Different Types Of Governing System in Steam Turbine

STEAM TURBINE GOVERNING SYSTEM

Steam turbine governing system is a method, used to maintain a constant steady speed of turbine. The importance of this method is, the turbine can maintain a constant steady speed irrespective of variation of its load. A turbine governor is provided for this arrangement. The purpose of the governor is to supply steam into the turbine in such a way that the turbine gives a constant speed as far as possible under varying the load. So, basically Steam turbine governing system is a process where turbine maintains a steady output speed irrespective of variation of load. The different types of steam turbine governor of are:

1. Throttle Governing Of Steam Turbine

2. Nozzle Control Governing Of Steam Turbine

3. Bypass Governing Of Steam Turbine

1. Throttle Governing Of Steam Turbine:-

Throttle Governing of steam Turbine is most popular and easiest way to control the turbine speed. When steam turbine controls its output speed by varying the quantity of steam entering the turbine is called Throttle Governing. It is also known as Servomotor methods.

In this system, a centrifugal governor is driven from the main shaft of turbine by belt or gear arrangement. A control valve is used to control the direction of oil flow which supplied by the pipe AA or BB. The servomotor or relay valve has a piston which moves towards left or right depending upon the pressure of oil flow through the pipes AA or BB. This cylinder has connected a needle which moves inside the nozzle.When the turbine is running at normal speed, everything in the turbine such as such control valve, servomotor, piston position, fly balls of centrifugal governor will be in their normal position as shown in the figure. The mouth of both pipes AA or BB is closed into the control valves.

Assume that the turbine's load increases. It will decrease its speed which will decrease the centrifugal force of the turbine. Now fly balls of the governor will come down thus decreasing their amplitude. These fly balls also bring down the sleeve. The sleeve is connected to a control valve rod through a lever pivoted on the fulcrum. This down word sleeve will raise the control valve rod. Now oil is coming from the from the oil sump, pumped by gear pump is just stay at the mounts of both pipes AA or BB which are closed by the two wings of control valves. So, raise of control valve rod will open the mouth of the pipe AA but BB is still closed. Now the oil pressure is coming from the pipe AA. This will rush from the control valve which will move the right side of the piston. As a result, the steams flow rate into the turbine increases which will bring the speed of the turbine to the normal range. When speed of the turbine will come to its normal range, fly balls will come into its normal position. Now, sleeve and control valve rod will back to its normal position.

2. Nozzle Control Governing Of Steam Turbine:-

It is another interesting method by which turbine's speed can be controlled. Nozzle control governing of steam turbine is basically used for part load condition. Some set of nozzles are grouped together (may be two, three or more groups) and each group of the nozzle is supplied steam controlled by valves. Every valve is closed by the corresponding set of nozzle. Steam's flow rate is also controlled by these nozzles. Actually, nozzle control governing is restricted to the first stage of turbine whereas the subsequent nozzle area in other stage remains constant. According to the load demand, some nozzles are in active and other inactive position. Suppose turbine holds ten numbers of nozzles. If the load demand is reduced by 50% then five numbers of nozzles are in open condition and rest is closed. This method is suitable for SIMPLE IMPULSE TURBINE. It is a process where rate of steam flow is regulated depending on the opening and closing of set of nozzles rather than regulating its pressure.

3. Bypass Governing Of Steam Turbine:-

Bipass governing of steam turbineis a method where a bypass line is provided for the steam. Especially this is used when turbine is running in overloaded condition. The bypass line is provided for passing the steam from first stage nozzle box into a later stage where work output increase. This bypass steam is automatically regulated by the lift of valve which is under the control of the speed of the governor for all loads within its range. Bypass valve is open to release the fresh stem into the later stage of the turbine. In the later stage output, work is increased and the efficiency is low due to the throttle effect.

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