Excessive Cyclic Operations in Firetube Boilers Only/Technical Presentations/2017-2...• Burner/...
Transcript of Excessive Cyclic Operations in Firetube Boilers Only/Technical Presentations/2017-2...• Burner/...
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Excessive Cyclic Operations in Firetube Boilers
The cause and effect of induced stress cycling
National Board Training
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Causes of induced stress cycling
• Burner/ boiler short cycling– An oversized boiler quickly satisfies load demand and
starts and stops frequently ( 4 or cycles per hour).• A cycle consists of a interval of pre-purge ( cold air) , firing
( hot air) , post purge ( cold air) and idle time.
– Low turn down ratio ( high fire versus low fire) and lack of modulation control contributes to this short cycling.
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Thermal Shock on the Water side
• The boiler manufacturers tried to prevent this thermal shock through controls and flow design with in the boiler.
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Thermally Induced stress cycling (Thermal Shock) in Firetube BoilersG Halley NB Bulletin Winter 1998.
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Causes of induced stress cycling
• Thermally induced stress ( Thermal shock from the waterside)– Primarily hot water boilers.– Night set backs cause a drop in temperature of the
system. This colder water enters the hot boiler when the system is released to the normal temperature.
– System temperature transients caused by human error.
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Causes of induced stress cycling
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Battelle Memorial Institute conducted a study of the tube leaks of firetube boilers in the 1950s.This report recommended the use of a fillet weld on the hot pass of the flues/fire tubes to reduce hoop stress caused by cyclic operations and to provide a heat sink to remove heat stress from the end of the tube extending out from the tube sheet.
• Fatigue cracking from on/off cycles (short cycling)of the burner
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Tube Attachment
• ASME Code requires some tube attachments to stick out of the tube sheet at least to the thickness of the tube. This end is not water cooled.
• On the first pass out of the furnace tube and into the flues/firetubes the gases can be as high as 2200 F.
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Affects of induced stress
• Metal fatigue causing tube cracks/leaks.
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Fatigue
• Fatigue is the progressive, localized, and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that have maximum values less than the tensile strength of the material .
• Fractures occur below the yield strength of the material.
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Fatigue limit or Endurance limit
• The endurance limit is the value of stress below which a material can presumably endure an infinite number of stress cycles.
• On a S-N diagram it is when the line becomes and remains horizontal
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Image results for s-n curves for steel
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Crack Propagation
• Cracks initiate on the surface.
• As the number of cycles increase the cracks continue to grow.
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• Why does a Commissioned Inspector want to know this ?
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Commissioned Inspectors use this information
• NBIC Part 2 -4.4.8.6– a) A fatigue evaluation should be performed if a
component is subject to cyclic operation. The allowable number of cycles (mechanical or thermal) at a given level of stress should be adequate for the specified duration of service to determine suitability for continued operation.
– b) Data requirements and history should follow Part 2 4.4.5 Data Requirements for remaining service life assessments.
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NBIC Part 3
• 3.3 Repairs to Pressure-Retaining Items– 3.3.1 Defect Repairs
– Before a repair is made to a defect in a welded joint or base metal, care should be taken to investigate its cause and to determine its extent and likelihood of recurrence.
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More Incentive to stop short cycling.
• US DOE Advanced Manufacturing Office– Steam Tip # 16, Minimize
Boiler Short Cycling Losses
– Short cycling reduces the efficiency of the boiler operation
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