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4B practice exam › Unit B-3

4B B-3: Boiler Safety Devices

82 practice questions in this unit. Each practice set pulls 20 at a time, so you keep seeing new questions as you go.

What this unit covers: Safety valves, combustion safety controls, feedwater control, required boiler fittings.

Exam theme: Boiler safety devices and operation. Commonly reported share of the 100-question 4B exam: about 10 questions (unofficial; SOPEEC and ABSA do not publish per-unit counts). See the 4B exam breakdown.

2 free 4B B-3 questions, explained

Original exam-style questions written to the public syllabus, from our free public sample. They are not questions from a real exam.

1. According to ASME PTC-25, which type of pressure relief valve is characterized by rapid opening and is normally used to relieve compressible fluids such as steam?

  • A. Safety relief valve
  • B. Relief valve
  • C. Safety valve
  • D. Temperature and pressure (T&P) relief valve
Show the answer and explanation

Answer: C. ASME PTC-25 defines a safety valve as a valve with rapid opening normally used for compressible fluids like steam. A relief valve opens gradually for incompressible fluids, a safety relief valve does either, and a T&P valve adds temperature actuation.

Why the other options are not the answer

  • A. A safety relief valve can act either way depending on the service; the rapid-opening valve for steam is the safety valve.
  • B. A relief valve opens gradually and is used for incompressible fluids (liquids).
  • D. A T&P valve adds temperature actuation; it is not the PTC-25 definition of a rapid-opening steam valve.

Question ID KE-B03-C01-Q001 · Unit B-3: Boiler Safety Devices · Report an error

2. What is the concern if a high-low boiler is started from cold with the HI-LO-AUTO switch set to AUTO?

  • A. The boiler may jump to high-fire, causing uneven expansion and excessive stress
  • B. The burner may fail to light because the pilot is starved of air during the pilot trial for ignition
  • C. The modulating motor may stall between the high and low fire positions
  • D. The low water cut-off may trip before the burner establishes a flame
Show the answer and explanation

Answer: A. A cold boiler's pressure is below the high-fire cut-in, so with the switch in AUTO it goes immediately to high-fire on start-up. The sudden heat input causes uneven thermal expansion and excessive stress, which is why a cold boiler should be warmed on low-fire first.

Why the other options are not the answer

  • B. Pilot ignition is not what the switch setting changes; the concern is the burner going straight to high fire.
  • C. A high-low burner moves between two fixed firing positions; a stalled modulating motor is not the hazard here.
  • D. The low water cut-off responds to water level, not to the HI-LO-AUTO switch position.

Question ID KE-B03-C05-Q009 · Unit B-3: Boiler Safety Devices · Report an error

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