Turbine

Turbine:

A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work. The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and impart rotational energy to the rotor.

Types of turbine:

1.      Steam turbine

2.      Gas turbine

3.      Water turbine

4.      Wind turbine

5.      Transonic turbine

6.      Ceramic turbine

Steam turbine:

Steam turbine is such type of turbine where steam is used as working fluid. When steam is injected over the blades it rotates at a certain speed. Since steam is used for rotation it is called steam turbine. Generally it is used at steam turbine power station.

In Ashuganj power station, steam turbines are used for steam turbine units. All the turbines are manufactured by BBC (Germany).

Main parts of steam turbine:                

Ø  Rotor

Ø  Blades (fixed and moving)

Ø  Bearings (thrust and journal)

Ø  Turbine casing

Ø  Valves (main stop valve, control valve etc).

   

                 Figure: Steam turbine (case opened)

Rotor assembly:

The turbine rotor assembly consists of the turbine shaft and the attached moving blade. The rotor assembly absorbs energy from working fluid (steam, gas, water etc) and converts that energy into mechanical energy.

          

                                 Figure: Rotor (ST).                                             

 

                                      Figure: Rotor (GT)  

Turbine blades:

            The energy conversion takes place through the turbine blades. A turbine consists of alternate rows of blades. This blades convert the chemical or thermal energy of working fluid into kinetic energy and then from kinetic energy to mechanical energy as rotation of the shaft.

         

                                        Figure: Turbine blades.

There are two types of blade, fixed and moving blade. Moving blade is also two types.

One is impulse blade and another reaction blade.

Fixed blade:

A fixed blade assembly is very important for turbine blading. It is also known as diaphragm. The shape of the blade is the key to the energy conversion process. Since the fixed blades have a conversing nozzle shape, it is also called nozzles. When steam is passed over the fixed blades, they increase the velocity of steam as an operation of nozzles. Here blades are converted the thermal energy of steam into kinetic energy by causing the steam to speed up and gain velocity.

               

                                             

Moving blade:

Moving blade can be shaped in either of two ways: reaction shaped or impulse shaped. The shape of the blade determines how the energy is actually converted. Either type of moving blades or a combination of both can be attached to the shaft of the rotor on dices, called wheels as shown in the figure. Along the outer rim of the blades is a metal band, called shrouding which ties the blades together. The moving blades convert the kinetic energy in the moving speed into the mechanical energy as rotor rotation.

                      
                                             

                                                 Figure: Moving blade

Impulse turbines:

These turbines change the direction of flow of a high velocity fluid or gas jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid or gas in the turbine rotor blades as in the case of a steam or gas turbine, all the pressure drop takes place in the stationary blades.

Before reaching the turbine, the fluid's pressure head is changed to velocity head by accelerating the fluid with a nozzle. Impulse turbines do not require a pressure casement around the rotor since the fluid jet is created by the nozzle prior to reaching the blading on the rotor. Newton's second law describes the transfer of energy for impulse turbines.

Reaction turbines: 

These turbines develop torque by reacting to the gas or fluid's pressure or mass. The pressure of the gas or fluid changes as it passes through the turbine rotor blades. A pressure casement is needed to contain the working fluid as it acts on the turbine stages or the turbine must be fully immersed in the fluid flow. The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages are usually used to harness the expanding gas efficiently. Newton's third law describes the transfer of energy for reaction turbines.

Blading stages:

Two successive fixed and moving blades are collectively known as blading stage. The effects of pressure and velocity of working fluids depend upon the stage conditions. In Ashuganj power station, the turbines which are used have 23 stages at HP turbine and 21 stages at IP & LP turbine. Now the effects of pressure and velocity on various blading stages are described in below:

                                 

                         Figure: Effects of stage on pressure and velocity.

For impulse blading velocity increases and pressure decreases across each row as the steam passes through the fixed blading. Again when steam passes through the impulse type moving blade, its velocity decreases, but its pressure remains constant as shown in the figure.

For reaction blading velocity increases and the pressure decreases across each row as the steam passes through the fixed blading. When steam passes through the reaction type moving blade, its pressure and velocity both decreases as shown.

Valves:

Steam from the boiler is routed to the turbine through a steam line that contains the main stop valves and the control valves.

Main stop valves:

            It is such a valve through which steam passes to the turbine blades. By controlling this valve steam flow can be controlled. Each main stop valve consists of a valve disk, a valve stem and a hydraulic actuator.

The hydraulic actuator contains a piston and a compression spring. Since the valve disk and stems are connected to the piston, movement of the piston causes movement of the valve disc. During normal turbine operation, hydraulic oil is directed into or out of the hydraulic actuator. Directing oil into the actuator opens the valve and compress the spring, as shown in figure.

    

            Figure : Main stop valve

As long as the amount of oil in actuator is held constant, the valve will remain in the same position. Bleeding oil from the actuator allows the spring to push on the piston, closing the valve. Tripping the turbine causes hydraulic oil to be bled quickly from beneath the piston, allowing the spring to quickly shut the valve. Steam pressure also helps to close the valve by forcing the disc back toward the seat. When the valve is closed as shown in figure (2), the flow of steam toward the HP turbine is shut off.

Control valves:

When the main stop valves are fully opened, the flow of steam into the HP turbine is usually regulated by four or more control valves. The control valves regulate the turbine speed or its power output. Steam from the main stop valve flows to the control valves through a steam line. The steam is sent to different sections of the turbines nozzle block through the four steam lines below the control valves. Each control valve feeds only one section of the nozzle block.

The control valves are operated by hydraulic actuators. The control valves regulate steam flow into the turbine by opening and closing in sequence. As each valve is opened, more steam is admitted to the turbine. During normal operation, the control valves are automatically positioned to compensate for changes in load. For example, if load increases, the control valves are opened more which increase the flow of steam into the turbine. If load decreases, the control valves are closed more which decrease the flow of steam into the turbine. At full condition, all the control valves are completely opened as shown in the figure.

                        

        Figure: control valves during full load condition.

Turbine governing system:

Mechanical governor:

                The purpose of a mechanical governor is to maintain the speed of the turbine at a desired value when the generator is disconnected from the power supply.

Main parts of mechanical governor:

Ø  Flyweights

Ø  Bracket

Ø  Spring etc.

Mechanism:

            When the turbine shaft rotates, the governor flyweights respond to the centrifugal forces created by the rotations. As turbine speed increases, the centrifugal force increases, causing the flyweights to move outward, overcoming the tension of the spring.

              

     Figure: Mechanical governors.                                                                 

The force of the spring tends to pull the flyweights toward the center of the governor. When turbine speed decreases, the centrifugal force also decreases, allowing the spring to   pull the flyweights inward.

Governing system at high speed:

When the speed of the turbine increases, the flyweights move outward, which causes the pilot valve stem to move upward. The movement of the stem and disc unblocks the port of the control oil line and allows oil to flow from the actuator, through the pilot valve, to the drain. The resulting decrease in pressure beneath the piston allows the actuator spring to expand, forcing the piston towards. This action decreases the opening of the control valve. Less steam is admitted to the turbine and turbine speed decreases.

  

   Figure: Governing system at high speed.      

Governing system at low speed:

When turbine speed decreases, the flyweights move inward and the connecting rod moves downward. As the rod moves downward, the pilot valve also moves downward. Then the pilot valve blocks the drain line and opens the lube oil supply line. As a result, oil from the supply oil line flows through the pilot valve and then into the control oil line to the actuator. Now the pressure of the lube oil causes the piston to move upward. Thus the opening of the control valves increase and mare steam is admitted to the turbine. Hence the turbine speed increases gradually until it reaches at desire speed.

Gas turbine:

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Gas turbine may also refer to just the turbine component. Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature of the exhaust gas.
         

                 

                           Figure: Gas turbine chamber.

Gas turbine works on the basis of Brayton cycle. Brayton cycle is called the backbone of the gas turbine. Here the T-S and P-V diagram is shown in below:

All four processes of the Brayton cycle are executed in steady flow devices so they should be analyzed as steady-flow processes. When the changes in kinetic and potential energies are neglected, the energy balance for a steady-flow process can be express, on a unit-mass basis, as − 

                       

                                                                                                                      Figure: 1^st stage blade of GT

    (Q _in –Q _out) + (W_in –W _out) = H _exit – H _inlet

Therefore, heat transfers to and form the working fluid are

                        Q _in = H₃ – H₂ = C_p (T₃- T₂)

            And, Q _out = H₄ – H₁ = C_p (T₄ – T₁)

Then the thermal efficiency of the ideal Brayton cycle is-

 Brayton efficiency = 1 – (Q _out / Q _in).
                               

            Figure: T-S and P-V diagram     of Brayton cycle.

Gas turbine protections:

Protection should be taken for safety operation when the following conditions occur:

Ø  Over speed of turbine (about 3300 rpm)

Ø  Temperature deviation between two furnaces is 40^oC or mare.

Ø  Lube oil temperature increases gradually.

Ø  Over vibration (maximum 6 mils or more).

Ø  Generator over current

Ø  Reverse current comes to generators.

Ø  Stator earth fault.

Ø  Standby earth fault.

Ø  Exciter over voltage.

Ø  Exciter over current.

Ø  Exciter under current.

Efficiency Calculation

We know, efficiency = output energy/ input energy.

To produce 1unit electricity, we need 0.465 m³ gas

Again we know, the equivalent heat of -

1 KW = 860 K Call/ hour.

1 m³ = 8425 K Call.

So the efficiency, η = [860/ (8425 * 0.465)] * 100%

                                = 22%

Hence the efficiency of gas turbine is 22 % at open cycle.

Turbine trip conditions:

Ø  Over speed when it is exceed 55 Hz.

Ø  Fuel valve close.

Ø  Main stop valve close.

Ø  Furnace temperature deviation at 60^oC or more.

Ø  Breaker open.

Comparison of gas and steam turbine:

Serial No:	Gas turbine	Steam turbine
01	Flue gas acts as working fluid.	Steam acts as working fluid.
02	Main components are compressor and combustor.	Main components are boiler and accessories.
03	Running cost is less and starts quickly.	Running cost is less and starts quickly
04	Its efficiency is lass.	Its efficiency is more.
05	It requires less space for installation.	It requires more space for installation.
06	The mass of gas turbine per KW developed is less.	The mass of gas turbine per KW developed is more.
07	It does not depend upon water supply.	It depends upon water supply.