Steam turbine governing

[1] The primary objective in the steam turbine operation is to maintain a constant speed of rotation irrespective of the varying load.

In throttle governing the pressure of steam is reduced at the turbine entry thereby decreasing the availability of energy.

[1] Low initial cost and simple mechanism makes throttle governing the most apt method for small steam turbines.

The valve is actuated by using a centrifugal governor which consists of flying balls attached to the arm of the sleeve.

With a reduction in the load the turbine shaft speed increases and brings about the movement of the flying balls away from the sleeve axis.

This results in an axial movement of the sleeve followed by the activation of a lever, which in turn actuates the main stop valve to a partially opened position to control the flow rate.

[2] In larger steam turbines an oil operated servo mechanism is used in order to enhance the lever sensitivity.

The decrease in load during operation of the turbine will bring about increase in the shaft speed thereby lifting the governor sleeve.

The variation of the steam consumption rate ṁ (kg/h) with the turbine load during throttle governing is linear and is given by the “willan’s line”.

Generally bypass and nozzle governing are used simultaneously to match the load on turbine as shown in figure 3.