict the information about the performance of a turbine. 

According to the specific speed, turbines will be classified as mentioned here. 

High specific speed turbine

Specific speed of such turbines will be in the range of 255 to 860 and hence such turbines will be termed as high specific speed turbines. 

Kaplan and propeller turbine are the best examples of high specific speed turbines. 

Medium specific speed turbine 

Specific speed of such turbines will be in the range of 50 to 250 and hence such turbines will be termed as medium specific speed turbines. 

Francis turbine is the best example of medium specific speed turbine. 

Low specific speed turbine 

Specific speed of such turbines will be in the range of 8 to 30 with single nozzle and up to 50 with multiple nozzles. Hence such turbines will be termed as low specific speed turbines. 

Pelton turbine is the best example of low specific speed turbine. 

Specific speed for various types of turbines 

Specific speed for various types of turbines are mentioned in following table as displayed here. 


Do you have any suggestions? Please write in comment box.  Further we will find out, in our next post, Governing of turbine.

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google 

Also read  

Wednesday, 14 August 2019

August 14, 2019

DRAFT TUBE IN TURBINES: TYPES, FUNCTION, EFFICIENCY AND PURPOSE OF A DRAFT TUBE

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine


Now we will focus here to understand the basics of draft tube and its classifications with the help of this post. We will also find out the efficiency of draft tube and its function in hydraulic turbine operation. 

Draft - Tube 

A draft - tube is basically a pipe of gradually increasing area that will connect the outlet of the turbine runner to the tail race where the water will be finally discharged from the turbine. Therefore, a draft tube will be used to discharge the water from the exit of the turbine to the tail race. 

One end of the draft tube will be connected to the outlet of the runner while other end of draft tube will be submerged below the level of water in the tail race. 

This pipe of gradually increasing area will be termed as draft tube. 

Function of Draft - Tube in turbine 

There are following important functions of a draft tube in the operation of a turbine as mentioned here. 

Primary function of a draft tube is basically to provide a passage for water discharge from the turbine. 

Draft tube will reduce the velocity of discharged water and hence it will help to minimize the loss of kinetic energy at the outlet. 

Draft tube will allow establishing a negative head at the outlet of the runner and hence a draft tube will help to increase the net head on the turbine. With the presence of draft tube, turbine may be placed above the tail race without any loss of net head. In doing so, it will be quite easy to inspect the turbine properly. 

A draft tube will convert a large portion of the kinetic energy, rejected at the outlet of the turbine, in to useful pressure energy. If a draft tube will not be used with the turbine, kinetic energy rejected at the outlet of the turbine will go waste to the tail race. 

With the application of draft tube, the net head on the turbine and efficiency of the turbine will be increased and therefore turbine will develop more power. 

A clear understanding of the function of the draft tube in any reaction turbine is very much important for the purpose of its design. 

Types of Draft - Tubes 

There are following types of draft tubes those are usually used in turbine operation and these are as mentioned here 
  1. Conical draft tubes 
  2. Simple elbow draft tubes 
  3. Moody spreading draft tubes 
  4. Elbow draft tubes with circular inlet and rectangular outlet 

Conical draft tubes 

Conical draft tube will have circular inlet and circular outlet. We can consider it as simple taper tube. Taper angle will vary from 4 degree to 7 degree as displayed in figure. 

Conical draft tube will be made by mild steel plates. Conical draft tubes will have efficiency up to 90%. 

Simple elbow draft tubes 

Simple elbow draft tubes will have circular cross-section throughout i.e. from inlet to outlet of draft tube. 

As name suggest, simple draft tube will be basically a tube with uniform cross-section turned into 90 degree. 

Simple elbow draft tube will be made by concrete with steel lining at its inlet in order to reduce the effect of cavitation. Simple elbow draft tubes will have efficiency up to 60%. 

Moody spreading draft tubes 

Moody spreading draft tubes are very identical to conical draft tubes and such type of draft tubes are used with vertical shaft turbine. 

Moody spreading draft tubes will have efficiency up to 85%. 

A Moody spreading draft tube will have a solid central core at center, as displayed in figure, to reduce the whirling. 

Elbow draft tubes with circular inlet and rectangular outlet 

As name suggest, Elbow draft tubes will have circular inlet and rectangular outlet and it is displayed in following figure. 

Elbow draft tubes with circular inlet and rectangular outlet will be made by concrete with steel lining at its inlet in order to reduce the effect of cavitation. Such draft tubes will have efficiency up to 60% to 80%. 

Out of above types of draft tubes, Conical draft tubes and Moody spreading draft tubes are most efficient. 

Efficiency of Draft – Tube 

The efficiency of a draft tube will be defined as the ratio of actual conversion of kinetic head into pressure head in the draft tube to the kinetic head available at the inlet of draft tube. 


Where, 
V1 = Velocity of water at the inlet of draft tube 
V2 = Velocity of water at the outlet of draft tube 
hf = Loss of head in the draft tube 

Do you have any suggestions? Please write in comment box.  Further we will find out, in our next post, Specific speed and its importance in turbine operation

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google 

Also read  

Wednesday, 7 August 2019

August 07, 2019

AXIAL FLOW REACTION TURBINE

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine


Now we will focus here to understand the basics of Axial flow reaction turbine with the help of this post. 

Axial flow reaction turbine 

Axial flow reaction turbine, as name suggest, consist two important terms i.e. axial flow and reaction turbine. We will first understand here the meaning of saying axial flow and then we will see the term reaction turbine. 

Once, we will come to know the above two important terms of axial flow reaction turbine, it will be quite easy to understand the fundamental of this type of turbine. 

If the water will flow in a direction parallel to the direction of rotation of the shaft, the turbine will be considered as axial flow turbine. 

If the head at the inlet of the turbine is the sum of pressure energy and kinetic energy and during the flow of water through the runner a part of pressure energy is converted in to kinetic energy, the turbine will be considered as reaction turbine. 

The shaft of the axial flow reaction turbine will be vertical and the bottom portion of the shaft will be made larger which will be termed as hub of the turbine. 

Vanes will be fixed over the surface of the hub and therefore hub of the turbine will be considered as the runner of the axial flow reaction turbine. 

Classification of Axial flow reaction turbine

On the basis of installation of vanes over the surface of hub of turbine, there are basically two types of axial flow reaction turbine.
  1. Propeller turbine
  2. Kaplan turbine

Propeller turbine

In case of propeller turbine, vanes fixed over the surface of hub are not adjustable.

Kaplan turbine

In case of Kaplan turbine, vanes fixed over the surface of hub are adjustable. Kaplan turbine is suitable where a large quantity of water at low head is available. 

Following figure, displayed here, indicates the runner of a Kaplan turbine. Kaplan turbine will have one hub over which adjustable vanes will be fixed. 
There are following main parts of a Kaplan turbine as displayed here in following figure.
  1. Scroll Casing
  2. Guide vanes mechanism
  3. Hub with vanes or runner of the turbine
  4. Draft tube 

Water from penstock will enter the scroll casing and then it will move to the guide vanes. From the guide vanes, the water will turn through 90 degree and will flow axially through the runner as shown in figure.
Discharge through the runner will be determined with the help of following equation or formula.

Where,
D0 = Outer diameter of the runner
Db = Diameter of the hub
Vf1 = Velocity of flow at inlet

Let us see some important relations in respect of Kaplan turbine

1. Peripheral velocity at inlet and outlet will be equal and will be given by following equation or formula.
u1 = u2 = πD0N/60
Where, D0 is the outer diameter of the runner 

2. Velocity of flow at inlet and outlet will be equal i.e. Vf1 = Vf2

3. Area of flow at inlet and area of flow at outlet will be equal and will be given by following equation as mentioned here
A = (π/4) x [D02-Db2]
Do you have any suggestions? Please write in comment box.  Further we will find out, in our next post, the basics of Draft tube

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google 

Also read