2CTC2 HRSG Cycling StudyIn 2001, Fern conducted a study for the Combustion Turbine Combined Cycle Users Group (CTC2)Issued CTC2 Report HSRG 20-14On CTC2 “best seller list” (www.ctc2.org)
3Study GoalsReview problems encountered when operating an HRSG in cycling modeIdentify “best practices” that are employed to avoid or minimize these problemsResults should be applicable to both existing plants and new units
4Major Cycling-Related Problems Four General CategoriesThermal stress – relatedWater-relatedExhaust gas sideOtherWill focus on first two categoriesReport covers all four
5Best Practices for Existing Units The Two Most Important Actions to Take:Conduct a design review of the HRSGDetermine cyclic design conditionsAssess remaining fatigue lifeDefine ramping limitsImplement effective water lay-up proceduresWet lay-up should use nitrogen or steam capDry lay-up: drain hot & use nitrogen capA design review can also identify any modifications which may be needed to accommodate the desired cycling schedule
6Other Actions Use slower ramps Gradually reduce superheated steam T at shutdownModerates impact of CT purge on SHAvoid or closely monitor Spin CoolingAdd motor-operated drain valves on superheater and automate drain sequenceIf 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.
7Other Actions Keep HP drum P as high as possible during shutdowns close all valves including blowdownimport steam from another unit or aux. BoilerAdd 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).
8Stack DamperStack 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 InletDesign 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.
10Other ActionsFor long-term shutdowns, add and circulate a octadecyl amine (ODA) to BFWForms a protective film on metal surfacesThen place unit in dry lay-upFilm resists corrosion even if surfaces get wetAdd on-line water quality analyzerspH of drum and conductivity of condensateOther on-line analyzers that are helpful include oxygen concentration of BFW and conductivity of drum water
11Summary: 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 practicesHint: buy nitrogenThe rest is detailsI.e., read the report!
12Background Information Causes of Thermal Stress During Cycling – See “notes” portion of Powerpoint presentation for narrative
13Thermal Stress All metals expand when heated Amount of expansion is directly proportional to the change in temperatureUnconstrained expansion does not generate stress, but…Constrained parts will be stressedNon-uniform temperatures also create stress
14Steel 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.
15Yield Strength vs TOne 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.
16Cyclic Stresses => Fatigue Fatigue is damage caused by repeated application of cyclical stressesFatigue will also cause a material to fail at stress levels below the yield strengthThe effects of fatigue are cumulativeFatigue is a function of the number of stress cycles and the magnitude of the cyclic stressFew 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.
17Fatigue 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.
18Fatigue-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.
19Thermal Stress-Related Problems Fatigue damage from rapid rampingHP Steam Drum is the most vulnerableRamp downs cause more damage to drum than ramp upsLess of a concern for steam systems <1500 psig (103 barg)Warm and hot starts can be faster due to smaller overall temperature changeThe HP drum has the thickest wall of any part of the HRSG, so it takes the longest for heat to soak through itRamp downs put the outer surface of the drum into tension