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Tuesday, 16 December 2014

Governing of Steam Turbines

Governing of Steam Turbines Points : Governing of Steam Turbines, Throttle Governing, Nozzle Control Governing, Bypass Governing, Combined Throttle and Nozzle Governing The major idea of governing a steam turbine is to keep turbine speed stable irrespective of load. The performance will depend; to a large expand, on method to control steam supply to turbine. The regular process of governing are:
(i) Throttle Governing
(ii) Nozzle Control Governing
(iii) Bypass Governing
(iv) Combined Throttle and Nozzle Governing
(i) Throttle Governing Throttle governing is mainly generally use mostly on small turbines as its initial cost is less and mechanism is easy. The mechanism of the throttle governing to throttle steam every time there is a reduction of load evaluate to economic or design load for maintain speed and vice versa. To start the turbine for complete load running valve A is open. The process of double heat valve B is carried out through an oil servo motor which is controlled through a centrifugal governer. As steam turbine gains speed valve B closes to throttle steam and decrease steam supply to nozzle. (ii) Nozzle Control Governing Nozzle control governing is one in which steam flow during nozzles is regulate through valves. Nozzle control governing is commonly working at first point of turbine due to sensible limits. Nozzle areas in remain stages continue constant. If someway nozzle governing is give for every nozzles in every and every stage then an perfect situation of turbine flow way areas conforming to mass flow speed at every loads shall exist. Below such ideal conditions pressure, velocities and nozzle and blade efficiencies would be stable by load. For such ideal form the Willan’s line possible straight line as indicate for throttle governing of turbine. In nozzle governing the nozzles of turbines are group in two, three or extra groups upto six or eight groups. As nozzle governing is working then pressure and temperature of steam entering first stage nozzles are independent of load. (iii) By Pass Governing In case of by-pass governing arrangement is made for by-passing over quantity of steam with no allowing total steam amount to add in turbine output while load reduces. The steam as of main line enters the major valve which is control with speed governor. Steam as of main valve enters nozzle box or steam chest. By pass valve is too give on nozzle box. By pass valve is attached to a passage which deliver steam being through passed to later end of turbine. By pass valve is actuate as load varies, therefore allow simply part of steam entering major valve to give in power output. By pass valve is controlled through speed governor for every loads in its range. In this type of governing depending upon turbine and its’ function there might be over one by pass valves. (iv) Combined Governing Some times while governing necessities are not met through any one kind of arrangements of governing that is throttle, nozzle control and by pass governing, then combination of two governing mechanisms may be employed. These popular combinations are throttle and nozzle control combined governing and throttle with by pass combined governing.

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