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GUJARAT TECHNOLOGICAL
UNIVERSITY
Vidhyadeep Institute of Engineering And
Technology
Governing of Steam Turbine
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Governing
• Steam turbine governing is the procedure of controlling the flow rate of steam into a steam turbine so as to maintain its speed of rotation as constant.
• The variation in load during the operation of a steam turbine can have a significant impact on its performance.
• In a practical situation the load frequently varies from the designed or economic load and thus there always exists a considerable deviation from the desired performance of the turbine.
• The primary objective in the steam turbine operation is to maintain a constant speed of rotation irrespective of the varying load. This can be achieved by means of governing in a steam turbine.
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Throttle governing
• In throttle governing the pressure of steam is reduced at theturbine entry thereby decreasing the availability of energy.
• In this method steam is allowed to pass through a restrictedpassage thereby reducing its pressure across the governingvalve.[
• The flow rate is controlled using a partially opened steamcontrol valve. The reduction in pressure leads to a throttlingprocess in which the enthalpy of steam remains constant,
• Low initial cost and simple mechanism makes throttlegoverning the most apt method for small steam turbines. Themechanism is illustrated in figure 1.
• The valve is actuated by using a centrifugal governor whichconsists of flying balls attached to the arm of the sleeve.
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Throttle governing
• A geared mechanism connects the turbine shaftto the rotating shaft on which the sleevereciprocates axially.
• With a reduction in the load the turbine shaftspeed increases and brings about the movementof the flying balls away from the sleeve axis.
• This result in an axial movement of the sleevefollowed by the activation of a lever, which inturn actuates the main stop valve to a partiallyopened position to control the flow rate.
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Nozzle governingIn nozzle governing the flow rate of steam is regulated byopening and shutting of sets of nozzles rather than regulatingits pressure.
• In this method groups of two, three or more nozzles form aset and each set is controlled by a separate valve.
• The actuation of individual valve closes the corresponding setof nozzle thereby controlling the flow rate.
• In actual turbine, nozzle governing is applied only to the firststage whereas the subsequent stages remain unaffected. Sinceno regulation to the pressure is applied.
• Figure 2 shows the mechanism of nozzle governing applied tosteam turbines. As shown in the figure the three sets ofnozzles are controlled by means of three separate valves.
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By pass governing
• Occasionally the turbine is overloaded forshort durations.
• During such operation, bypass valves areopened and fresh steam is introduced into thelater stages of the turbine.
• This generates more energy to satisfy theincreased load.
• The schematic of bypass governing is asshown in figure3.
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A
B
d
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By pass governing
• The total amount of steam entering the turbine passesthrough the valve A which is under the control of speedgovernor.
• B is a nozzle box or steam chest .• For all loads greater than the economic load , a by pass
valve c is opened, allowing steam to pass from the firststage nozzle box in to the steam belt D and so in to thenozzle of downstream stage.
• The valve c is designed such that it is not opened untilthe lift of the valve a diminishes.
• The by pass valve c remains under control of a speedgovernor for all loads within its range.
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Thank You
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Steam Power Plant Process
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Steam Turbine• Steam turbine convert a part of the energy of the steam evidenced by high
temperature and pressure into mechanical power-in turn electrical power
• The steam from the boiler is expanded in a nozzle, resulting in the emission of a high velocity jet. This jet of steam impinges on the moving vanes or blades, mounted on a shaft.
• Here it undergoes a change of direction of motion which gives rise to a change in momentum and therefore a force.
• the motive power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate.
• The conversion of energy in the blades takes place by impulse, reaction or impulse reaction principle.
• Steam turbines are available in a few kW(as prime mover) to 1500 MW
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Rankine Cycle
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Reheat on T-s diagram
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Schematic of Rankine Reheat Cycle
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Compounding of Steam Turbines
• This is done to reduce the rotational speed of the impulse turbine to Compoundingof Steam Turbines practical limits.
• Compounding is achieved by using more than one set of nozzles, blades rotors in aseries keyed to a common shaft; so that either the blades, rotors, series, steampressure or the jet velocity is absorbed by the turbine in stages.
Three main types of compounded impulse turbines are:
a. Pressure compounded
b. Velocity compounded
c. Pressure and velocity compounded impulse turbines.
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Pressure compounded
• Involves splitting up of the whole pressure drop into a series of smaller pressure drops across several stages of impulse turbine.
• The nozzles are fitted into a diaphragm locked in separates one wheel chamber from the casing that another. All rotors are mounted on the same shaft.
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Velocity compounded &Pressure and velocity compounded impulse turbines.
• Velocity drop is achieved through many moving rows of blades instead of a single row of moving blades. It consists of a nozzle or a set of nozzles and rows of moving blades attached to the rotor or the wheel and rows of fixed blades attached to the casing.
• Pressure velocity compounding gives the advantage of producing a shortened rotor compared to pure velocity compounding. In this design steam velocity at exit to the nozzles is kept reasonable and thus the blade speed (hence rotor rpm) reduced.
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THANK YOU