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PowerPoint Presentation:

DISCUSSION ON BOILER TUBE FAILURES TYPES CAUSES & REMADIES

Boiler Tube Failures:

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

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Design stage: Selection of material Compatible for working pressure and temperature Steam flow, velocity / pressure – tube size Heat transfer characteristics Metal temperature / Thermal expansion / constraints Radius of bends 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

Failure Mechanisms which cause Boiler Tube Failure:

Failure Mechanisms which cause Boiler Tube Failure Stress rupture Water-side corrosion Fire-side corrosion Erosion Fatigue Lack of quality control

Boiler Tube Failures (22 Primary Mechanisms):

Boiler Tube Failures (22 Primary Mechanisms)

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

PowerPoint Presentation:

Source: EPRI Manual

Lack of Quality:

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

STRESS RUPTURE FAILURES:

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

STOH:

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.

Short Term Overheating:

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

SHORT TERM OVERHEATING:

SHORT TERM OVERHEATING Fish mouth opening with gradual reduction in thickness

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SHORT TERM OVERHEATING

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

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

High temperature Creep (Long term overheating):

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: Tubes often contain significant deposit Long term system operating problem Reduced coolant flow Excessive heat input (nearer to burner)

High temperature creep (Long term overheating):

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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