cycling hrsgs jeffrey phillips [email protected]
TRANSCRIPT
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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
0
5
10
15
20
25
600 700 800 900 1000 1100 1200
Metal Temperature (deg F)
Allo
wab
le S
tres
s (k
si)
SA-213-T91SA-213-T22SA-213-T11SA-192SA-178-A
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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