In this article we will discuss about:- 1. Operation of Steam Turbines 2. Classification of Steam Turbines 3. Compounding.
Operation of Steam Turbines:
The operation of a steam turbine is based upon the principle that the steam issuing from a small opening attains a high velocity. This velocity attained during the expansion of the steam depends on the initial and final heat contents of the steam. The difference of the two (the initial and final heat contents) represents the heat energy converted into kinetic energy.
The basic construction of a steam turbine is simple. There is no need of piston rod mechanism and slide valves and no flywheel is required with steam turbine. No wearing action being involved, maintenance of a steam turbine is comparatively much simpler. Problems of vibrations is also much less since high operating speeds result in lower weight of rotating parts for the same power. Governors are used to maintain speed constant when there is a change in load on the system. The governors may be centrifugal or hydraulic type.
Classification of Steam Turbines:
There are different ways of classifying steam turbines and according to the action of steam on moving blades, the steam turbines, are of two types namely impulse and reaction types. These two types differ in working.
ADVERTISEMENTS:
In an impulse turbine, the steam expands completely in the stationary nozzles, there being no pressure drop over the moving blades or runner. In doing so, the steam attains a high velocity and impinges against the blades fixed on the rotor periphery. This results in the impulsive force on the moving blades which sets the rotor rotating. The rotor may be a built-up rotor or integral rotor.
A built-up rotor consists of a forged steel shaft on which separate forged steel discs are shrunk and keyed. In an integral rotor the wheels and shaft are formed from one solid forging. The built-up rotor is cheaper and easier to manufacture. The high pressure and intermediate pressure rotors are always of integral type. The blades have symmetrical profile.
Impulse turbine has high speed and provides an ample clearance in between rotor or runner and stationary blades or casing (stationary blades are mounted in the casing). It also gives optimum utilisation of steam with a simple clear design. Owing to its simple and sturdy construction of all steam carrying parts, it has a long life.
In a reaction turbine, the steam does not expand in nozzles but expands as flows over the rotor blades, the blade will, therefore, act also as nozzles. The expansion of steam as it flows over the blades is adiabatic, any friction losses between the steam and blades are converted into heat, which, in turn will reheat the steam. The effect of this is to dry or superheat the steam as it flows over the blades. Reaction turbines are characterised by a relatively low rpm.
ADVERTISEMENTS:
Commercial turbines use series combination of impulse and reaction types because steam can be used more efficiently by using impulse and reaction blading on the same shaft. The steam is expanded through the turbine from a high pressure at the throttle valve to a back pressure corresponding to a vacuum of 71 to 73.5 cm Hg or an absolute pressure of 5 to 2.5 cm Hg. The standard speeds are 3,000 rpm and 1,500 rpm for coupling to 50 Hz alternators. Although all modern turbines are basically impulse-reaction turbines (partial expansion taking place in a nozzle), but are designated as reaction turbines.
According to the type of flow of steam, the steam turbines used are of two types, namely axial flow and radial flow type. In axial flow type turbines, the steam flows over the blades in a direction parallel to the axis of the wheel. In radial flow turbines the blades are arranged radially so that the steam enters at the blade tip nearest the axis of the wheel and flows towards the circumference. Majority of the steam turbines are of ‘axial flow’ type.
Steam turbines can also be classified in other ways, as given below:
Steam turbines are classified as condensing or non-condensing depending upon whether the back pressure is below or above atmospheric pressure.
ADVERTISEMENTS:
Where there is no use of exhaust steam, the turbine is built as a pure condensing turbine in which steam is reduced in pressure down to a vacuum pressure which is in accordance with the cooling water temperature. The steam inlet in this case is regulated; to suit the output required for the machine, by means of a governor in such a way that the oil pressure equipment opens and closes as required, a number of nozzle valves which admit steam to individual groups of nozzles of first turbine stage i.e., regulating stage.
i. Central-Station Turbines:
These turbines are employed for driving alternators at synchronous speed (usually 3,000 rpm) and have capacities ranging from 16 to 1,500 MW.
ii. Reheating Turbines:
ADVERTISEMENTS:
In these turbines, steam is returned after partial expansion to the boiler for re-superheating and then allowed to expand to back pressure.
iii. Superposed or Topping Turbines:
These are high pressure non-condensing turbines installed in existing low pressure steam plants. They exhaust into the existing low pressure turbines, thus, increasing plant capacity and overall thermal efficiency.
iv. Back Pressure Type Steam Turbines:
ADVERTISEMENTS:
Where process heating steam is required at low pressures back pressure steam turbines are used, which utilizes the pressure difference (i.e., heat gradient) available between live steam and back pressure of the process steam. The output of such a turbine is dictated by the amount of process steam required and the boiler steam condition which are determined by economical considerations.
Back pressure turbines are controlled by a pressure governor which operates in the same way as the speed governor of a condensing turbine, being affected by the volume of back pressure and opening or closing the group of valves regulating the steam supply to the turbine.
v. Bleeder or Extraction Turbines:
In such turbines part of the steam leaves the turbine casing before the exhaust, for feed water heating.
vi. Extraction-Induction Turbines:
Such turbines have provisions for both removing and introducing steam into the turbine at intermediate points.
As regards the size of the units to be installed, it depends upon several factors such as capital cost, spares required, load factor and the peak load of the system.
Compounding of Steam Turbines:
In a single stage turbine, the steam is expanded in a single ring of nozzles from the boiler pressure down to the exhaust pressure, resulting in an extremely high velocity (of the order of 30,000 rpm). With such a turbine, the blade tip stresses and the disc friction losses will be very high, resulting in poor efficiencies. In thermal power plants where the generators usually run at 3,000 rpm, single stage turbines are undesirable.
Compounding is, therefore, necessary for obtaining reasonable blade tip speeds in turbines. In compounding, a number of rotors in series, keyed on the same shaft, are employed and the steam pressure or the jet velocity is absorbed in steps as it flows over the moving blades.
There are two types of compounding—velocity compounding and pressure compounding. In velocity compounding, the steam is expanded from boiler pressure to condenser pressure in one set of stationary blades or nozzles. However, the total flow energy is absorbed in a number of rows (not in one) of moving blades (2, 3 or even 4), with a row of fixed guide blades between every pair of them.
The turbine is impulse one and is called the ‘Curtis stage’, because the pressure remains constant as the steam flows over the blades. Pressure compounding is equivalent to a number of simple impulse stages in series. The pressure drop occurring in each stage is only a portion of total pressure drop.
Combined velocity-pressure compounding makes use of both types of compounding. The overall pressure drop of steam is divided into stages and the velocity attained in each stage is also compounded.
Steam Nozzles:
The function of a steam nozzle is to convert the heat energy of steam into kinetic energy and the chief use of a steam nozzle is to develop a high velocity jet of steam for driving a steam turbine. This is achieved by allowing the steam to expand from a region of high pressure at the inlet to a region of low pressure at the outlet. With the expansion of steam through the nozzles, its velocity and specific volume both increase. Its dryness fraction will also vary due to condensation of steam.
Since the weight of steam per second flowing across any section of the nozzle is constant, the cross section of the nozzle will vary in accordance with velocity, specific volume and, the dryness fraction of steam. Depending upon its shape, the nozzles are of two types, viz., convergent-diver- gent nozzle and convergent nozzle. The nozzle is designed for discharge of maximum weight of steam for a given pressure drop.
In the convergent-divergent nozzles, the x-section first decreases until it reaches the minimum at a section called the nozzle throat and then it increases as shown in Fig. 3.19 (a), the largest x-section being at the exit end. In the convergent type nozzles, the nozzle exit is the throat itself, as shown in Fig. 3.19 (b).
