Boiler Control

Question 1

What are the four major components of a control loop?

Answer 1

sensor, transmitter, controller & actuator

Question 2

What does a sensor do?

Answer 2

Measures the actual value of the controlled variable

Question 3

What does a transmitter do?

Answer 3

Sends the signal to the controller

Question 4

What does the controller do?

Answer 4

Compares the signal with the set point and emits a corrective signal to the actuator

Question 5

What does the actuator do?

Answer 5

Applies power to adjust the variable so as to reduce the deviation

Question 6

What are the three types of control signal?

Answer 6

1. Pneumatic air pressure
2. Electronic millivolts/milliamps (analog)
3. Digital

Question 7

What mediums may be used to move an actuator?

Answer 7

Compressed air, electric motor, electro-magnetic & hydraulic

Question 8

What are three methods of final steam temperature control?

Answer 8

1. Gas recirculation
2. Desuperheater attemperation
3. Tilting burners

Question 9

What are the three main problems of poorly conditioned feedwater?

Answer 9

1. Formation of scale inside the tubes
2. Carryover of water droplets
3. Internal corrosion

Question 10

What are the three main contaminants of feedwater?

Answer 10

1. Magnesium & calcium salts
2. Silica
3. Products of corrosion (iron & copper)

Question 11

What losses are there from continuous blowdown?

Answer 11

Heat & water

Question 12

What are three possible causes of carryover?

Answer 12

1. High solids content
2. High drum level
3. High pH

Question 13

What problems may be caused by silica?

Answer 13

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

Question 14

Does phosphate injection increase or decrease solids?

Answer 14

Increase

Question 15

What are the three main causes of corrosion?

Answer 15

1. Acid attack
2. Caustic attack
3. Presence of dissolved gases

Question 16

What is the range of pH allowed for boiler water?

Answer 16

8.5-9.5

Question 17

What is the negative effect of DO & CO2?

Answer 17

Causes pitting on internal metal surfaces

Question 18

What is the allowable concentration of DO in the feedwater?

Answer 18

<10ppb

Question 19

Three forms of pollution from the boiler?

Answer 19

1. Liquid waste
2. Flue gas
3. Solid waste

Question 20

What are three of the most troublesome emissions from the boiler?

Answer 20

1. Particulate matter
2. SOx
3. NOx

Question 21

What are the sources of pollution in boiler related waste effluent?

Answer 21

Boiler blowdown, airheater washing, chemical cleaning, FGD slurry, storm water run off from coal piles etc

Question 22

What is the most common reagent used in FGD systems?

Answer 22

Lime/limestone

Question 23

What chemical reaction takes place in the FGD system?

Answer 23

Calcium reacts with SOx to form calcium sulfite

Question 24

How is calcium sulfite removed in the dry process?

Answer 24

In a precipitator or fabric filter

Question 25

How is calcium sulfite removed in the wet process?

Answer 25

Drained from the absorption tower as a slurry for treatment and disposal

Question 26

What types of particulate removal equipment can be found in a coal fired boiler?

Answer 26

Electrostatic precipitator & fabric filter