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Boiler Drum Level Fluctuation – BOE Exam Oriented Notes (Causes, Effects & Remedies)

Boiler drum level fluctuation is a very important topic for BOE (Boiler Operation Engineer) examinations, frequently asked in written exams, oral interviews, and safety viva. Questions are commonly framed on shrink & swell phenomenon, drum level control systems, and troubleshooting during operation.

Definition of Boiler Drum Level (BOE Point of View)

Boiler drum level is the height of water maintained inside the steam drum of a water-tube boiler, ensuring proper separation of steam and water for safe boiler operation.

Exam Tip: Always mention steam-water separation and boiler safety in definition-based questions.

Importance of Drum Level Control (Exam Answer)

Maintaining correct boiler drum level is important because:

  1. Prevents overheating of boiler tubes

  2. Avoids water carryover with steam

  3. Protects superheater and turbine

  4. Prevents boiler explosion

  5. Ensures continuous and safe steam generation

Normal Boiler Drum Level

  • Normal drum level: 50–60% of gauge glass

  • Alarm levels:

Exam Tip: Mention interlocks if asked about drum level safety.

Main Causes of Boiler Drum Level Fluctuation (BOE Important)

1. Shrink and Swell Phenomenon (Most Important for BOE Exam)

Swell

  • Occurs when steam demand suddenly increases

    Boiler drum level fluctuation diagram showing shrink and swell phenomenon in high pressure power plant boiler for BOE exam preparation
    Boiler drum shrink–swell effect

  • Drum pressure decreases

  • Steam bubbles expand

  • Drum level indicates high falsely

Shrink

  • Occurs when steam demand suddenly decreases

  • Drum pressure increases

  • Steam bubbles collapse

  • Drum level indicates low falsely

Exam Question: Explain shrink and swell phenomenon with neat sketch.

2. Sudden Load Change on Boiler

  • Rapid increase or decrease in steam demand

  • Causes pressure fluctuation

  • Results in unstable drum level

Example: Sudden turbine load change in thermal power plant.

3. Improper Feed Water Control

  • Feed water valve hunting

  • Improper PID tuning

  • Oversized or sticky control valve

Result: Continuous rise and fall of drum level.

4. Faulty Drum Level Measurement

  • Choked impulse lines

  • DP transmitter calibration error

  • Condensate pot level mismatch

Exam Tip: Mention false high and false low level indication.

5. Feed Water Temperature Variation

  • Cold feed water → Shrink

  • Hot feed water → Swell

Occurs due to deaerator temperature or level fluctuation.

6. Boiler Pressure Fluctuation

  • Poor combustion control

  • Fuel quality variation

  • Faulty pressure control loop

Directly affects drum level indication.

7. Blowdown Maloperation

  • Sudden intermittent blowdown

  • Continuous blowdown valve passing

Causes rapid drop in drum level.

8. Poor Boiler Water Chemistry

  • High TDS

  • Foaming and priming

  • Oil contamination

Leads to false high drum level.

Effects of Improper Drum Level (Very Important for BOE)

Effects of Low Drum Level

  1. Overheating of riser tubes

  2. Tube failure

  3. Boiler explosion risk

  4. Boiler trip on low-low level

Effects of High Drum Level

  1. Water carryover with steam

  2. Superheater damage

  3. Turbine blade erosion

  4. Poor steam quality

Boiler Drum Level Control Systems (BOE Exam Favorite)

1. Single Element Drum Level Control

  • Input: Drum level only

  • Used in: Low-pressure boilers

  • Disadvantage: Cannot handle shrink & swell

2. Two Element Drum Level Control

  • Inputs: Drum level + steam flow

  • Advantage: Better response to load change

  • Limitation: Feed water disturbances not handled

3. Three Element Drum Level Control (Best & Most Asked)

  • Inputs:

    1. Drum level

    2. Steam flow

    3. Feed water flow

Advantages:

  • Compensates shrink and swell

  • Stable operation during load change

  • Used in high-pressure and utility boilers

Exam Question: Why three-element control is preferred in power plant boilers?

Boiler Drum Level Fluctuation – Troubleshooting Table (BOE Ready)

ObservationPossible CauseEffectCorrective Action
Level increases during load riseSwellFalse high levelUse three-element control
Level drops during load reductionShrinkFalse low levelStabilize load
Continuous oscillationValve huntingUnstable operationPID tuning
False high readingFoamingCarryoverControl TDS
False low readingChoked impulse lineBoiler tripClean impulse line
Level drops suddenlyBlowdownLow level alarmReduce blowdown

Safety Interlocks Related to Drum Level (BOE Viva Point)

  • Drum level low – Alarm

  • Drum level low-low – Master fuel trip

  • Drum level high – Alarm

Short Exam Questions & Answers

Q1: What is shrink in boiler?

Ans: Apparent decrease in drum level due to collapse of steam bubbles when pressure increases.

Q2: What is swell in boiler?

Ans: Apparent increase in drum level due to expansion of steam bubbles when pressure decreases.

Q3: Which drum level control is best?

Ans: Three-element drum level control.

Important BOE Exam Questions (Repeated & High-Probability)

The following questions are frequently repeated in BOE written exams, viva, and departmental competency tests. Preparing these thoroughly can significantly improve your BOE exam score.

1. Explain boiler drum level fluctuation.

Hint: Define drum level, mention causes, effects, and control methods.

2. What is shrink and swell phenomenon in boiler drum?

Hint: Explain with load change, pressure change, and level indication.

3. Why does drum level rise when steam load increases?

Hint: Swell phenomenon due to pressure drop and bubble expansion.

4. Why does drum level fall when steam load decreases?

Hint: Shrink phenomenon due to pressure rise and bubble collapse.

5. What are the effects of low boiler drum level?

Hint: Tube overheating, tube failure, boiler explosion risk.

6. What are the effects of high boiler drum level?

Hint: Water carryover, superheater damage, turbine blade erosion.

7. What is the normal drum level maintained in a boiler?

Answer: Normally 50–60% of gauge glass, depending on boiler design.

8. Explain single-element drum level control.

Hint: Drum level only, used for low-pressure boilers, limitations.

9. Explain two-element drum level control.

Hint: Drum level + steam flow, advantage and limitation.

10. Explain three-element drum level control with advantages.

Hint: Drum level + steam flow + feed water flow; best for high-pressure boilers.

11. Why is three-element control preferred in power plant boilers?

Hint: Handles shrink & swell, stable during load changes.

12. What are the causes of false high drum level indication?

Hint: Foaming, priming, dirty impulse lines, faulty transmitter.

13. What are the causes of false low drum level indication?

Hint: Choked impulse line, condensate pot imbalance.

14. What safety interlocks are provided for boiler drum level?

Hint: Low level alarm, low-low level master fuel trip, high level alarm.

15. How will you control drum level during sudden load change?

Hint: Maintain steady firing, proper feed water control, three-element control.

Conclusion (BOE Perspective)

Boiler drum level fluctuation is mainly caused by shrink and swell phenomenon, load variation, and improper control systems. For safe boiler operation, proper instrumentation, correct control logic, and three-element drum level control are essential.

BOE Exam Tip: Always explain drum level answers with cause → effect → corrective action structure.

📦 BOE Preparation Hub – Must-Read Internal Links

Use this BOE Preparation Hub to revise high-scoring topics before exams, renewals, and viva.
These articles are written specifically for Boiler Operation Engineer (BOE) candidates and power plant professionals.

🔥 Core BOE Boiler Subjects

🔧 Boiler & Auxiliaries (BOE Important)

💧 Water & Steam System (High Weightage)

🔥 Combustion, Safety & Controls

📘 Exam Strategy Tip: Study this Drum Level topic along with Blowdown, Water Chemistry, and Safety Interlocks—these are often asked together in BOE exams.


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