Instrumentation

What is Governor and how is it used to control turbine speed?

What is the governor in a Steam Turbine?

  • A governor in a steam turbine is an electronic component to monitor and regulates the rotational speed of the turbine.
  • The main objective is to maintain a constant speed in response to variation in load.
  • Governor in the turbine is a type of feedback device.
  • It senses fuel supply and loads to the engine accordingly to keep pace with the load.
  • The load variation in a turbine operation makes a remarkable impact on the performance.
  • The output signal generated by the governor handles the steam inlet valve or nozzle position to control the flow of main steam from the boiler to the steam turbine.
  • Turbine governing is necessary because the turbine is directly coupled to an electric
  • The load and speed are inverse proportion to each other.

Case I: When the turbine load increases the speed of the engine will decrease proportionally.

Then the governor increases the fluid supply to maintain constant speed within specified limits.

Case II: When the turbine load decreases the speed of the engine will increase proportionally.

Then the governor reduces the fluid supply to maintain a constant speed as much as possible over a long period.

What Are The Various Types Of Governors Used In Steam Turbines?

A steam turbine uses three types of governors.

1. Throttle Governing of Steam Turbine:

  • To maintain turbine speed constant the main steam is throttled the load falls below the design load conditions
  • The pressure is reduced at the inlet side of the turbine to minimize the availability of steam
  • The steam is passed through a narrow path to reduce pressure across the governing valve.
  • The flow rate of the main steam is controlled by partially opening the main steam control valve.
  • The main shaft of the turbine drives the centrifugal governor by belt or gear arrangement. And the control valve controls the direction of oil flow.
  • It is cheaper and its mechanism is simple.
  • This system is used for small turbines.

2. Nozzle Control Governing of Steam Turbine:

  • In Nozzle control, the steam flow rate is controlled by opening and closing sets of nozzles instead of regulating the steam pressure.
  • A nozzle set consists of more than two nozzles. And these are controlled by a separate valve.
  • The flow rate is controlled by the actuation of the individual valves to close the nozzles.
  • This nozzle governing is applied only at the first stage of the turbine.
  • Advantage  of nozzle control governing is to utilize full boiler pressure and temperature

3. Bypass Governing of Steam Turbine:

  • The bypass line in the steam turbine provides steam passing from the first stage nozzle box to a later stage for increasing work output.
  • These valves are opened and fresh steam is introduced to the next stage of the turbine.
  • The bypass steam is controlled automatically by valve lift under the management of the governor for all loads within the specified range.
  • The advantage of this is more energy is generated for satisfying the increased load.

4. Combination Governing:

  • This combination governing type makes use of any two types.
  • Generally for matching the loads in the turbine, nozzle, and bypass governing methods are considered.

5. Emergency Governing:

An over speeding of the turbine occurs in rare cases. But it is necessary to test the over-speed governor periodically to determine proper working during turbine-rated operation without bringing it to excess speed at which the emergency governor is set to trip.

The emergency governing mechanism of a turbine consists of.

  • Main emergency governor for stopping the turbine at excessive speed.
  • Mechanically actuated trip valve to control the flow of working fluid.

Every steam turbine consists of emergency governors which come into action if.

  1. Shaft speed rises above 110% of the rated value.
  2. Balancing of the turbine is disturbed.
  3. Failure in the lubrication system of the turbine.
  4. Vacuum in the condenser goes low.
  5. Inadequate supply of coolant to the condenser.

 

How does the Turbine Governing System work?

GOVERNOR

The turbine governing system is classified as

Case 1:  When point B is fixed

  • The speed controlling lever is varied based on load and demand.
  • The steam requirement must be raised or higher load to increase generator mechanical input.
  • When the lever is lowered and point A shifts to the down position. But point B is fixed. That makes points C and D rise.
  • The pilot valve is attached to Point D the upper pilot valve opens on the upward movement of point D.
  • The high pressure pushes the piston in the hydraulic amplifier downward.
  • The upper end of the piston is connected to the main steam valve to allow the steam to enter to provide the required mechanical input to meet the steam demand.

Case 2: When point A is fixed

  • The power system is said to be in a steady state.
  • Frequency is directly proportional to load conditions.
  • When the load in the system increases, the frequency increases and vice versa.
  • The change in frequency is sensed by the governor.

Working of Fly Ball Governor:

  • The fly balls move outward if the frequency is increased
  • The fly balls move inward if the frequency is decreased
  • Here frequency is in direct proportion to shaft speed.

 Let us consider a situation for reducing load.

  • When the load has reduced this results in an increase in frequency that makes the balls fly outward.
  • When the balls fly outward point B is pushed downwards which makes both points C and D pull downward.
  • When point D is moved down the lower pilot valve is opened to allow high pressure in the hydraulicamplifier to push the piston upward to close the steam valve and reduce input steam.

A Summary is described as

GOVERNOR 2

Describe the parts of the turbine governing system:

The turbine governing system consists of

1. SPEED CHANGER:

  • This provides a constant power setting to the turbine to get a constant output under steady-state conditions
  • It consists of a lever that can be lowered or raised.
  • Steam Inlet valve position depends upon lever position.
  • The position of the fly balls is not dependent upon the lever position.

2. GOVERNOR MECHANISM:

  • It is known as a centrifugal governor.
  • This governing system uses fly ball 
  • A change in frequency is detected in this mechanism

3. HYDRAULIC AMPLIFIER:

  • Pilot valve and piston arrangement are two sub-parts
  • This amplifier has an inlet valve to allow high-pressure oil inside.

4. LINKAGE MECHANISM:

  • This is the principal part of the governing system.
  • The position of the piston depends on the linkage mechanism.
  • The linkage mechanism controls the Steam Inlet Valve position.

How is the Governor used to control the speed of the Turbine/Pump?

Generally, the speed of the turbine is 9000 RPM for a full load.

  • Woodward governor is used to controlling turbine speed.
  • These include a pilot valve and controller to modulate the inlet valve of the turbine to maintain the shaft speed at the set point.
  • Proximity probes are used to measure the turbine speed.
  • The speed PID controller receives the signal from probes and compares it with a given set point to generate an output signal.
  • This controller receives a programmable droop feedback signal to increase the stability of the system.
  • This droop feedback signal is proportional to the controller’s output signal or generator load in MW.

The output signal is selected from two control modes

  1. Start-Up Speed Controller and
  2. Speed and Load Controller
  • This controller produces overrule control during the start-up sequence of the steam turbine.
  • The start-up speed controller continues to ramp to its maximum set point value when the unit reaches synchronous speed.
  • The ramp rates are slow between these states but the ramp rates are managed by start-up speed controller from state to state.
  • During the warm-up period, uneven expansion of the steam turbine components is avoided.
  • Torsional and lateral analysis determines the critical speed of steam turbines.
  • The critical speed of the system is defined as when the frequency of rotation meets or becomes equal to the critical frequency, and thus high vibrations occur.
  • When this start-up speed controller brings the steam turbine to the rated speed, the control is handed over to the speed and load controller.

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