Identifying and correcting the root cause of tube failures is essential to help lessen the chance of future problems. A comprehensive assessment is the most effective method of determining the root cause of a failure. A tube failure is usually a symptom of other problems. To fully understand the cause of the failure, you must investigate all aspects of boiler operation leading to the failure in addition to evaluating the failure itself. When you experience tube failures, take advantage of B&W’s expertise to help you determine and eliminate the root cause of the problem. Our experienced field service engineers can help gather all the pertinent information. Better yet, let us help put together a complete condition assessment program to help eliminate tube problems before failures occur. The following boiler tube failure mechanisms are some of the most common that occur on modern operating boilers. We’ve organized these into three sections and have included their symptoms, possible causes, the components typically affected and solutions: Waterside failure mechanisms Fireside failure mechanisms General failure mechanisms Waterside failure mechanisms Caustic attack

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DISCUSSION
ON
BOILER TUBE FAILURES
TYPES
CAUSES
&
REMADIES

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Boiler Tube Failures
 Boiler Tube Failures - main cause of forced outages and
availability in steam generating boilers
 One tube failure in a 500 MW unit  Rs. 5 to 5.5Crores
(replacement, generation charges for 3-4 days to repair)
besides affecting Plant Morale.

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1. Sudden or abnormal loud noise
2. Continuous hissing noise
3. Steam / Water leakage
4. Furnace draft fluctuations / pressurization
5. Flame out
6. Falling in drum level
7. Increase in quantum of make-up water
8. Uncontrolled Boiler Feed Regulation / Feed Pump Trip
Tube failure indications

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Design stage: Selection of material
1. Compatible for working pressure and temperature
2. Steam flow, velocity / pressure – tube size
3. Heat transfer characteristics
4. Metal temperature / Thermal expansion /
constraints
5. Radius of bends
6. Attachments
Manufacturing • Material mix-up
• Welding defects
• Forming defect / Radius of bend
• Heat treatment
Aspects to be considered at various stages

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Transportation /
Handling
• Length of panel/walls
• Packing, bottom support
Storage • Foreign material entrapment
• Corrosion attack
• Atmospheric corrosion
Erection • Handling, temporary supports/lugs for lifting,
• Welding, plugging of tubes
• Material mix-up
• Expansion restriction ,
• Supports & hangers etc
Commissioning • Initial commissioning activities
• Boiler thermal expansion
Operation and
Maintenance
Fatigue, creep, erosion, corrosion etc.
Aspects to be considered at various stages

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Failure Mechanisms which cause Boiler
Tube Failure
1. Stress rupture
2. Water-side corrosion
3. Fire-side corrosion
4. Erosion
5. Fatigue
6. Lack of quality control

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Boiler Tube Failures (22 Primary Mechanisms)
Stress Rupture
 Short Term Overheating
 High Temperature Creep
 Dissimilar Metal Welds
Fatigue
 Vibration
 Thermal
 Corrosion
Water-side Corrosion
 Caustic Corrosion
 Hydrogen Damage
 Pitting
 Stress Corrosion Cracking
Erosion
 Fly Ash
 Falling Slag
 Soot Blower
 Coal Particle
Fire-side Corrosion
 Low Temperature
 Waterwall -$
 Coal Ash - $
 Oil Ash
Lack of Quality Control
 Maintenance cleaning damage
 Chemical excursion damage
 Material Defects
 Welding Defects
$ - indicates that such problems have not been reported in India

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Source: EPRI Manual
This is the highest metal temperature for which maximum allowable stress values are given.

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Source: EPRI Manual

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Lack of Quality
• Raw material defect
• Tube manufacturing defects
• Test Certificate not in order
• Quality requirement (non adherence)
• Inspection – Visual & NDT

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STRESS RUPTURE FAILURES
STOH occurs when the tube temperature rises above the design limits
for a brief period.
Depending on the temperature, the failure might occur in a very short
time.
STOH – mostly due to an upset over a short period.
Operating procedures or parameters are to be analysed to find out the
reason for the failure.
Short term overheating

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STOH
Identification:
• Mostly a fish mouth opening with gradual reduction in thickness
• Uniform tube expansion.
• Absence of large amount of thermally formed magnetite
• No significant internal deposit
Metallographic examination will reveal the reason.

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Short Term Overheating
Location: Steam & water cooled
sections
Reasons:
 Plugged by debris, scale etc.
 Drum level low
 High Heat Transfer / Improper
firing
 Low water/steam flow due to
poor circulation /upstream leak
Corrective Action:
 Prevent Blockage
 Maintain Drum level
 Assure Coolant flow
 Reduce over firing
 Redesign tubing to promote
flow
 Relocation of horiz. /
inclined tubes to avoid film
boiling

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SHORT TERM OVERHEATING
Fish mouth opening with gradual reduction in thickness

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SHORT TERM OVERHEATING

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1. STOH - ORIGINAL STRUCTURE
2. STOH - TRANSFORMED MARTENSITE
1 2

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High temperature Creep
(Long term overheating)
 Metal temperature exceed the design limit for days, weeks,
months or longer.
 LTOH is the cause for more failures because the metal loses its
strength at elevated temperatures
Causes:
o Tubes often contain significant deposit
o Long term system operating problem
o Reduced coolant flow
o Excessive heat input (nearer to burner)

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High temperature creep
(Long term overheating)
Identification:
 A thick, brittle oxide layer near the failure
 Bulging and plastic deformation at the failure
 Rupture is always longitudinal with a fish-mouth shape
 Rupture edge can be thick or knifelike depending upon the time
temperature and stress level.
 Multiple bulges might also be there in case of deposit inside.
Metallograhic inspection will ascertain the failure.

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