What is Steam Turbine Degree of Reaction (Reaction Ratio)?

One of steam turbine components are the turbine blades that act as nozzles. Those nozzles, either on the stator or rotor side, convert heat of the steam into kinetic energy. The nozzled shape of turbine blade on the rotor side also serves to convert the steam kinetic energy into mechanical energy as rotation of the rotor.

The convertion of heat energy to kinetic energy is always followed by enthalpy drop at isentropic condition. This enthalpy drop may occur on the side of the stator or rotor blade depending on the turbine design.

Reaction Ratio (also known as the

Simply put, the Reaction Ratio becomes a number indicating the type of a turbine whether it is an impulse turbine, reaction turbine or mixture.

Reaction Ratio (R) = \dfrac {\Delta h_{rotor}}{\Delta h_{stage}}

Where Δhrotor is the amount of decreased enthalpy converted to kinetic energy on the side of the rotor blade, and Δhstage is the total amount of the enthalpy drop in one stage.

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Speed Profile and Vector ​​of Impulse and Reaction Turbine


Reaction Ratio (R) = 0
At R = 0 means 100% enthalpy drop due to change into kinetic energy occurs on the stator blades. This process is a pure impulse process characterized by constant pressure at the point before and after the rotor blade, the steam stream only changes the direction only. The rotor blades alter the direction of the steam impulse directed at it and transfer high torque to the turbine shaft. Therefore, this type of turbine is also called an impulse turbine.

The advantage of this type of turbine is the large drop of enthalpy on a single stage of the blades, so the generation of energy by one turbine is greater. So the number of stages from the turbine will be less, and the turbine size will be shorter. But the disadvantage of this type is the loss of too much steam flow due to the larger flow velocity.

Reaction Ratio (R) = 0.5
A turbine of design R = 0.5 means that half of the enthalpy drop at one stage of the turbine blade occurs on the side of the stator blade, and the other half occurs on the turbine rotor blade. Turbine with this design is also called reaction turbine. Decrease in pressure and enthalpy of steam occurs on the stator side and turbine rotor. The steam pressure in the rotor blade inlet is larger than the outlet side. The flow of the steam is not only accelerated on the stator side, but also on the rotor side of the turbine.

The difference in steam pressure on the rotor blade side, causing the axial force on the whole turbine. An axial force means a force that is in line with the direction of the shaft. The axial force of the turbine rotor is opposite to the direction of the steam stream, and is also called axial thrust. Axial thrust must be compensated by the use of thrust bearing or against the force using a balance piston.

The advantage of using this type of turbine is the loss of steam flow due to the increase in flow velocity at every small stage. But the disadvantage is that the longer the turbine design, because the need for more stage than the impulse turbine.

In practice, turbines with a design R = 0.7 are more widely used at present. This means more enthalpy drops occur on the rotor side of the turbine than the stator side.

Steam Turbine Components – All Components in Detail

Here I will explain the steam turbine components.

1. Stop Valve
The stop valve of the steam turbine serves to isolate the turbine from the steam stream and also to quickly stop the supply of steam to the turbine under certain conditions. For example in case of loss of electrical load at a steam power plant from the the grid – we know it as a load rejection – the stop valve will quickly close in a split second. This is useful for avoiding overspeed on the turbine due to the presence of pressurized steam entering the turbine but no electrical load on the generator. The stop valve opens by the working of the hydraulic actuator and closes by the spring.



2. Control Valve
Control Valve is to control the flow of steam into the turbine in accordance with the existing load. In the steam power plant, the control valve opening depends on the amount of electrical load in the generator.

3. Electrohydraulic actuators on Stop and Control Valve
The actuators for stop and control valve on steam turbine power plant use the “fail-safe” principle. That is, the valves are opened by the hydraulic actuator and closed by force from the spring. The differences between the stop valve and control valve actuators is on the stop valve does not need to use the valve position sensor as in the control valve. The stop valve uses only a limit switch sensor.

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Working Principles of Hydraulic Actuators with Spring

4. Extraction Steam Line and its Check Valve
Extraction steam is a steam taken from certain stages in a steam turbine that is used for many things, such as preheating water (feedwater before boiler entry), turbine sealing system, sootblower system, etc.

In the pipeline extraction steam shall be installed check valve to prevent backflow from the steam. For example in the case of load rejection above, the flow of water back into the turbine, especially on the side of the superheater turbine will cause a termal stress on the steam turbine components.

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Swing Check Valve and Power Assisted Swing Check Valve

The check valve is commonly using Swing Check Valve or Power Assisted Swing Check Valve type. Swing Check Valve opens up by the large differences in steam pressure. And in the event of steam flow interuption (as in the case of load rejection) this check valve will close as the result of the weight of the valve itself. While the Power Assisted Swing Check Valve uses additional actuators to close the valve. Otherwise, to open the valve does not need to use the actuator. It will open by the pressure difference of the steam flow in the pipe.



5. Bearing
The steam turbine use bearing as a part to reduce friction between the shaft (the rotating part) with the casing (stator). Bearings are equipped with circulating and pressurized lubricating oil. To compensate the gravity of the turbine the journal bearing is used, whereas to compensate for the axial forces arising from the steam flow inside the turbine, thrust bearing is used.

6. Hydraulic Turning Gear
It is a mechanism to rotate the turbine rotor at initial start or at the time after shut down to prevent distortion/bending resulting from uncoordinated heating or cooling process on the rotor. This system uses a hydromatic motor whose rotating power comes from a high pressure hydraulic system.

7. Balance Piston
Balance Piston on steam turbine is to compensate the emergence of axial forces due to the flow of the steam insidenthe turbine. This component mitigates the work of thrust bearing.

Steam Turbine Working Principle

The steam turbine working principle lies in the change of heat energy contained in steam which is converted into mechanical energy transmitted to the turbine rotor. This happens in several different steam turbine stages. Each turbine stage always consists of a stationary circular blade and a rotating blade.

Heat energy in steam is shown by the amount of enthalpy (h).

h = u + p.V
u = internal energy, p.V = work flow

Steam Turbine Working Principle
Converting Heat Energy from Steam into Kinetic Energy


First, heat energy must be converted to kinetic energy, this process occurs in the nozzle (see picture above). In steam turbines, nozzles are mounted on the sides of the turbine stator and also at the rotor blades, hereinafter known as reaction stage. In this nozzle, water steam increases the speed (kinetic energy increase), and this acceleration causes differential pressure between the upstream sides nozzle and downstream nozzle.

Second, the kinetic energy is transformed into a rotary energy of the turbine rotor that occurs only on the rotating blade (rotor side).

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Velocity Vector On Steam Turbine Reaction Stage

 

Stages on the turbine has a speed difference, as shown in the picture above. At each level, a velocity triangle is drawn, one on the rotating inlet side of the blade, and the second at the outlet side. The absolute speed (c) in the inlet and outlet have different magnitude, since the kinetic energy of water vapor is converted to mechanical energy in the rotor.