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

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

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