Cycling HRSGs

1. Cycling HRSGs Jeffrey Phillips Jphillips@FernEngineering.com

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” (www.ctc2.org)

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