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T91 Reheater Tube Arrangement AT RAMAGUNDAM STPP

T91 Reheater Tube Arrangement AT RAMAGUNDAM STPP. T91 Reheater Coil Arrangement AT RAMAGUNDAM STPP. PROBLEM FACED AT RAMAGUNDAM STPP IN T91 WELDS. TUBE SIZE : 44.5x8mm : 44.5x5.6mm : 54x4mm TOTAL NO.OF WELDS : 888 RT TAKEN : 888 No. OF WELDS ARE CLEAR : 340

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T91 Reheater Tube Arrangement AT RAMAGUNDAM STPP

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  1. T91 Reheater Tube Arrangement AT RAMAGUNDAM STPP

  2. T91 Reheater Coil Arrangement AT RAMAGUNDAM STPP

  3. PROBLEM FACED AT RAMAGUNDAM STPP IN T91 WELDS TUBE SIZE : 44.5x8mm : 44.5x5.6mm : 54x4mm TOTAL NO.OF WELDS : 888 RT TAKEN : 888 No. OF WELDS ARE CLEAR : 340 OXIDATION OBSERVED : 518 OTHER REPAIRS : 30

  4. REASONS FOR USAGE OF T91 NIPPLES IN PLACE OF T22 FOR NTPC 500 MW PROJECTS RHO Header Nipples in all 210/250/500MW boilers engineered by BHEL are with SA335 P22 headers, T22 nipples and T22 for the ineffective portion of tubing above SH roof. NTPC tender stipulates the following limits for selection of material: NTPC LimitIBR Limit Carbon Steel400ºC 427ºC Alloy Steel (SA213 T11, T22) 550ºC 577ºC Even though IBR code permits use of SA213T22 material up to temp. limit of 577ºC, NTPC spec. restricts the limit to 550ºC as specified above. In view of this, wherein the penthouse region, metal temp. are in the range of 555ºC, T91 material has to be necessarily used for header nipples. In case of Simhadri and Talcher, the temp. were exceeding marginally by 2ºC and NTPC accepted SA213 T22 material. But, in case of Ramagundam and Rihand, temp. limit is exceeded by 5ºC and hence T91 material has to be used.

  5. CORRECTIVE ACTION Out of 888 welds 340 welds are clear in all aspects. Radiographs of balance welds were examined jointly by site, NTPC-CQA and BHEL-Tiruchy and all defects like porosity, root mis-match, incomplete penetration and uneven surface etc. which are not permissible as per codes were repaired.

  6. PREVENTIVE ACTION For further projects like BELLARY, SIPAT etc. Even though procedure had been qualified without backing gas (purging), to achieve uniform smooth root weld, it was decided to retain backing gas in future application also. Welding Technology Centre, Trichy had been requested to qualify a procedure using RT 21/4 Cr 1Mo filler wire for root welding in the case of T91 welds and subsequent run by RT 9Cr1Mo filler wire i.e. matching filler wire of T91. Strength portion also to be acceptable to Engineering.

  7. PREVENTIVE ACTION If existing filler wire of T91 i.e. matching combination (9Cr1Mo) is to be continued then, -      Method of purging to be re-looked Argon being heavier gas, if the entry is from bottom in a 2G weld, it will ensure that the atmospheric air is completely purged out through the root gap before starting welding & welding can be continued with argon backing. Argon entry can be at erection welds between RH front and RH rear coils- i.e. 6 circuits above the roof (in pent house) and 6 circuits below the roof.

  8. PREVENTIVE ACTION If existing filler wire of T91 i.e. matching combination (9Cr1Mo) is to be continued then, This will ensure argon entry is from bottom in a 2G weld and purging of air can be better in this case. Trials can be conducted before starting the actual job. Even though this is not maintenance friendly, this will definitely improve the situation.

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