National Board Inservice Inspector (NB-
IS) Certification Exam Questions And
Correct Answers With Rationales 2026
1. A boiler is considered a high-pressure boiler when it operates at
pressures exceeding:
A. 50 psi
B. 100 psi
C. 15 psi for steam or 160 psi for hot water
D. 200 psi
Rationale: High-pressure boilers are defined as those exceeding 15 psi for
steam or 160 psi for hot water, according to the National Board Inspection
Code (NBIC).
, 2. Which of the following is a common sign of boiler tube corrosion?
A. Blue flame
B. Pitting and thinning of the tube wall
C. Low water level
D. Excessive steam production
Rationale: Pitting and wall thinning are indicators of corrosion, which can
compromise tube integrity.
3. The purpose of a fusible plug in a boiler is to:
A. Regulate water level
B. Control steam pressure
C. Provide a safety feature that melts under high temperature
D. Enhance combustion efficiency
Rationale: Fusible plugs melt at high temperatures to release pressure and
prevent overheating of the boiler.
4. When inspecting a pressure vessel, the inspector notices a dent with a
depth of 5% of the vessel thickness. The correct action is:
A. Reject the vessel immediately
B. Record and monitor if within allowable limits
C. Weld over the dent
D. Paint over the dent
Rationale: Small dents below the allowable depth per code may be
acceptable but require monitoring.
, 5. What is the primary reason for conducting hydrostatic testing on a
pressure vessel?
A. Check valve function
B. Verify structural integrity under pressure
C. Calibrate pressure gauges
D. Clean internal surfaces
Rationale: Hydrostatic testing ensures the vessel can safely hold the design
pressure without leaking or failing.
6. Which NDE method is most suitable for detecting surface cracks in
boiler tubes?
A. Radiographic Testing
B. Dye Penetrant Testing
C. Ultrasonic Testing
D. Magnetic Particle Testing
Rationale: Dye penetrant testing is highly effective for detecting surface-
breaking cracks in non-porous materials.
7. The allowable working pressure of a boiler is determined by:
A. The thickness of insulation
B. Operator preference
C. Design code and material strength
D. Fuel type
, Rationale: The allowable working pressure is calculated using design codes
considering material strength, temperature, and safety factors.
8. Which safety device protects a boiler from overpressure?
A. Safety valve
B. Blowdown valve
C. Check valve
D. Feedwater regulator
Rationale: Safety valves automatically release pressure to prevent boiler
rupture.
9. In welding inspection, a “slag inclusion” refers to:
A. Excess weld metal
B. Non-metallic material trapped in the weld
C. Undercut along the weld edge
D. Porosity in the weld
Rationale: Slag inclusions occur when flux or non-metallic material gets
trapped in the weld, weakening it.
10. The primary function of a boiler water level indicator is to:
A. Measure pressure
B. Control fuel input
C. Allow the operator to monitor water level
D. Vent steam
IS) Certification Exam Questions And
Correct Answers With Rationales 2026
1. A boiler is considered a high-pressure boiler when it operates at
pressures exceeding:
A. 50 psi
B. 100 psi
C. 15 psi for steam or 160 psi for hot water
D. 200 psi
Rationale: High-pressure boilers are defined as those exceeding 15 psi for
steam or 160 psi for hot water, according to the National Board Inspection
Code (NBIC).
, 2. Which of the following is a common sign of boiler tube corrosion?
A. Blue flame
B. Pitting and thinning of the tube wall
C. Low water level
D. Excessive steam production
Rationale: Pitting and wall thinning are indicators of corrosion, which can
compromise tube integrity.
3. The purpose of a fusible plug in a boiler is to:
A. Regulate water level
B. Control steam pressure
C. Provide a safety feature that melts under high temperature
D. Enhance combustion efficiency
Rationale: Fusible plugs melt at high temperatures to release pressure and
prevent overheating of the boiler.
4. When inspecting a pressure vessel, the inspector notices a dent with a
depth of 5% of the vessel thickness. The correct action is:
A. Reject the vessel immediately
B. Record and monitor if within allowable limits
C. Weld over the dent
D. Paint over the dent
Rationale: Small dents below the allowable depth per code may be
acceptable but require monitoring.
, 5. What is the primary reason for conducting hydrostatic testing on a
pressure vessel?
A. Check valve function
B. Verify structural integrity under pressure
C. Calibrate pressure gauges
D. Clean internal surfaces
Rationale: Hydrostatic testing ensures the vessel can safely hold the design
pressure without leaking or failing.
6. Which NDE method is most suitable for detecting surface cracks in
boiler tubes?
A. Radiographic Testing
B. Dye Penetrant Testing
C. Ultrasonic Testing
D. Magnetic Particle Testing
Rationale: Dye penetrant testing is highly effective for detecting surface-
breaking cracks in non-porous materials.
7. The allowable working pressure of a boiler is determined by:
A. The thickness of insulation
B. Operator preference
C. Design code and material strength
D. Fuel type
, Rationale: The allowable working pressure is calculated using design codes
considering material strength, temperature, and safety factors.
8. Which safety device protects a boiler from overpressure?
A. Safety valve
B. Blowdown valve
C. Check valve
D. Feedwater regulator
Rationale: Safety valves automatically release pressure to prevent boiler
rupture.
9. In welding inspection, a “slag inclusion” refers to:
A. Excess weld metal
B. Non-metallic material trapped in the weld
C. Undercut along the weld edge
D. Porosity in the weld
Rationale: Slag inclusions occur when flux or non-metallic material gets
trapped in the weld, weakening it.
10. The primary function of a boiler water level indicator is to:
A. Measure pressure
B. Control fuel input
C. Allow the operator to monitor water level
D. Vent steam
