STEAM TURBINE AND ITS GOVERNING y
INTRODUCTION: A steam turbine is rotary machine which is designed to convert the energy of
high pressure and high temperature steam into mechanical power. The operation of steam turbine wholly depends upon the dynamic action of the steam. In this the steam is first expanded in a set of nozzles or passages up to exit
pressure where in the pressure energy of the steam is converted into kinetic energy. The nozzles are fixed to the casing. If the resultant high velocity steam is passed
over the curved vanes or blades, the steam changes its direction and it would leave vanes. Due to this there is a change in momentum and it will exert a resultant force on
the blades as shown. If these blades are attached to disc on a rotor or shaft which is free to rotate, the resultant force would cause the rotor to rotate. Thus the motive power is developed. A pair of ring of nozzles fixed to the casing and a ring of moving blades fixed to
the turbine rotor is called a stage or a turbine pair. Both the fixed and moving blades are so designed that the steam jet shall not strike the blades but it should glide over in the direction of blade surfaces. y
Types of the steam turbine: These are mainly of two types: Impulse turbine: These turbines use the principle of impulse in which the kinetic energy of
steam obtained after passing over a ring of fixed nozzles is used to exert a force on a ring of moving blades. The pressure of steam while passing over the moving blades remain
constant and its kinetic energy is converted into mechanical work. The examples of impulse turbines, are De-Laval, Curtis and Rateau etc. Reaction turbine: In case of reaction turbines, there is a continuous pressure drop of steam
while passing over the rings of fixed and moving blades. Accordingly, the moving blades passages are suitably designed for steam to expand; therefore, these blades also act as nozzles.
The reactive force along with that due to change in momentum of the
steam provides the motive force for the turbine to develop power. It follows that these turbines are basically the impulse-reaction turbine but in practice these are called reaction turbines. Example of such a turbine s Parsons reaction turbine. Mostly the steam turbines are axial flow type in which the steam flows
over the blades in direction parallel to the axis of turbine rotor. The only important radial flow is Lungstorm reaction turbine in which the
steam enters at the blades tip nearest to the axis of the wheel and flows towards the circumference. y
Advantages of Steam Turbines: The main advantages of steam turbines are: 1. It is rotary high speed machine. 2. It is compact and it has low weight to power ratio. 3. It has perfect balance and runs vibration free. 4. It needs less floor area. 5. It has low initial and maintenance cost. 6. It needs no internal lubrications, therefore, its condensate is not contaminated and it is suitable as feed water. 7. It is suitable for electrical generators.
Compounding of steam turbine y
Velocity compound impulse turbine (Curtis turbine) In this type of turbine the steam expand in a set of nozzle, from the boiler pressure
up to the condenser pressure, which converts its energy into kinetic energy. This high velocity steam is passed over the rings of moving blades, each ring of
moving blades being separated by a ring of fixed blades as shown in fig. A part of high velocity steam is absorbed in the first ring of moving blades and the
remainder is passed to the next ring of fixed blades. The function of fixed blades is to change the direction of flow of steam so that it
can glide over the second ring of moving blades. The velocity of steam while passing over a fixed blade is constant except for the
energy lost for overcoming the friction losses. Again a part of steam velocity is absorbed in the second ring of moving blades and
the process of absorbing the steam velocity continues till it is finally wasted to exhaust.
Fig. represents the variation in pressure and velocity of steam passing over the set
of fixed and moving blades along the axis of the turbine.
Advantages : 1. Due to relatively large heat drop, a velocity compounded impulse turbine requires comparatively small number of stages and less space. 2. Optimum blades speed ratio decreases with increase in number of stages. 3. Cost of turbine is low. 4. The pressure drop take place in nozzle, hence turbine casing not be designed to withstand high pressure.
Disadvantages : 1.
Friction
losses are large since the velocity of steam high.
2. It efficiency is low and keeps on decreasing with number of stages. 3. Power developed in later stage keeps on decreasing while these stages require same material spaces and fabrication. y
Pressure compounded impulse turbine (Rateau turbine) In this type of turbine the total pressure drop does not take place in a single ring of
nozzle but it is divided up in between the set of nozzle rings. Steam from boiler is partially expanded in the first ring of nozzle and then it is
passed over the ring of moving blades till its velocity is absorbed. Exhaust from the moving blade ring is passed over the secondary ring of nozzle for
further expansion partially and its increases velocity is absorbed in the secondary ring of moving tables. Similar process is repeated till the steam is expanded up to the condenser pressure. Since the steam is partially expand in each ring of nozzle, the velocity of steam
would not be very high, hence, the turbine velocity would be low. The pressure and velocity variation of the steam along the axis of turbine is shown in fig. y
Pressure velocity compound impulse Turbine : In this method, both the previous method of velocity and pressure compounding
are utilized. The total pressure drop of steam is divided into stages and velocity of steam
obtained due to expansion in each stage is also compounded. Since the number of stages are less we get a more a compact turbine than a
pressure compounded turbine. This method is well illustrated in fig. will pressure-velocity variation along the axis
of the turbine.
Governing of Steam Turbine: y
Throttle Governing :
In this type of governing the steam is throttled down to a lower pressure according to the
load on the turbine it is supplied to the turbine. It reduces the enthalpy drop i.e. the available energy of the steam. Such a method is
useful for small capacity power plants since the mechanism is simple with initial low cost. The schematic arrangement employing throttle governing is shown in Fig. The throttling
of steam is achieved with the help of a balanced throttle valve which is controlled by a centrifugal governor. In case of small turbines the throttle valve can be actuated directly with the help of
governor through linkages since the steam flow rates would be small and the valves needed are light.
However,
in case of medium and large power plants the effort of the governor may not
be sufficient to actuate the throttle valve, due to this an oil operated relay is incorporated in the circuit as shown in
Fig.2.13.
This relay magnifies the small force
produced by the governor for a small change of speed to produce a large force to move the throttle valve. The working of the mechanism is as follows. Consider the case when the load on turbine shaft is equal to the power developed by the
turbine, the speed is constant and the system is in equilibrium. Let us assume that the load on the turbine is reduced suddenly. At this stage since the
power developed is more than the load, turbine and governor speed will increase due to the excess energy developed by the turbine. The governor balls will fly out and it will raise the sleave of the governor, consequently,
the differential level will cause the pilot piston to be raised. The oil which is supplied under a pressure of 3-4 bar will flow through the pipe A to the
cylinder of relay piston and it would force the relay piston to move downwards, while the oil from relay piston cylinder is drained out through the pipe B. The downward movement of the relay piston operates the throttle valve which in turn
closes the steam ports partially. It throttles the steam and the steam pressure at inlet to the turbine is reduced. When the power developed by the turbine equals to the load on the turbine, the oil ports C-D are covered and the relay pistons locked. It should be noted that this methods of governing is though simple but it reduces the efficiency of power plants at parts loads because a part of the available energy is lost in the irreversible throttling process. The Willans straight line relationship between load and steam consumption is also followed. y
Nozzles Governing : This method of governing is more efficient than the throttle governing since in this
method only the floe rate of steam supplied to the turbine in controlled. The method is illustrated in fig. Total numbers of nozzle required are divided into the number of sets N 1, N2, N3.
Each set consists of the number of nozzles required to affect the require control on the mass flow rate corresponding to the require power control. Due to this, different sets of nozzles will have different number of nozzles. The supply of the steam to nozzles is controlled by opening and closing of valves
V1, V2, V3. At full load all the valves will supply the steam to all the nozzle and a part loads or more numbers of valves are closed. There are several mechanical possible arrangement of nozzles and control valves
but in each method the nozzle control is only applied for the first stage of the turbine and the nozzle area for the subsequence stages of the turbine remains constant. It can be seen that in case of nozzle governing the total enthalpy drop is available
unlike the case of throttle governing in which the enthalpy drop is reduce at parts loads. Due to this efficiency at part loads remains unaffected in case of nozzle governing. y
By-pass governing : Modern system power plants employing impulse turbine at very high pressure of
steam admission are usually design at an economical load which is about 75-80% of the maximum load. In such case it is desirable to have full admission of the steam in first few stages of
the turbines operating at very high pressure since the enthalpy drop is very small in the initial stage and the nozzle governing cannot be use effectively. Therefore the load regulation is achieved by throttle governing up to the stage of
economical loads.
However at the maximum loads the additional amount of steam required cannot
be passed through the first stage since the require additional number of nozzles are not available. The difficult of steam regulation is overcome by employing by-pass governing as
shown in fig. Steam requires up to economical load is passed through the inlet valve and it is
collected in the nozzle chest. The governing is affected by throttle valve. However at loads more than economical load, the by-pass valve lifts and a part of
the steam is by passed into the steam belt and this steam mixed with the steam of high pressure turbine stages and it is supplied to lower stages to lower stages of the turbine. The movement of the by-pass valves is controlled by the turbine governor.
