Boiler Types, Operation and Maintenance
- Before Boiler Operation Engineering exams, you can study the following articles
What is a boiler?
A boiler is a closed pressure vessel used to convert water into steam by transferring heat generated from fuel combustion.
What are the main types of boilers?
| Boiler Type | Description |
|---|---|
| Fire Tube Boiler | Hot gases pass inside tubes and water surrounds tubes |
| Water Tube Boiler | Water flows inside tubes and hot gases flow outside |
| Packaged Boiler | Factory-assembled compact boiler |
| Fluidized Bed Boiler | Fuel burns in fluidized bed of sand/ash |
| Waste Heat Recovery Boiler | Uses waste exhaust heat |
| Once-Through Boiler | No steam drum, direct steam generation |
Difference between fire tube and water tube boiler
| Fire Tube Boiler | Water Tube Boiler |
|---|---|
| Low pressure | High pressure |
| Small steam capacity | Large steam capacity |
| Simple construction | Complex construction |
| Slow steam raising | Fast steam raising |
| Lower efficiency | Higher efficiency |
What is boiler efficiency?
Boiler efficiency is the ratio of useful heat output in steam to heat input from fuel.
η = Heat Output/Heat Input
What is evaporation ratio?
It is the amount of steam generated per kg of fuel burned.
Evaporation Ratio = Mass flow rate of steam /mass fuel burnt in boiler
What is equivalent evaporation?
Equivalent evaporation is the amount of water evaporated from feedwater at 100°C to dry saturated steam at atmospheric pressure.
What is boiler mountings?
Boiler mountings are safety fittings required for safe boiler operation.
Examples:
- Safety valve
- Water level indicator
- Pressure gauge
- Blowdown valve
- Fusible plug
What is boiler accessories?
Accessories improve boiler efficiency and performance.
Examples:
- Economizer
- Superheater
- Air preheater
- Feed pump
What is the function of economizer?
Economizer recovers waste heat from flue gas to preheat feedwater and improve efficiency.
What is the function of superheater?
Superheater increases steam temperature above saturation temperature to produce dry steam.
What is boiler blowdown?
Blowdown removes impurities and dissolved solids from boiler water.
Types:
- Continuous blowdown
- Bottom blowdown
Why is boiler water treatment necessary?
To prevent:
- Scaling
- Corrosion
- Foaming
- Carryover
- Tube failure
What is scaling in boilers?
Scaling is hard deposit formation on tube surfaces due to dissolved salts.
Effects:
- Reduced heat transfer
- Tube overheating
- Efficiency loss
What is boiler priming and foaming?
| Term | Meaning |
|---|---|
| Priming | Water particles carried with steam |
| Foaming | Formation of stable bubbles on water surface |
What is circulation ratio?
CR = Mass of Water Circulated/ Mass of Steam Generated
Typical natural circulation ratio:
- 5 to 30
What is furnace draft?
Draft is the pressure difference used to supply air and remove flue gases.
Types:
- Natural draft
- Forced draft
- Induced draft
- Balanced draft
Difference between FD fan and ID fan
| FD Fan | ID Fan |
|---|---|
| Supplies combustion air | Removes flue gas |
| Positive pressure | Negative pressure |
| Located before furnace | Located after boiler |
What is excess air?
Extra air supplied beyond theoretical air for complete combustion.
Typical values:
- Gas firing: 10–15%
- Coal firing: 20–40%
What causes black smoke from boiler stack?
Possible causes:
- Insufficient air
- Poor atomization
- Burner malfunction
- High fuel rate
- Incomplete combustion
What are causes of low boiler efficiency?
- High excess air
- High stack temperature
- Scale formation
- Incomplete combustion
- Air leakage
- Poor insulation
What is furnace purging?
Purging removes combustible gases from furnace before burner ignition.
Why is low water level dangerous?
Low water exposes tubes/drum causing overheating and possible explosion.
What is carryover in boilers?
Carryover is water and impurities carried with steam into steam lines.
What is soot blowing?
Removal of ash and soot deposits from heat transfer surfaces using steam or air.
What are common boiler tube failure reasons?
| Cause | Effect |
|---|---|
| Overheating | Tube rupture |
| Corrosion | Wall thinning |
| Erosion | Tube wear |
| Scaling | Heat transfer reduction |
| Thermal fatigue | Cracking |
What is deaerator function?
Deaerator removes dissolved oxygen and CO₂ from feedwater to prevent corrosion.
What is boiler light-up procedure?
Typical sequence:
- Check water level
- Start FD fan
- Furnace purging
- Ignite pilot burner
- Main burner ignition
- Raise pressure gradually
What is boiler banking?
Banking means maintaining boiler in standby condition with minimum firing.
What is refractory in boiler furnace?
Refractory is heat-resistant lining protecting furnace walls from high temperatures.
What is AFBC and CFBC boiler?
| AFBC | CFBC |
|---|---|
| Atmospheric Fluidized Bed Combustion | Circulating Fluidized Bed Combustion |
| Lower efficiency | Higher efficiency |
| Smaller capacity | Larger capacity |
| Lower fuel flexibility | High fuel flexibility |
What is a safety valve?
A safety valve automatically releases steam when pressure exceeds safe limit.
Why is superheated steam preferred in turbines?
- Reduces moisture
- Prevents blade erosion
- Improves efficiency
What is boiler furnace explosion?
Explosion caused by accumulation and ignition of unburnt fuel in furnace.
Common causes:
- Improper purging
- Flame failure
- Fuel leakage
What is TDS in boiler water?
TDS = Total Dissolved Solids.
High TDS causes:
- Foaming
- Carryover
- Scaling
What is the importance of boiler insulation?
- Reduces heat loss
- Improves efficiency
- Protects personnel
What are boiler inspection points?
- Tube thickness
- Drum condition
- Refractory damage
- Safety valves
- Water level controls
- Burner condition
What is hydrostatic test?
Pressure test using water to check boiler leakage and strength.
Typical test pressure:
- 1.5 × working pressure
What is steam trap?
Steam trap removes condensate while preventing steam loss.
What are advantages of water tube boiler?
- High pressure capability
- High efficiency
- Fast steam generation
- Large steam capacity
What are important daily boiler operation checks?
| Parameter | Check |
|---|---|
| Drum level | Stable |
| Steam pressure | Within limit |
| Flue gas temperature | Normal |
| Fuel pressure | Stable |
| O₂ in flue gas | Controlled |
| Feedwater quality | Acceptable |
| Blowdown | Proper |
| Safety valves | Functional |
Important BOE Interview Topics
- Boiler mountings and accessories
- Combustion basics
- Draft systems
- Boiler water chemistry
- Boiler efficiency calculations
- Steam tables
- Safety procedures
- Boiler startup/shutdown
- Boiler tube failures
- AFBC/CFBC operation
- Boiler emission control
- IBR and safety regulations
References
- Solved Questions and Answer forBOE