DC THIRD CLASS STATIONARY BOILER EXAM 300
ACTUAL QUESTIONS AND CORRECT ANSWERS
WITH RATIONALE LATEST UPDATE ALREADY
GRADED A+
This comprehensive 300-question study guide covers all major content areas tested
on the DC Third Class Stationary Boiler licensing examination, including boiler
fundamentals and construction, safety devices and controls, water treatment and
chemistry, combustion and efficiency, boiler operations and procedures, DC
regulations and licensing, pumps, turbines, and mechanical equipment, and
refrigeration and HVAC systems . Each question includes a correct answer and
detailed rationale . Key topics include ASME Section I and IV requirements ,
safety valve and LWCO testing , water treatment chemistry (pH, TDS, oxygen
scavengers) , combustion efficiency calculations , boiler horsepower formulas ,
DCRA licensing requirements and exam structure , and essential mathematical
formulas for the exam . This guide serves as a complete review tool for stationary
engineer candidates preparing for the DC Third Class license examination.
SECTION 1: BOILER FUNDAMENTALS AND CONSTRUCTION (Questions
1-40)
Question 1
What is the minimum required diameter for a fusible plug on a low-pressure
heating boiler according to ASME Section IV?
A) 1/2 inch
B) 5/8 inch
C) 3/4 inch
D) 1 inch
Answer: B
,Rationale: 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 .
Question 2
What 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
Rationale: 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 .
Question 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
Rationale: 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 .
Question 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
,Rationale: 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 .
Question 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
Rationale: Most high-pressure industrial boilers require blowdown when TDS
reaches 3500 ppm to prevent carryover, foaming, and scale formation .
Question 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
Rationale: 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 .
Question 7
What is the formula to find the volume of a cylinder?
A) V = π × r² × h
B) V = D² × 0.7854 × Length
C) V = Base × Height × Length
D) V = π × D × Length
Answer: B
, Rationale: The formula for cylinder volume is V = Area of a circle (D² × 0.7854) ×
Height or Length. This is a fundamental calculation for determining boiler drum
capacity and water volume .
Question 8
How many cubic inches are in one gallon?
A) 128
B) 231
C) 1728
D) 144
Answer: B
Rationale: There are 231 cubic inches in one gallon of water. This constant is
essential for calculating pump capacities and water volumes in boiler systems .
Question 9
What is the total force acting on a 2-inch pressure relief valve with a popping
pressure of 125 psi?
A) 157.1 lbs
B) 392.7 lbs
C) 785.4 lbs
D) 196.3 lbs
Answer: B
Rationale: Force = Pressure × Area. Area = π × r² = 3.14 × 1² = 3.14 in². Force =
125 psi × 3.14 in² = 392.7 lbs .
Question 10
What is the formula for Maximum Allowable Working Pressure (MAWP) on a
boiler?
A) MAWP = (TS × t × E) / (FS × r)
B) MAWP = (P × D) / 2t
C) MAWP = (TS × t) / r
D) MAWP = (P × D × L) / 2t
Answer: A
ACTUAL QUESTIONS AND CORRECT ANSWERS
WITH RATIONALE LATEST UPDATE ALREADY
GRADED A+
This comprehensive 300-question study guide covers all major content areas tested
on the DC Third Class Stationary Boiler licensing examination, including boiler
fundamentals and construction, safety devices and controls, water treatment and
chemistry, combustion and efficiency, boiler operations and procedures, DC
regulations and licensing, pumps, turbines, and mechanical equipment, and
refrigeration and HVAC systems . Each question includes a correct answer and
detailed rationale . Key topics include ASME Section I and IV requirements ,
safety valve and LWCO testing , water treatment chemistry (pH, TDS, oxygen
scavengers) , combustion efficiency calculations , boiler horsepower formulas ,
DCRA licensing requirements and exam structure , and essential mathematical
formulas for the exam . This guide serves as a complete review tool for stationary
engineer candidates preparing for the DC Third Class license examination.
SECTION 1: BOILER FUNDAMENTALS AND CONSTRUCTION (Questions
1-40)
Question 1
What is the minimum required diameter for a fusible plug on a low-pressure
heating boiler according to ASME Section IV?
A) 1/2 inch
B) 5/8 inch
C) 3/4 inch
D) 1 inch
Answer: B
,Rationale: 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 .
Question 2
What 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
Rationale: 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 .
Question 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
Rationale: 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 .
Question 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
,Rationale: 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 .
Question 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
Rationale: Most high-pressure industrial boilers require blowdown when TDS
reaches 3500 ppm to prevent carryover, foaming, and scale formation .
Question 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
Rationale: 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 .
Question 7
What is the formula to find the volume of a cylinder?
A) V = π × r² × h
B) V = D² × 0.7854 × Length
C) V = Base × Height × Length
D) V = π × D × Length
Answer: B
, Rationale: The formula for cylinder volume is V = Area of a circle (D² × 0.7854) ×
Height or Length. This is a fundamental calculation for determining boiler drum
capacity and water volume .
Question 8
How many cubic inches are in one gallon?
A) 128
B) 231
C) 1728
D) 144
Answer: B
Rationale: There are 231 cubic inches in one gallon of water. This constant is
essential for calculating pump capacities and water volumes in boiler systems .
Question 9
What is the total force acting on a 2-inch pressure relief valve with a popping
pressure of 125 psi?
A) 157.1 lbs
B) 392.7 lbs
C) 785.4 lbs
D) 196.3 lbs
Answer: B
Rationale: Force = Pressure × Area. Area = π × r² = 3.14 × 1² = 3.14 in². Force =
125 psi × 3.14 in² = 392.7 lbs .
Question 10
What is the formula for Maximum Allowable Working Pressure (MAWP) on a
boiler?
A) MAWP = (TS × t × E) / (FS × r)
B) MAWP = (P × D) / 2t
C) MAWP = (TS × t) / r
D) MAWP = (P × D × L) / 2t
Answer: A
