DC THIRD CLASS STATIONARY
BOILER ENGINEER EXAM 200
SOLVED MCQS & RATIONALES
DC Third-Class Stationary Boiler Practice Exam
1. What is the minimum frequency for testing a low-water fuel cut-off
on an operating low-pressure steam boiler?
A. Once a month
B. Once a week
C. Once a day
D. Once per shift
Rationale: Low-water fuel cut-offs must be tested at least once a
day to ensure the float or probe functions correctly to prevent a
dry-firing disaster.
2. Which valve is required to be located closest to the boiler shell on a
steam boiler blowdown line?
A. A quick-opening valve
B. A slow-opening valve
C. A globe valve
D. A non-return valve
Rationale: ASME code dictates that when a quick-opening and
slow-opening valve combination is used, the quick-opening valve
is placed closest to the boiler shell.
3. What is the boiling point of water at sea level under standard
atmospheric pressure?
A. 100°F
B. 212°F
C. 14.7°F
D. 32°F
Rationale: At standard atmospheric pressure (14.7 psi), water
changes state from liquid to vapor at exactly 212°F (100°C).
4. A boiler safety valve is designed to pop open using what type of
mechanism?
, A. Motorized actuator
B. Diaphragm pneumatic pilot
C. Huddled chamber spring loading
D. Hydraulic piston
Rationale: Safety valves utilize a huddling chamber to trap
escaping steam, creating a sudden increase in upward force that
pops the valve fully open.
5. What type of boiler passes the hot gases of combustion through
tubes surrounded by water?
A. Water-tube boiler
B. Cast iron sectional boiler
C. Fire-tube boiler
D. Superheater boiler
Rationale: In a fire-tube boiler, the products of combustion travel
through the inside of the tubes, which are submerged in the boiler
water.
6. What is the primary purpose of a boiler deaerator?
A. To strain out solid particulate matter
B. To remove dissolved oxygen and non-condensable
gases
C. To lower the temperature of the feedwater
D. To add chemical treatment into the steam lines
Rationale: Oxygen causes severe pitting and corrosion inside a
boiler. Deaerators heat feedwater to liberate dissolved oxygen
and carbon dioxide before it enters the drum.
7. If a boiler safety valve fails to open at its set pressure, what should
the operator do immediately?
A. Increase the spring tension manually
B. Shut down the burner and isolate the fuel source
C. Tap the valve body hard with a hammer
D. Wait for the pressure to drop naturally
Rationale: An overpressure condition with a failed safety valve is
an extreme emergency. The heat source must be eliminated
immediately by shutting down the fuel supply.
8. What device is used to change the linear motion of a high-pressure
steam piston into rotary motion?
A. Crosshead guide
B. Eccentric rod
, C. Crankshaft and connecting rod
D. Governor valve
Rationale: The connecting rod links the crosshead/piston rod to
the crankshaft, transforming reciprocating linear motion into
rotational mechanical energy.
9. Which scale-forming mineral is most commonly found in
untreated boiler feedwater?
A. Sodium chloride
B. Calcium carbonate
C. Potassium permanganate
D. Iron oxide
Rationale: Calcium and magnesium carbonates readily
precipitate out of hard water when heated, forming a hard
insulating scale on boiler heating surfaces.
10. What is the function of a dry pipe in a steam boiler?
A. To drain condensate back to the mud drum
B. To vent air during a cold startup
C. To separate moisture droplets from the steam before it
exits the boiler
D. To supply dry air to the burner assembly
Rationale: Located in the top of the steam drum, the dry pipe
forces steam to change direction rapidly, separating liquid water
droplets so only dry steam exits.
11. What force causes natural circulation of water within a water-tube
boiler?
A. The difference in density between hot and cold water
B. The mechanical action of the feedwater pump
C. Steam velocity through the downcomers
D. Induced draft fan pressure
Rationale: Natural circulation relies on the density differential;
hotter, less dense water and steam rise in the risers, while cooler,
denser water sinks in the downcomers.
12.How often should a boiler operator blow down a boiler water
column glass to ensure accuracy?
A. Weekly
B. At least once per shift
C. Once a month
D. Only during annual inspections
, Rationale: Gauge glasses must be blown down at least once per
shift to clear out any accumulated sediment or sludge that could
cause a false high-water reading.
13.What pressure constitutes the boundary between a low-pressure
and a high-pressure steam boiler according to the ASME code?
A. 5 psi
B. 15 psi
C. 100 psi
D. 150 psi
Rationale: Boilers operating at a steam pressure over 15 psi are
classified as high-pressure (power) boilers; those at 15 psi and
below are low-pressure.
14.What occurs when a boiler operator opens the bottom blowdown
valve too quickly?
A. The steam pressure increases rapidly
B. Water hammer can occur and damage the piping
C. The water level stabilizes instantly
D. The safety valves will pop open
Rationale: Opening blowdown valves rapidly creates sudden
volumetric shifts and high-velocity fluid momentum, risking
catastrophic water hammer and pipe rupture.
15. What type of draft is produced by a fan pulling gases out of the
boiler furnace and pushing them up the stack?
A. Forced draft
B. Induced draft
C. Balanced draft
D. Natural draft
Rationale: Induced draft fans are located between the boiler
outlet and the stack, creating a negative pressure (vacuum) inside
the furnace to draw out flue gases.
16.What is the specific term for the heat required to change a liquid
into a vapor without changing its temperature?
A. Sensible heat
B. Superheat
C. Specific heat
D. Latent heat of vaporization
Rationale: Latent heat of vaporization is the energy absorbed
BOILER ENGINEER EXAM 200
SOLVED MCQS & RATIONALES
DC Third-Class Stationary Boiler Practice Exam
1. What is the minimum frequency for testing a low-water fuel cut-off
on an operating low-pressure steam boiler?
A. Once a month
B. Once a week
C. Once a day
D. Once per shift
Rationale: Low-water fuel cut-offs must be tested at least once a
day to ensure the float or probe functions correctly to prevent a
dry-firing disaster.
2. Which valve is required to be located closest to the boiler shell on a
steam boiler blowdown line?
A. A quick-opening valve
B. A slow-opening valve
C. A globe valve
D. A non-return valve
Rationale: ASME code dictates that when a quick-opening and
slow-opening valve combination is used, the quick-opening valve
is placed closest to the boiler shell.
3. What is the boiling point of water at sea level under standard
atmospheric pressure?
A. 100°F
B. 212°F
C. 14.7°F
D. 32°F
Rationale: At standard atmospheric pressure (14.7 psi), water
changes state from liquid to vapor at exactly 212°F (100°C).
4. A boiler safety valve is designed to pop open using what type of
mechanism?
, A. Motorized actuator
B. Diaphragm pneumatic pilot
C. Huddled chamber spring loading
D. Hydraulic piston
Rationale: Safety valves utilize a huddling chamber to trap
escaping steam, creating a sudden increase in upward force that
pops the valve fully open.
5. What type of boiler passes the hot gases of combustion through
tubes surrounded by water?
A. Water-tube boiler
B. Cast iron sectional boiler
C. Fire-tube boiler
D. Superheater boiler
Rationale: In a fire-tube boiler, the products of combustion travel
through the inside of the tubes, which are submerged in the boiler
water.
6. What is the primary purpose of a boiler deaerator?
A. To strain out solid particulate matter
B. To remove dissolved oxygen and non-condensable
gases
C. To lower the temperature of the feedwater
D. To add chemical treatment into the steam lines
Rationale: Oxygen causes severe pitting and corrosion inside a
boiler. Deaerators heat feedwater to liberate dissolved oxygen
and carbon dioxide before it enters the drum.
7. If a boiler safety valve fails to open at its set pressure, what should
the operator do immediately?
A. Increase the spring tension manually
B. Shut down the burner and isolate the fuel source
C. Tap the valve body hard with a hammer
D. Wait for the pressure to drop naturally
Rationale: An overpressure condition with a failed safety valve is
an extreme emergency. The heat source must be eliminated
immediately by shutting down the fuel supply.
8. What device is used to change the linear motion of a high-pressure
steam piston into rotary motion?
A. Crosshead guide
B. Eccentric rod
, C. Crankshaft and connecting rod
D. Governor valve
Rationale: The connecting rod links the crosshead/piston rod to
the crankshaft, transforming reciprocating linear motion into
rotational mechanical energy.
9. Which scale-forming mineral is most commonly found in
untreated boiler feedwater?
A. Sodium chloride
B. Calcium carbonate
C. Potassium permanganate
D. Iron oxide
Rationale: Calcium and magnesium carbonates readily
precipitate out of hard water when heated, forming a hard
insulating scale on boiler heating surfaces.
10. What is the function of a dry pipe in a steam boiler?
A. To drain condensate back to the mud drum
B. To vent air during a cold startup
C. To separate moisture droplets from the steam before it
exits the boiler
D. To supply dry air to the burner assembly
Rationale: Located in the top of the steam drum, the dry pipe
forces steam to change direction rapidly, separating liquid water
droplets so only dry steam exits.
11. What force causes natural circulation of water within a water-tube
boiler?
A. The difference in density between hot and cold water
B. The mechanical action of the feedwater pump
C. Steam velocity through the downcomers
D. Induced draft fan pressure
Rationale: Natural circulation relies on the density differential;
hotter, less dense water and steam rise in the risers, while cooler,
denser water sinks in the downcomers.
12.How often should a boiler operator blow down a boiler water
column glass to ensure accuracy?
A. Weekly
B. At least once per shift
C. Once a month
D. Only during annual inspections
, Rationale: Gauge glasses must be blown down at least once per
shift to clear out any accumulated sediment or sludge that could
cause a false high-water reading.
13.What pressure constitutes the boundary between a low-pressure
and a high-pressure steam boiler according to the ASME code?
A. 5 psi
B. 15 psi
C. 100 psi
D. 150 psi
Rationale: Boilers operating at a steam pressure over 15 psi are
classified as high-pressure (power) boilers; those at 15 psi and
below are low-pressure.
14.What occurs when a boiler operator opens the bottom blowdown
valve too quickly?
A. The steam pressure increases rapidly
B. Water hammer can occur and damage the piping
C. The water level stabilizes instantly
D. The safety valves will pop open
Rationale: Opening blowdown valves rapidly creates sudden
volumetric shifts and high-velocity fluid momentum, risking
catastrophic water hammer and pipe rupture.
15. What type of draft is produced by a fan pulling gases out of the
boiler furnace and pushing them up the stack?
A. Forced draft
B. Induced draft
C. Balanced draft
D. Natural draft
Rationale: Induced draft fans are located between the boiler
outlet and the stack, creating a negative pressure (vacuum) inside
the furnace to draw out flue gases.
16.What is the specific term for the heat required to change a liquid
into a vapor without changing its temperature?
A. Sensible heat
B. Superheat
C. Specific heat
D. Latent heat of vaporization
Rationale: Latent heat of vaporization is the energy absorbed
