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Cycling HRSGs Jeffrey Phillips [email protected] 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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ctc2 hrsg cycling study
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
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
major cycling related problems
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
best practices for existing units
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
other actions
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
other actions7
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”
other actions10
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
summary remember 2 things
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!
background information

Background Information

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

thermal stress
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
cyclic stresses fatigue
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
thermal stress related problems
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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