DC Third Class Stationary Boilers (323)
Exam Questions With Correct Solutions,
Already Passed!!
1. What is the minimum required diameter for a fusible plug on a low-pressure heating
boiler?
A) 1/2 inch
B) 5/8 inch
C) 3/4 inch
D) 1 inch
Answer: B
*Explanation: ASME Section IV requires a minimum fusible plug diameter of 5/8 inch for
low-pressure heating boilers to ensure sufficient metal mass for reliable melting and
blow-through during low-water conditions.*
2. Which type of draft is produced by a tall stack without mechanical assistance?
A) Forced draft
B) Induced draft
C) Natural draft
D) Balanced draft
Answer: C
Explanation: Natural draft relies on the buoyancy of hot flue gases inside a tall stack,
creating a pressure difference that draws air through the boiler without fans.
3. What is the primary purpose of a try lever on a safety valve?
A) To adjust the set pressure
B) To test the valve manually
C) To lock the valve closed
,D) To increase boiler pressure
Answer: B
Explanation: The try lever allows manual lifting of the valve disc to verify free movement
and proper seating, preventing stuck valves that could cause overpressure explosions.
4. The water column on a steam boiler must be connected directly to the boiler shell at
what points?
A) One connection above the water line and one below
B) Both connections above the water line
C) Both connections below the water line
D) One connection at the steam nozzle and one at the blowdown
Answer: A
Explanation: One connection goes to the steam space (above NWL) and the other to the
water space (below NWL) to allow accurate water level sensing via equal pressure.
5. What is the recommended maximum concentration of dissolved solids in a high-
pressure boiler before blowdown?
A) 500 ppm
B) 1500 ppm
C) 3500 ppm
D) 5000 ppm
Answer: C
*Explanation: Most high-pressure industrial boilers require blowdown when TDS reaches
3500 ppm to prevent carryover, foaming, and scale formation.*
6. Which device automatically shuts off the fuel supply if the flame goes out?
A) Low-water cutoff
B) Flame scanner / flame failure device
C) High-limit pressure control
D) Stack temperature switch
Answer: B
,Explanation: A flame scanner (UV or IR) senses the flame; if no flame is detected during
the ignition trial or after established flame, it de-energizes the fuel safety shutoff valves.
7. In a fire-tube boiler, the hot gases pass:
A) Inside the tubes, water outside
B) Outside the tubes, water inside
C) Through a separate heat exchanger
D) Only around the furnace
Answer: A
Explanation: Fire-tube boilers have combustion gases flowing through tubes that are
surrounded by water, transferring heat through the tube walls.
8. A hydrostatic test of a boiler should be performed at what pressure relative to the
MAWP?
A) 1.5 × MAWP
B) 2.0 × MAWP
C) MAWP + 50 psi
D) MAWP + 100 psi
Answer: A
Explanation: ASME Code requires a hydrostatic test at 1.5 times the maximum allowable
working pressure to verify integrity without plastic deformation.
9. What is the function of a deaerator in a boiler feedwater system?
A) Increase feedwater temperature
B) Remove dissolved oxygen and carbon dioxide
C) Filter suspended solids
D) Add chemicals for pH control
Answer: B
*Explanation: Deaerators use steam stripping and heating to reduce dissolved gases (O₂
and CO₂) to very low levels (typically <0.005 cc/L) to prevent corrosion.*
, 10. Which type of boiler has the highest steam generation rate per unit floor area?
A) Horizontal fire-tube
B) Vertical fire-tube
C) Water-tube
D) Cast iron sectional
Answer: C
Explanation: Water-tube boilers have water inside small-diameter tubes exposed to
radiant heat, allowing very high heat transfer rates and rapid steam generation.
11. The lowest permissible water level in a steam boiler during operation is:
A) Bottom of the gauge glass
B) Midpoint of the gauge glass
C) Top of the visible water column
D) As determined by the lowest safe water level (LSWL)
Answer: D
Explanation: The LSWL is marked on the boiler or water column; it is the minimum safe
level to prevent overheating of furnace tubes or crown sheet.
12. What color indicates a properly adjusted neutralized boiler water (pH between 8.5
and 10.5) with phenolphthalein indicator?
A) Colorless
B) Pink
C) Blue
D) Yellow
Answer: B
Explanation: Phenolphthalein turns pink in alkaline solutions (pH >8.2). Boiler water
should be slightly alkaline to prevent acidic corrosion.
13. A pressure gauge on a boiler must have a dial range of at least:
A) 1.5 × MAWP
B) 2 × MAWP
C) 2.5 × MAWP
Exam Questions With Correct Solutions,
Already Passed!!
1. What is the minimum required diameter for a fusible plug on a low-pressure heating
boiler?
A) 1/2 inch
B) 5/8 inch
C) 3/4 inch
D) 1 inch
Answer: B
*Explanation: ASME Section IV requires a minimum fusible plug diameter of 5/8 inch for
low-pressure heating boilers to ensure sufficient metal mass for reliable melting and
blow-through during low-water conditions.*
2. Which type of draft is produced by a tall stack without mechanical assistance?
A) Forced draft
B) Induced draft
C) Natural draft
D) Balanced draft
Answer: C
Explanation: Natural draft relies on the buoyancy of hot flue gases inside a tall stack,
creating a pressure difference that draws air through the boiler without fans.
3. What is the primary purpose of a try lever on a safety valve?
A) To adjust the set pressure
B) To test the valve manually
C) To lock the valve closed
,D) To increase boiler pressure
Answer: B
Explanation: The try lever allows manual lifting of the valve disc to verify free movement
and proper seating, preventing stuck valves that could cause overpressure explosions.
4. The water column on a steam boiler must be connected directly to the boiler shell at
what points?
A) One connection above the water line and one below
B) Both connections above the water line
C) Both connections below the water line
D) One connection at the steam nozzle and one at the blowdown
Answer: A
Explanation: One connection goes to the steam space (above NWL) and the other to the
water space (below NWL) to allow accurate water level sensing via equal pressure.
5. What is the recommended maximum concentration of dissolved solids in a high-
pressure boiler before blowdown?
A) 500 ppm
B) 1500 ppm
C) 3500 ppm
D) 5000 ppm
Answer: C
*Explanation: Most high-pressure industrial boilers require blowdown when TDS reaches
3500 ppm to prevent carryover, foaming, and scale formation.*
6. Which device automatically shuts off the fuel supply if the flame goes out?
A) Low-water cutoff
B) Flame scanner / flame failure device
C) High-limit pressure control
D) Stack temperature switch
Answer: B
,Explanation: A flame scanner (UV or IR) senses the flame; if no flame is detected during
the ignition trial or after established flame, it de-energizes the fuel safety shutoff valves.
7. In a fire-tube boiler, the hot gases pass:
A) Inside the tubes, water outside
B) Outside the tubes, water inside
C) Through a separate heat exchanger
D) Only around the furnace
Answer: A
Explanation: Fire-tube boilers have combustion gases flowing through tubes that are
surrounded by water, transferring heat through the tube walls.
8. A hydrostatic test of a boiler should be performed at what pressure relative to the
MAWP?
A) 1.5 × MAWP
B) 2.0 × MAWP
C) MAWP + 50 psi
D) MAWP + 100 psi
Answer: A
Explanation: ASME Code requires a hydrostatic test at 1.5 times the maximum allowable
working pressure to verify integrity without plastic deformation.
9. What is the function of a deaerator in a boiler feedwater system?
A) Increase feedwater temperature
B) Remove dissolved oxygen and carbon dioxide
C) Filter suspended solids
D) Add chemicals for pH control
Answer: B
*Explanation: Deaerators use steam stripping and heating to reduce dissolved gases (O₂
and CO₂) to very low levels (typically <0.005 cc/L) to prevent corrosion.*
, 10. Which type of boiler has the highest steam generation rate per unit floor area?
A) Horizontal fire-tube
B) Vertical fire-tube
C) Water-tube
D) Cast iron sectional
Answer: C
Explanation: Water-tube boilers have water inside small-diameter tubes exposed to
radiant heat, allowing very high heat transfer rates and rapid steam generation.
11. The lowest permissible water level in a steam boiler during operation is:
A) Bottom of the gauge glass
B) Midpoint of the gauge glass
C) Top of the visible water column
D) As determined by the lowest safe water level (LSWL)
Answer: D
Explanation: The LSWL is marked on the boiler or water column; it is the minimum safe
level to prevent overheating of furnace tubes or crown sheet.
12. What color indicates a properly adjusted neutralized boiler water (pH between 8.5
and 10.5) with phenolphthalein indicator?
A) Colorless
B) Pink
C) Blue
D) Yellow
Answer: B
Explanation: Phenolphthalein turns pink in alkaline solutions (pH >8.2). Boiler water
should be slightly alkaline to prevent acidic corrosion.
13. A pressure gauge on a boiler must have a dial range of at least:
A) 1.5 × MAWP
B) 2 × MAWP
C) 2.5 × MAWP
