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Tuesday, 20 August 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. So, lets see governing of Pelton turbine. 

Before going to understand the governing of Pelton turbine, We must recall 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. 

Important points to consider while governing of turbine 

There are two very important points that we have to consider while governing of turbine. 

First, we need to do the governing of turbine in such a way that it should not alter the operating parameters of turbine otherwise efficiency of turbine will be reduced. 

Second, there should not be any water hammer problems during governing of 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  

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