Start Up and Commissioning of Supercritical Power Stations: Risk Management Plan

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Start Up protocols play a major part in risk management for modern Supercritical power plants especially during start-up and commissioning. EPC contractors must be alert to contract conditions set by …

Start Up protocols play a major part in risk management for modern Supercritical power plants especially during start-up and commissioning. EPC contractors must be alert to contract conditions set by owners and start up engineers on deviation of steam and water chemistry conditions especially.

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  • 1. Supercritical Units: Efficiency AdvantageImprovements in the efficiency of pulverized coal (P.C.) fired boiler/steamturbine power plants have been pursued since the introduction of P.C. firedboilers in the 1920’s.This led eventually in the late 1960’s to the introduction of supercritical boilersoperating at about 570 Deg C (1060qF) superheat/reheat temperatures and 24MPa (3400 psi) pressure for the steam.Presently boilers with 300bar and 700 Deg C are being adopted for many of therecent power plants.Since at these pressures and temperatures there is no distinct differencesbetween the water and steam phases, no drums are required in the same. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 1
  • 2. The new generation of power plants operate atConventional steam pressures higherpower plants operate at a than the criticalsteam pressures in the pressure.range of 170 bar and 565Deg C.These are Subcritical These arepower plants. Supercritical power plants. The operating pressures are in the range of 230 to 265 bar. And 565 Deg CThe next generation ofpower plants willoperate with SteamPressures in the rangeof 300 bar and 630-Temperatures 630-700 Deg Centigrade.These are the UltraSuper Critical PowerPlants.Plants. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 2
  • 3. The lower part of the boiler furnace is arranged in a spiralconfiguration such that the fluid path wraps around the boileras it travels up the furnace. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 3
  • 4. Comparison of Allowable Stresses between Conventional and AdvancedStainless Steel Tubes, Advanced CrMo Steel Pipes Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 4
  • 5. Advantages of Oxygenated Treatment OT of Supercritical BoilersThe benefits from a once-through supercritical boiler on OT include:• Lowering overall corrosion rates by forming a protective, double-oxide layerwith a controlled amount of oxygen present in the condensate (This protectivelayer is considered to be more stable than the oxide layer formed using AVT.)• Decreasing boiler chemical cleaning frequency due to reduced amounts ofiron transport and deposition• Allowing quicker, cleaner start ups and reduced corrosion product transportrates during cold and hot start ups• Allowing boiler operation at lower pH with overall objective of minimizingchemical costs• Eliminating feeding, handling, and storage of oxygen scavenger products
  • 6. EPC Contractor’s Chemistry ChallengesMost once-through supercritical boilers have been converted frompreviously predominant AVT to OT, with new facilities almostexclusively using OT. all-This chemistry change requires all-ferrous metallurgy in the feed copper-water train, and precludes copper or copper-based alloy feed waterheat exchangers in system design and bronze impellers incondensate pumps and valve trims in the condensate system. This isprimarily the challenge to the engineering group og the EPCcontractor to ensure that this is strictly adhered..all steels containtraces of copper and zinc. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 6
  • 7. coal-Shows the pressure drop history of a coal-fired supercritical plant in Japan that changed the waterchemistry from AVT to OWT in 1996.After 7 years operation since the change, there has been no significant increase in pressure drop.This plant has not required acid cleaning since the change, and no future cleaningis planned. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 7
  • 8. RISK MANAGEMENT Plant CommissioningSUPERCRITICAL UNITS: EPC CONTRACTOR’S CHEMISTRY CONTROL PARADIGM Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 8
  • 9. EPC Contractor’s Chemistry ChallengesSome once-through supercritical boiler manufacturers have instituted penaltiesagainst the allowable pressure drop during initial boiler performance testing, anadditional complication that may impact start up and commissioning activities.These penalties are based on extended operation on all volatile treatmentreducing (AVT[R]) during start up and commissioning.The reducing environment (negative oxidation reduction potential [ORP])present when operating on AVT(R) may contribute to increased iron transport,subsequently increasing the pressure drop through the boiler.These pressure drop correction penalties will be fervently debated by the EPCcontractor during commissioning and challenged by both owners and plantoperators. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 9
  • 10. WARRANTY IMPLICATIONSSteam turbine suppliers are also setting limits, in theequipment contract, on the number of hours a turbine can beoperated with out-of-specification chemistry. These limits aretypically listed in an action-level These limits are typicallylisted in an action-level format where minor chemistryexcursions are allowable for predetermined timeperiods without violating the equipment warranty. up— Steam Purity Limits During Start up— EPC Once- Contractor Recommendation for Once-Through Boilers...usually one week Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 10
  • 11. EPC Contractor’s Chemistry ChallengesDuring steam-side start up and commissioning, the EPC contractor is mostlyinterested in main and reheat steam chemistry.Table below lists EPRI recommendations for once-through boilers operating underOT, including normal target value and action levels 1, 2, and 3. Although recommendations listed in Table are acceptable for targeted chemistry limits during operation, EPC contractors would like to see the table: following two columns added to this table: • Allowable chemistry excursions during hot start up • Allowable chemistry excursions during cold start up Feed Water Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 11
  • 12. What Are Action Levels 1, 2, 3 Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 12
  • 13. EPC CONTRACTOR’S CHEMISTRY CONTROL PROGRAMStart up chemistry guidelines should primarily focus on main steam chemistrytargets, includingcation conductivity: it warns of salts and acids that may cause turbine corrosionsilica : silicate scaling may contribute to turbine capacity and efficiency losses.sodium: critical for avoiding corrosion because uncontrolled sodium hydroxide concentrations are known to cause corrosion damage failures in boiler tubes.Targets for chlorides, sulphates, and organic compounds should bedeferred until the end of the commissioning cycle.Degassed cation conductivity is the preferred conductivity to bemeasured during commissioning since system air leaks are stillbeing discovered and sealed during the start up and commissioningphase. The measurement of degassed cation conductivity will aid indifferentiating between air leaks and other contamination sources Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 13
  • 14. EPC Contractor’s Chemistry ChallengesThe two most important parameters in Table are feed water cationconductivity and pH.Cation conductivity should be maintained below 0.15 µS/cm duringoperation on OT. Normal pH range for feed water under OT is 8.0 to 8.5. The EPC contractor is challenged with controlling pH when feed water cation conductivity increases to concentration levels listed in Table , action levels 1, 2, and 3 (≤0.3 µS/cm, ≤0.6 µS/cm, >0.6 µS/cm, respectively). Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 14
  • 15. EPC Contractor’s Chemistry ChallengesThe pH/conductivity relationship is crucial for once-through cycles on OT;thus, the EPC contractor implements the chemistry control at its own risk.Important issues to be addressed when implementing OT include:• At what point during the start up and commissioning process should thechemistry regime be switched from AVT to OT to prevent frequent switching backand forth between a reducing and an oxidizing environment?• What would be the “detrimental effects” of going from an oxidizing atmosphere toa reducing (or close to reducing) atmosphere, for temporary periods?• How can these “detrimental effects” be quantified and addressed during designand equipment procurement? Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 15
  • 16. Stringent steam quality limits implemented by steam turbinesuppliers, boiler manufacturers have tightened limits on feed waterchemistry.Yet EPC contractor requires standards to be relaxed duringcommissioning to permit timely unit start up. The challenge also is in determining the appropriate time to switch from AVT to OT Once cation conductivity levels are stable below 0.15 µS/cm, EPRI recommends operation on OT with oxygen injection in a pH range of 8.0 to 8.5. EPRI guidelines also state that oxygen injection into feed water may continue with pH controlled between 9.2 and 9.6 µS/cm µS/cm. and cation conductivity between 0.15 µS/cm and 0.3 µS/cm. However, µS/cm, at cation conductivity levels greater than 0.3 µS/cm, EPRI recommends that oxygen injection be terminated and AVT resumed. Upsets in cation conductivity may lead to serious corrosion problems if oxygen is continuously fed during upset conditions. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 16
  • 17. Comparison of Start-up Systems Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 17
  • 18. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 18
  • 19. The turbine bypass system was designed to minimize the start-uptime by controlling the main steam pressure and temperature beforeturbine rolling, and enabling the steam to flow through the superheater sections at a short time after light-off.The low-load recirculation system was designed to recover residualheat during start-up by circulation of the un-evaporated water fromthe furnace back to the economizer inlet, which also can assist inreducing start-up time.As this system is automatically operated, the start-up process is assimple as with a natural circulation (NC) boiler. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 19
  • 20. EPC Contractor Guidelines on Feed Water Chemistry StartupStart up Steam Chemistry Limits Turbine Rolling Continued AVT(O) Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 20
  • 21. Turbine Rolling ContinuedBalance of Commissioning Period Chemistry Limits Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 21
  • 22. Operation Guidelines with AVT R and AVT OAVT(R) Feed water Chemistry Control Guidelines AVT(O) Feed water Chemistry Control Guidelines Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 22
  • 23. Suggested Start up Feed water Chemistry Guidelines for Once-Through Cycles Without Deaerators Once-Practical start up chemistry guidelines should be established byconsensus among the turbine manufacturer, boiler manufacturer,and EPC contractor early on in project development and outlinedin equipment contracts. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 23
  • 24. ROLE OF CONDENSATE POLISHERSDURING COMMISSIONING Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 24
  • 25. Once-through supercritical boilers are commonly installed withfull-flowcondensate polishers to control corrosive impurities concentrationin condensate and feed water systems. The presence of impurities in feed water will significantly affect feed water chemistry, potentially exceeding boiler supplier feed water limits and turbine supplier steam purity specifications. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 25
  • 26. Impurities shaken loose during start up may cause a chemistry hold, whereplant load increases are temporarily halted until these impurities areremoved from the system.For a once-through supercritical boiler, impurities are removed exclusivelyby condensate polishers subsequent to chemical cleaning and boiler flush.Once impurities are removed, the chemistry hold is lifted and the plant isallowed to continue to ramp up to full load without exceeding allowableboiler or turbine chemistry limits. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 26
  • 27. Steam and Water Analysis System (SWAS) shall be furnished for continuousmonitoring and control of water and steam purity in the plant cycle Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 27
  • 28. Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 28
  • 29. EPC Contractor Responsibility includes that Chemistry limits are before, duringand after once-through supercritical boilers commissioning• Control system component cleanliness during shop fabrication• Control system component cleanliness during construction• Flush system components prior to start-up• Implement stringent water quality requirements for hydro testing• Perform boiler and feed water system chemical cleaning• Flush system components thoroughly following chemical cleaning• Perform steam blows to obtain steam cycle cleanliness• Implement time-based, progressively improving feed water and steamchemistry targets Dr. Himadri Banerji:Conference on Plant Commissioning Kuala Lumpur 1st and 2nd Nov 2010 10/29/2010 29