All steam turbines have at least two independent governors that control the flow of steam. The first shuts off the steam supply if the turbine speed exceeds a predetermined maximum.

It is often called an emergency trip. The second, or main, governor throttles the flow of steam to maintain constant speed (in units not synchronized to a grid) or to vary the load (in units synchronized to a grid). The governors of extraction, mixed-pressure, and back pressure turbines control the steam flow while the speed and pressures are varying.

These governors are usually extremely complex.

Speed Governors
Speed governor systems consist of the following:e
● Speed-sensitive element
● Linkage or force-amplifying mechanism that transmits motion from the governor to the steam control valves
● Steam control valves (governing valves)

Figure below illustrates a centrifugal, or flyball, governor.

Mechanical speed governor. A simple arrangement such as this using a flyball governor is suitable for many small turbines.

The weights, mounted on opposite sides of the spindle and revolving with it, move outward by centrifugal force against a spring when the turbine speed increases. This action actuates the steam admission valve by one of the following:

● Mechanical linkage
● Operation of the pilot valve of a hydraulic system, which admits or releases oil to opposite sides of a power piston, or to one side of a spring-loaded piston (movement of the power piston opens or closes the steam valves)

Moderate and large units are equipped with a double-relay hydraulic system to boost the force of a centrifugal governor and to reduce the response time of the system. Intercept valves are installed upstream of the intermediate-pressure turbine.

They are closed by the governor system upon a load rejection (opening of the circuit breaker as a result of a disturbance in transmission) or a sudden load reduction. The intercept valves interrupt the flow of steam from the high-pressure turbine, the reheater, and the piping to the intermediate pressure turbine, thus preventing over speeding of the turbine.

Pressure Governors
The governors of back-pressure and automatic extraction turbines are designed to maintain constant extraction or exhaust pressure, regardless of the load.

The signal from a pressure transducer is communicated to the steam extraction control valves and the speed governor, which controls the steam flow to the turbine.

On automatic extraction turbines, the governor coordinates the signals from the pressure and speed transducer to maintain constant speed.

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