Governor is a device which senses the speed of an engine or turbine and controls the fuel (or steam) to the engine or turbine to maintain the speed at a desired level to meet changes in load or horse power o/p required.
For further understanding take an example:
- A way set to desire the speed (The driver set the desired speed mentally.)
- A way to sense the actual speed (The driver refers to speedometer)
- A way to compare the actual speed to the desire speed (The driver compares two item mentally )
- A way for the governor to change the fuel to the engine or turbine(Moving the rack or fuel valve)(The driver moves the throttle)
- A way to stabilize the engine after a fuel change has been made
- The governor functions the same as the car driver
- It automatically changes the fuel flow to maintain the desired speed or load.
- Closed Loop Definition: When used as an automatic control system for operation or process in which feedback in a closed path or group of paths to maintain output at a desired level.
- If parameters of the loop change, it will affect the entire loop and fuel will automatically be corrected to maintain the desired set point.
Types of Governor:
Following types of governor are mostly used :
- MECHANICAL GOVERNOR
- HYDRAULIC GOVERNOR
- PNEUMATIC GOVERNOR
- ELECTRONIC GOVERNOR
A mechanical governor uses gears and flyweights inside the crankcase as a speed-sensing device that detects changes in the load and adjusts the throttle accordingly.
As the crankshaft spins, centrifugal force causes the flyweights to open. As they open, they apply pressure to the governor cup and governor crank, which are linked to the throttle. The throttle is pulled toward the closed position.
As the load on the crankshaft increases, the flyweights spin more slowly. The reduced centrifugal force on the flyweights results in less pull on the throttle toward the closed position. But since the governor spring tension remains, the throttle reopens until the desired governed speed is achieved.
Hydraulic Amplifier is a linear electro-hydraulic actuator designed to operate steam turbine control valves or valve racks.
This integrated actuator is intended for use on mechanical-drive or generator-drive steam turbines, and uses a low-pressure hydraulic oil source (typically turbine lube oil) to provide its output shaft force.
The Hydraulic Amplifier’s superb accuracy and resolution make it ideal for steam turbine valve control and related turbine speed and load control.
- Output: linear, 76 mm (3″) max stroke, 184 mm (7.25″) bore max, 54.5 kN (12,250 lbf) force max
- Input Power: 690 to 3448 kPa (100 to 500 psi), supply oil, 0-200mA – position demand
Pneumatic Governor :
A pneumatic governor uses a movable air vane, made of metal or plastic, as a speed-sensing device by registering the change in air pressure around the spinning flywheel.
The pneumatic governor design is simpler and parts are easier to access. It is also a slightly less reliable design, since small particles of debris can interfere with the pneumatic governor’s operation.
The pneumatic governor also relies on one or two springs to pull the throttle toward the open position. As the load lessens and engine speed increases, air blown by the flywheel also begins to increase, causing the governor blade to pull the throttle plate toward the closed position in its effort to maintain a steady engine speed.
Electronics Governor :
The electronic governor uses a combination of electrical and mechanical components. Speed probe sensing the speed and desired signal output is used in conjunction to operate a hydraulic unit.
Then the fuel controls are moved in the appropriate direction to control the engine speed.
505 Digital Governors:
Woodward 505 Digital Governor for steam turbines with single or split-range actuators. The 505 is field programmable which allows a single design to be used in many different control applications and reduces both cost and delivery time.
The 505 can be configured to operate as a stand-alone unit or in conjunction with a plant’s Distributed Control System.
Example of pump or compressor discharge pressure control with turbine inlet pressure limiting:
Woodward Governor 505
This is an example of a typical pump or compressor application. 505 is configured to normally control pump/compressor discharge pressure, extraction pressure, and limit governor valve position based on low turbine inlet steam pressure.
Both the Auxiliary and Cascade modes were used for this application.
Extraction pressure is controlled by the Extr/Adm PID. This PID controller can be enabled automatically or manually depending on configuration.
In all cases the 505 starts up with the Extr/Adm PID disabled and the LP valve at its maximum open position. This allows a turbine to warm-up in a uniform manner. The Extr/Adm set point is only varied through the 505 front panels.
505E could be programmed to have the Extr/Adm set point varied through discrete inputs; a 4-20mA signal .Pump/compressor discharge pressure control is performed within the 505 through the Cascade controller.
Because the discharge pressure is being controlled, typically affects many other plant processes, a plant Distributed Control System (DCS) may be used to monitor plant process conditions and set the Cascade set point position. This can be performed through Analog set point signal.
For this application a limiting type of control function was required to help preserve inlet header pressure in case of system header problems. Because the Auxiliary PID is the only controller that has this capability, it is used to sense turbine inlet pressure and limit governor valve position, based on a low inlet pressure setting.
If a plant Distributed Control System is used to sense and control a process by positioning the load of multiple pumps or compressors (load sharing), the DCS may interface directly to the 505 Speed PID set point through a programmed Remote Speed Set point analog input.
This allows a DCS to monitor and compensate for plant and system conditions by directly changing the speed of multiple pumps or compressors simultaneously.
All 505 PID controller set points (Speed, Extr/Adm, Auxiliary, Cascade) may be changed through programmed raise and lower contacts (From field), programmed 4-20mA inputs.
This is only a reference example we may configure 505 as per our requirement.
Overview of 505 Integrated Control System
The 505 model is designed for simple single valve steam turbine applications where basic turbine control, protection, and monitoring are required.
The 505 controller’s integrated operator control panel, overspeed protection, and trip event recorder make it ideal for small steam turbine applications.
Image Courtesy : govconsys
The 505-XT model is designed for more complies single valve, single extraction or single admission steam turbine applications where more analog or discrete I/O are required.
The 505E’s control capabilities are:
- Speed/Frequency Control
- Extraction / Admission Header Pressure Control
- Turbine or Generator Load Control or Limiting
- Turbine Inlet Header Pressure Control or LimitingTurbine Exhaust Header Pressure Control or Limiting
- Plant Import/Export Power Control or Limiting
- Isochronous Load Sharing between units (with DSLC™control)
- Critical Speed Avoidance (3 speed bands)
- Auto Start Sequence (hot & cold starts)
- Valve Limiter(s)
- Security (Program is Password Protected)
- Dual Speed/Load Dynamics
- First-Out Indication (Shutdowns)
- Zero Speed Detection with proximity probe (< 0.5 Hz)
- Peak Speed Indication for overspeed trip
- Two Programmable Functions Keys on the 505E’s frontpanel
- Two independent Modbus comm links
- Remote analog setpoints for Speed/Load, Ext/Adm, Aux, and Cascade
- Program upload/download capability
- Feed-forward decoupling algorithm for compressors
505 Woodward governors Panel key Functions :
SCROLL: The large diamond shaped button in the middle of the keypad with arrows at each of its four corners. The < > (scroll left, right) moves the display left or right through the function blocks of the Program or Run Mode. (Scroll up, down) moves the display up or down within a function block of the Program or Run Mode.
SELECT: The Select key is used to select control of the 505 display’s top or bottom line variable. The @ sign is used to indicate which line (variable) can be adjusted by the adjust keys. Only when there is a changeable variable on both lines (dynamics, valve calibration modes) does the “select key” and @ sign determine which line variable can be adjusted. When there is only one variable per screen the “select key” and @ sign’s position are irrelevant. ADJ (adjust):
In the Run Mode, “ “ moves any adjustable parameter up (larger) and “ “ moves any adjustable parameter down (smaller).
PRGM (program): When the control is shutdown this key selects the Program Mode. While in the Run Mode this key selects a Program Monitor Mode. In the Program Monitor Mode the program can be viewed but not changed.
RUN: Initiates a turbine run or start command from the (CONTROLLING PARAMETER/PUSH RUN or PRGM) state.
STOP: Initiates a controlled turbine shutdown (Run Mode) once verification is given. The “Stop” command can be disabled through a Service Mode setting (under ‘Key Options’).
RESET: Resets/clears Run Mode alarms and shutdowns. Pressing the key also returns the control to the (CONTROLLING PARAMETER/PUSH RUN OR PRGM) status after a shutdown.
0/NO: Enters 0/NO or disable.
1/YES: Enters 1/YES or enable.
2/ACTR (actuator): Enters 2 or displays the actuator position (Run Mode).
3/CONT (control): Enters 3 or displays the parameter which is in control (Run Mode); press the Scroll down arrow to display the control’s last trip cause, highest speed reached, and local/remote status (if used).
4/CAS (cascade): Enters 4 or displays the cascade control information (Run Mode).
5/RMT (remote): Enters 5 or displays the remote speed set point control information (Run Mode).
6/LMTR (valve limiter): Enters 6 or displays the valve limiter information (Run Mode)
7/SPEED: Enters 7 or displays the speed control information (Run Mode).
8/AUX (auxiliary): Enters 8 or displays the auxiliary control information (Run Mode).
9/KW (load): Enters 9 or displays the kW/load or first stage pressure information (Run Mode).
CLEAR: Clears Program Mode and Run Mode entries and takes control out of it’s present mode.
ENTER: Enters new values in the Program Mode, and allows the “direct entry” of specific set point values in the Run Mode.
DYNAMICS (+/–): Accesses the dynamic settings of the parameter controlling the actuator position in the Run Mode. The dynamics adjustments can be disabled through a Service Mode setting (under ‘Key Options’). This key will also change the sign of a value being entered.
(DECIMAL KEY) : Enters decimal point in number being entered from front panel.
ALARM (F1): Displays the reason for any alarm condition (last/newest alarm) when the key’s LED indicator is illuminated. Press the Scroll down arrow (diamond key) to display additional alarms.
OVERSPEED TEST ENABLE (F2): Permits the speed reference to be raised beyond the maximum controlling speed set point to test either the electrical or mechanical over speed trip.
F3 (function key): Programmable function key for enabling or disabling programmable control functions.
F4 (function key): Programmable function key for enabling or disabling programmable control functions.
EMERGENCY SHUTDOWN BUTTON: Large red octagonal button on the front of the enclosure. This is an Emergency Shutdown command for the control.
Watchdog Timer/CPU Fault Control A watchdog timer and CPU fault circuit monitors the operation of the microprocessor and microprocessor memory. If the microprocessor fails to reset the timer within 15 milliseconds of the last reset, the CPU fault-control will activate the reset output. This resets the CPU, de-energizes all relay outputs and turns off all milliamp outputs.
Image Courtesy : woodward