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BOE Exam MCQs – Boiler Drum Level Fluctuation

BOE MCQs with Explanations

Topic: Boiler Drum Level Fluctuation (Shrink & Swell)

1. Boiler drum level fluctuation is mainly due to:

A. Feedwater pump failure
B. Shrink and swell phenomenon
C. Steam separator design
D. Economizer choking

✅ Correct Answer: B

Explanation:
Shrink and swell are dynamic effects caused by pressure changes inside the drum during load variation. They give apparent level change without actual change in water mass.

2. Swell in a boiler drum occurs when:

A. Steam demand decreases
B. Boiler pressure increases
C. Steam demand increases suddenly
D. Feedwater temperature decreases

✅ Correct Answer: C

Explanation:
Sudden increase in steam demand reduces drum pressure. Steam bubbles expand, causing apparent rise in drum level, called swell.

3. Shrink phenomenon is caused by:

A. Expansion of steam bubbles
B. Collapse of steam bubbles
C. Excess feedwater
D. Boiler blowdown

✅ Correct Answer: B

Explanation:
When steam demand decreases, pressure increases. Steam bubbles collapse, reducing volume and causing apparent fall in drum level.

4. During swell condition, drum level rise is:

A. Actual increase in water quantity
B. Due to feedwater excess
C. Apparent increase only
D. Due to boiler leakage

✅ Correct Answer: C

Explanation:
Water mass remains constant; only bubble volume increases. Hence level rise is false or apparent.

5. Shrink and swell phenomenon is significant in:

A. Fire tube boilers
B. Low-pressure boilers
C. High-pressure water tube boilers
D. Package boilers

✅ Correct Answer: C

Explanation:
High-pressure boilers have more steam bubble formation, making shrink and swell effects more pronounced.

6. Which drum level control cannot handle shrink and swell properly?

A. Three-element control
B. Two-element control
C. Single-element control
D. Cascade control

✅ Correct Answer: C

Explanation:
Single-element control uses only drum level and reacts incorrectly to apparent level changes.

7. Three-element drum level control uses:

A. Drum level only
B. Drum level + feedwater flow
C. Drum level + steam flow
D. Drum level + steam flow + feedwater flow

✅ Correct Answer: D

Explanation:
Three-element control compensates for load changes and feedwater disturbances, making it ideal for power plant boilers.

8. High boiler drum level can result in:

A. Tube overheating
B. Furnace explosion
C. Water carryover
D. Feed pump cavitation

✅ Correct Answer: C

Explanation:
High drum level allows water droplets to carry over with steam, damaging superheaters and turbines.

9. Low boiler drum level may cause:

A. Wet steam
B. Turbine erosion
C. Tube overheating
D. Foaming

✅ Correct Answer: C

Explanation:
Insufficient water exposes tubes to high temperature, leading to overheating and tube failure.

10. False high drum level indication is commonly caused by:

A. Choked impulse line
B. Foaming and priming
C. Feed pump trip
D. Economizer leakage

✅ Correct Answer: B

Explanation:
Foaming traps steam bubbles in water, giving false high level indication.

11. False low drum level indication occurs due to:

A. High TDS
B. Oil contamination
C. Choked impulse line
D. High feedwater temperature

✅ Correct Answer: C

Explanation:
Blocked impulse lines prevent correct pressure sensing, showing lower-than-actual level.

12. Normal boiler drum level is maintained at:

A. 20–30%
B. 30–40%
C. 50–60%
D. 80–90%

✅ Correct Answer: C

Explanation:
Maintaining mid-level ensures safe margin for load fluctuations and shrink–swell effects.

13. Low-low drum level condition leads to:

A. Alarm only
B. Feed pump trip
C. Master fuel trip
D. Pressure relief valve opening

✅ Correct Answer: C

Explanation:
Low-low level is a critical safety condition and initiates boiler trip to prevent explosion.

14. During sudden load increase, correct operator action is:

A. Reduce feedwater immediately
B. Stop firing
C. Allow three-element control to act
D. Open blowdown

✅ Correct Answer: C

Explanation:
Three-element control anticipates steam flow changes and maintains stable drum level.

15. Shrink and swell phenomenon indicates:

A. Actual change in water mass
B. Faulty level transmitter
C. Dynamic pressure effect
D. Feedwater pump malfunction

✅ Correct Answer: C

Explanation:
Shrink and swell are dynamic effects due to pressure variation, not actual water loss or gain.



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