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Cycling HRSGs Jeffrey Phillips

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1 Cycling HRSGs Jeffrey Phillips

2 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” (

3 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

4 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

5 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

6 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

7 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”

8 Stack Damper

9 “Garage Door” on Inlet

10 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

11 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!

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

13 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

14 Steel Stress-Strain Curve

15 Yield Strength vs T

16 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

17 Fatigue Curves for Steel

18 Fatigue-driven Life Expenditure

19 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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