GUJARAT TECHNOLOGICAL

UNIVERSITY

Vidhyadeep Institute of Engineering And

Technology

Governing of Steam Turbine

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.

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.

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.

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.

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.

A

B

d

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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Steam Power Plant Process

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

Rankine Cycle

Reheat on T-s diagram

Schematic of Rankine Reheat Cycle

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.

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.

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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