Failure Analysis of Waterside Boiler Damages and Risk from Insufficient Maintenance & Inspection

Transcription

1 Page 1 Failure Analysis of Waterside Boiler Damages and Risk from Insufficient Maintenance & Inspection Ludwig Höhenberger, TÜV SÜD, IS-ATW1, Munich Ludwig.Hoehenberger@tuev-sued.de Waterside damages on steam generators e.g. on condensate or feedwater pre-heater, economiser, evaporator, super-heater and re-heater are mostly caused by Deposition (Scaling) and Corrosion or combinations of both effects. Possible, but more rare are damages due to Operational Faults and Design Defects Deposition or Scaling is a common problem on steam generators and may lead to Lack in material cooling due to reduced heat transfer and/or reduced mass flow Material overheating and consequently loss of material strength Damage - due to rapid overheating, characterised by thin-lipped ruptures Photo 1 and complete micro-structural transformation at temperatures > 720 C. - due to long-term overheating, characterised by thick-lipped ruptures Photo 2 and carbide speroidisation or grain growth at temperatures > ca. 480 C. Attention: Scaling appears first at spots with high heat transfer (heat flux) and high steam production Silicate scale has 10 x lower heat transfer than common carbonate or phosphate scale Porous scale with its steam content is more dangerous than compact and dense scale Causes and Risks: Insufficient Boiler Feedwater (BFW) treatment that includes - Make up water treatment, - Condensate polishing, Inadequate chemical dosing and conditioning of BFW and Boiler Water (BW) Fig. 1-3 Coordinated phosphate dosing is not anymore recommended, Fig 4 All Volatile Treatment (AVT) is permitted only with demineralised BFW! Contaminated BFW as injection water for desuperheating leads to scale in super-heater & turbines or caustic stress corrosion cracking in super-heater Photo 3 6 or steam pipework for extracted steam or back pressure steam. Chemical cleaning must be done properly and profound Photo 7 9. TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 1

2 Page 2 Deposition or Scaling may also lead to Localised concentration of impurities within or underneath of deposits at heated surfaces On Load Corrosion with acidic or alkaline attack Photo Attention: Scaling appears first at spots with high heat transfer and high steam production Even non corrosive deposits e.g. Photo iron oxides may lead to porous, thermal critical scale, - silicates as Ca-, Mg, Al-silicates to over-heating Boiler operation according to the BFW and BW requirements for some years lead mostly to some deposits of iron oxide, which should be preventively removed Causes and Risks: Insufficient on-line quality control of make-up water, condensate return or BFW Measurement of Acid Conductivity amplifies detection of ingress of impurities like cooling water and some process media but do not indicate ingress of free caustics! Lack in Inspection of not safety related components like heat exchangers and condensers Preventive cleaning during operation or at shut down must be done according to the specifications Corrosion on boiler steel (carbon or low alloyed steel) can be mainly avoided, if the BFW and BW requirements were kept. Deviations may leads to Dissolution of the important protective layer, material attack and loss in form of - Uniform or General Corrosion or - more dangerous - to - Localised Corrosion (Pitting, On-load corrosion, etc.) Figs. 2-4 Crack formation in specific cases Attention: Uniform or General Corrosion isn t problem if the material loss is less than 0.1 mm/year (is easy to achieve with normal protective iron oxide layer) Localised corrosion during operation underneath of deposits and within heated gaps as well as during stand still in form of oxygen corrosion (pitting formation) Crack formation due to high concentration of free caustics in presence of high tensile stress (Caustic SCC) Photos 3-6 Causes and Risks: Insufficient BFW and BW composition, which do not meet the requirements of the boiler manufacturer or approved standards Inadequate preservation during shut down Localised concentration of acidic acting impurities e. g. cooling water or sea water ingress Localised concentration of caustic acting impurities or free caustic underneath deposits within heated gaps or at heated phase boundaries TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 2

3 Page 3 Caustic SCC in case of contamination of super-heater due to mechanical carry over of BW or contaminated injection water for desuperheating Photo 3-6 Corrosion due to non-corrosive BFW and BW conditions but elevated temperatures or distinct temperature fluctuations may lead to Excessive Iron Oxide formation particularly > 570 C Photo Hydrogen production Hydrogen damage that includes - Decarburisation - Hydrogen Embrittlement - Fissuring (micro cracks) - Sudden brittle rupture Photo 21 Thermo shock Photos Iron reacts very fast at temperatures > 570 C with water or steam [1] Fe + H 2 0 FeO + 2 {H} to wustite (FeO) and atomic hydrogen {H} [2] 2 {H} H 2 that later recombines to molecular hydrogen (H 2 ) Causes and Risks: Insufficient water (mass) flow e.g. due to Steam/Water separation in tubes with low slope (Design problem) or Too much refractory material (Design or repair problem) Insufficient mass flow due to deposits within headers Excessive localised heat input e. g. due to misalignment of burners Operational Faults may lead to Boiler / Super-Heater damages for instance Short-term increasing steam production or over-load may lead to Fast pressure drop Increasing BW level BW carry over Immediate boiler pressure drop may lead to BW carry over steam contamination super-heater/turbine damages Operation significantly below design pressure leads to Increasing specific steam volume and poor steam/water separation that may lead to steam contamination BW carry over SH/turbine deposits Quick start up from cold shut down may lead to - Thermal stress on components with thick wall thickness (may cause crack) - Local overheating due to insufficient BW circulation (lack in steam production) Quick start up from hot stand by may lead to Local overheating due to insufficient BW circulation (lack in steam production) BFW Temperature significant below design temperature may lead to - Material over-heating due to lack of steam production - Under cooled boiling ( erosion corrosion, FAC) Design Problems may lead to several problems or damages, for instance Low slope riser tubes (on the way to the drum) heated also from top may lead to steam/ water separation Steam-side tube over-heating Loss in material strength TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 3

4 Page 4 Steamside excessive iron oxidation Material loss Hydrogen damage Photos 19 & 21 Heated down-comer tubes may cause circulation problems like stagnant flow Insufficient water supply for riser tubes Lack in cooling of evaporator tubes Material overheating Heavily fluctuating water level in the boiler drum may lead to Fluctuating steam space load and problems with the steam purity Refractory material reduces heat input to evaporator tubes but may also affect the steam production and water circulation (mass flow) Defects at drum internals e. g leaks at baffle sheets, incorrect installed demisters or cyclones may affect the steam purity Annexes: Photos 1 23 Figures 1 4 Photographs Photo 1 TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 4

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15 Page 15 Photo 22 Photo 23 Figures : TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 15

16 Page 16 Fig.1 Fig.2 TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 16

17 Page 17 Fig. 3 TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 17

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