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Cycling HRSGs Jeffrey Phillips Jphillips@FernEngineering.com CTC2 HRSG Cycling Study In 2001, Fern conducted a study for the Combustion Turbine Combined Cycle Users Group (CTC2) Issued CTC2 Report HSRG 20-14 On CTC2 “best seller list” ( www.ctc2.org ) Study Goals

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Cycling HRSGs

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Cycling hrsgs l.jpg

Cycling HRSGs

Jeffrey Phillips

Jphillips@FernEngineering.com


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CTC2 HRSG Cycling Study

  • In 2001, Fern conducted a study for the Combustion Turbine Combined Cycle Users Group (CTC2)

  • Issued CTC2 Report HSRG 20-14

  • On CTC2 “best seller list” (www.ctc2.org)


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Study Goals

  • Review problems encountered when operating an HRSG in cycling mode

  • Identify “best practices” that are employed to avoid or minimize these problems

  • Results should be applicable to both existing plants and new units


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Major Cycling-Related Problems

  • Four General Categories

    • Thermal stress – related

    • Water-related

    • Exhaust gas side

    • Other

  • Will focus on first two categories

    • Report covers all four


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Best Practices for Existing Units

  • The Two Most Important Actions to Take:

    • Conduct a design review of the HRSG

      • Determine cyclic design conditions

      • Assess remaining fatigue life

      • Define ramping limits

    • Implement effective water lay-up procedures

      • Wet lay-up should use nitrogen or steam cap

      • Dry lay-up: drain hot & use nitrogen cap


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Other Actions

  • Use slower ramps

  • Gradually reduce superheated steam T at shutdown

    • Moderates impact of CT purge on SH

  • Avoid or closely monitor Spin Cooling

  • Add motor-operated drain valves on superheater and automate drain sequence


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Other Actions

  • Keep HP drum P as high as possible during shutdowns

    • close all valves including blowdown

    • import steam from another unit or aux. Boiler

    • Add a stack damper or inlet “garage door”


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Stack Damper


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“Garage Door” on Inlet


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Other Actions

  • For long-term shutdowns, add and circulate a octadecyl amine (ODA) to BFW

    • Forms a protective film on metal surfaces

    • Then place unit in dry lay-up

    • Film resists corrosion even if surfaces get wet

  • Add on-line water quality analyzers

    • pH of drum and conductivity of condensate


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Summary: Remember 2 Things

  • Know what your HRSG is capable of withstanding!

    • Conduct a design review (or life cycle analysis for new units)

  • Implement good water lay-up practices

    • Hint: buy nitrogen

  • The rest is details

    • I.e., read the report!


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Background Information

Causes of Thermal Stress During Cycling – See “notes” portion of Powerpoint presentation for narrative


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Thermal Stress

  • All metals expand when heated

  • Amount of expansion is directly proportional to the change in temperature

  • Unconstrained expansion does not generate stress, but…

    • Constrained parts will be stressed

    • Non-uniform temperatures also create stress


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Steel Stress-Strain Curve


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Yield Strength vs T


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Cyclic Stresses => Fatigue

  • Fatigue is damage caused by repeated application of cyclical stresses

  • Fatigue will also cause a material to fail at stress levels below the yield strength

  • The effects of fatigue are cumulative

  • Fatigue is a function of the number of stress cycles and the magnitude of the cyclic stress


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Fatigue Curves for Steel


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Fatigue-driven Life Expenditure


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Thermal Stress-Related Problems

  • Fatigue damage from rapid ramping

    • HP Steam Drum is the most vulnerable

    • Ramp downs cause more damage to drum than ramp ups

    • Less of a concern for steam systems <1500 psig (103 barg)

    • Warm and hot starts can be faster due to smaller overall temperature change


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