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
• 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
• 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
• 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
• 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 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”
Stack Damper
“Garage Door” on Inlet
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
• 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
Causes of Thermal Stress During
Cycling – See “notes” portion of
Powerpoint presentation for narrative
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
Steel Stress-Strain Curve
Yield Strength vs T
25
Allowable Stress (ksi)
20
15
SA-213-T91
SA-213-T22
SA-213-T11
SA-192
SA-178-A
10
5
0
600
700
800
900
Metal Temperature (deg F)
1000
1100
1200
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
Fatigue Curves for Steel
Fatigue-driven Life Expenditure
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