What are the four major components of a control loop?
sensor, transmitter, controller & actuator
What does a sensor do?
Measures the actual value of the controlled variable
What does a transmitter do?
Sends the signal to the controller
What does the controller do?
Compares the signal with the set point and emits a corrective signal to the actuator
What does the actuator do?
Applies power to adjust the variable so as to reduce the deviation
What are the three types of control signal?
- Pneumatic air pressure
- Electronic millivolts/milliamps (analog)
- Digital
What mediums may be used to move an actuator?
Compressed air, electric motor, electro-magnetic & hydraulic
What are three methods of final steam temperature control?
- Gas recirculation
- Desuperheater attemperation
- Tilting burners
What are the three main problems of poorly conditioned feedwater?
- Formation of scale inside the tubes
- Carryover of water droplets
- Internal corrosion
What are the three main contaminants of feedwater?
- Magnesium & calcium salts
- Silica
- Products of corrosion (iron & copper)
What losses are there from continuous blowdown?
Heat & water
What are three possible causes of carryover?
- High solids content
- High drum level
- High pH
What problems may be caused by silica?
At certain concentrations silica vaporizes and passes along with the steam through the turbine, until ncondensing and forming a hard scale. This results in a reduction of the turbine output and loss of efficiency
Does phosphate injection increase or decrease solids?
Increase
What are the three main causes of corrosion?
- Acid attack
- Caustic attack
- Presence of dissolved gases
What is the range of pH allowed for boiler water?
8.5-9.5
What is the negative effect of DO & CO2?
Causes pitting on internal metal surfaces
What is the allowable concentration of DO in the feedwater?
<10ppb
Three forms of pollution from the boiler?
- Liquid waste
- Flue gas
- Solid waste
What are three of the most troublesome emissions from the boiler?
- Particulate matter
- SOx
- NOx
What are the sources of pollution in boiler related waste effluent?
Boiler blowdown, airheater washing, chemical cleaning, FGD slurry, storm water run off from coal piles etc
What is the most common reagent used in FGD systems?
Lime/limestone
What chemical reaction takes place in the FGD system?
Calcium reacts with SOx to form calcium sulfite
How is calcium sulfite removed in the dry process?
In a precipitator or fabric filter
