1. Jeffrey Phillips Jphillips@FernEngineering.com
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
A design review can also identify any modifications which may be needed to accommodate the desired cycling schedule

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
If Spin Cooling must be used, allow the unit to first cool naturally to moderate temperatures. The damage then inflicted by rapidly cooling the rest of the way with spin cooling will then be moderated.
Even if a unit already has SH drain valves, they may not be adequately sized to fully drain the condensate during a start-up.

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”
Maintaining the HP drum at 200 psig (14 bar) instead of letting it cool to ambient T will reduce the temperature change of the start-up and shutdown ramps by 300 F (167 C).

8. Stack Damper
Stack damper design of Stejasa, SA of Spain. The twin dampers are electrically actuated. Uninstalled price for an F-class turbine would be circa $52,000 and $25,000 for an LM2500.

9. “Garage Door” on Inlet
Design of inlet system supplied by Freudenberg Vliesstoffe KG of Germany for Alstom GT13E2s. The design includes a “closure shutter” (item 8) that looks similar to a roll-up garage door. Three sensors are used to verify position and turbine start-up requires two out of three to indicate that the door is open. Budgetary cost of closure door only is $22,000.

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
Other on-line analyzers that are helpful include oxygen concentration of BFW and conductivity of drum water

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
How much thermal stress is too much?
To answer that question, one must first have an understanding of strength of materials. The next few slides provide some background.
This stress-strain curve shows what happens to steel when it is subjected to different levels of stress.
When placed in tension, the material will expand (strain)
Y is yield point of the material. If stressed beyond Y, the steel will not snap back to its original shape when stress is released. It will be permanently deformed.
U is ultimate strength of the material. At that point the steel will be pulled apart and eventually rupture.
R is the rupture point or tensile strength of the material.
Boiler design codes call for pressure containing parts to have yield points that are 150% higher than the highest expected stress and rupture points that are more than 200% higher than the highest expected stress.
Unfortunately, the designer may only look at the stresses caused by normally operating conditions and not have taken into account stresses caused by thermal expansion during transients.

15. Yield Strength vs T
One also needs to recognize that temperature has a significant impact on yield strength. The designer must use the value of yield strength that corresponds to the highest expected temperature. During transients tubes may run hotter than during normal full or peak load because of lack of flow through the tubes. If the highest thermal stresses are also occurring at this time, the design margin between yield strength and actual stress may be reduced to zero and the material will yield.

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
Few people, if any, could pull a spoon apart. To do so, one would have to pull on the spoon hard enough to exceed the rupture strength of the metal. However, most people could break a spoon by repeatedly bending the handle back and forth. That is an example of how fatigue can cause a material to fail under stresses that are far lower than its yield point.

17. Fatigue Curves for Steel
This chart shows the number of cycles of a given stress level that are needed to fail steel due to fatigue.
Note that the greater the magnitude of the cyclic stress, the fewer cycles are needed.
If transient events place portions of an HRSG under high thermal stress for even a short period of time, they could rapidly use up the “fatigue life” of the part.

18. Fatigue-driven Life Expenditure
This chart was generated by Foster Wheeler as part of a Life Cycle Analysis of a new HRSG FW was building for Florida Power Corp (7FA combined cycle).
It shows that rapid ramp rates and large temperature changes over the course of the ramp will use up fatigue life faster. Why? Because large temperature changes rapidly incurred will generate large thermal gradients and therefore high thermal stresses within the HRSG.
However, the chart also shows that large temperature changes that occur slowly (<200 F/hr), will have much less impact on fatigue life. Slow changes in temperature allow more time for heat transfer to spread the thermal energy across the material and make the temperature more uniform.
Similarly, rapid temperature changes over a short range do not cause much damage because the different in temperature across the material cannot be greater than the total temperature change, which is small.

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
The HP drum has the thickest wall of any part of the HRSG, so it takes the longest for heat to soak through it
Ramp downs put the outer surface of the drum into tension