Excessive Cyclic Operations in Firetube Boilers

The cause and effect of induced stress cycling

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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.

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

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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