DESIGN AND FABRICATION OF SERPENTINE TUBE TYPE SODIUM TO AIR HEAT EXCHANGERS FOR PFBR SGDHR CIRCUITS Aravinda Pai, T.K.Mitra, T. Loganathan and Prabhat Kumar Prototype Fast Breeder Reactor Project Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI) Department of Atomic Energy Kalpakkam, India
500MWe Prototype Fast Breeder Reactor (PFBR) flow chart
In PFBR, two diverse and independent Decay Heat Removal (DHR) systems are provided Operational Grade Decay Heat Removal (OGDHR) System Safety Grade Decay Heat Removal (SGDHR) System
SAFETY GRADE DECAY HEAT REMOVAL SYSTEM (SGDHR) There are 4 SGDHR loops Each loop consists of 8MWt heat removal capacity Diversity in design to avoid common cause failure 2 loops - Type-A design 2 loops - Type-B design
Sodium to Air Heat Exchanger (AHX) Type A Sodium to Air Heat Exchanger (AHX) is serpentine tube design heat exchanger. AHX transfers heat from the intermediate circuit sodium to atmospheric air in Safety Grade Decay Heat Removal (SGDHR) loop One of the most critical component The principal material of construction is modified 9Cr-1Mo steel. Intermediate sodium inlet/outlet: Temp. during poised state : 540 0 C/ 540 0 C Nominal temp. during SGDHR : 494 0 C/303 0 C Air inlet and outlet: Temp. during poised state : 40 0 C/ 537 0 C Nominal temp. during SGDHR : 40 0 C/291 0 C
CRITICAL AREAS IN FABRICATION Hot forming of pullouts on headers Header welding and fabrication Tube to tube welding Header pullout to tube welding Post Weld Heat Treatment Testing and surface treatment
Hot forming of pullouts on headers There are 116 nos. of formed pullouts exists on each header. Each pullout has outside diameter of 38.1mm and wall thickness of 2.6 mm with a height of 25mm. Hot forming of pullouts in conventional method is not possible due to metallurgical and practical limitations. Hot forming of pullout is carried out at 900-1100 0 C by heating the local area of the header shell segment using induction heating process followed by die & punch pressing. The headers after pullout forming undergoes normalizing at 1050-1090 0 C followed by tempering at 780±10 0 C to restore the material properties.
Conventional nozzle concept Pullout concept Χ
HEADER WELDING AND FABRICATION After pullout forming, the longitudinal & circumferential seam welding of header shell segments is carried out. All modified 9Cr-1Mo joints are preheated & interpass temperature is maintained at 200-250 0 C. After welding, post heating is carried out at 200 0 C for 2 hours and PWHT carried out at 760±10 o C. Even though Shielded Metal Arc Welding (SMAW) process is permitted as per PFBR specification, 100% GTAW process alone is executed to meet impact properties.
The welding procedure is qualified with stringent destructive and non-destructive examinations & testing before welding on the actual job. During qualification, weld joints are subjected to Thorough visual examination Liquid Penetrant Examination (LPE) Radiography Examination (RE) Longitudinal tensile test at ambient temperature Bend tests Charpy V notch impact test Hardness survey Metallographic examination at 200X magnification for the complete transverse section of the weld Each production weld joint undergoes thorough visual examination, LPE and Radiography Examination. If RE is not possible, soundness of weld is evaluated by ultrasonic examination (UE) After completion of PWHT, thorough visual examination, LPE and RE/UE is repeated on the weld joints.
CHALLENGES IN HEADER WELDING & FABRICATION The welding and fabrication of 12mm thick slender header shell having outer diameter of 457mm is difficult and challenging task. Due to small inside diameter, the internal fixtures/spiders cannot be used at many locations for distortion control during welding. Due to many formed pullouts on the outer surface, fixtures/spiders cannot be placed directly on the headers during welding for distortion control. Even though distortion tendency is less due to smaller diameter and existence of formed pullouts, utmost care is inevitable during welding to avoid distortion and dimensional deviations.
CHALLENGES IN HEADER WELDING & FABRICATION As no sufficient access from inside of header shells, the welding has to be carried out only from the outside. Therefore single V type Weld Edge Preparation (WEP) exists for welding from outside. In case of distortion, re-rolling after welding for shape correction is not permitted as per PFBR specification. Many welding trials were conducted to understand the behavior of shells. Tremendous efforts were put to achieve less than 1% ovality.
TUBE BUNDLE FABRICATION There are 116 nos. of Modified 9Cr-1Mo tubes of OD 38.1mm and 2.6mm wall thickness. Tube bundle fabrication mainly consists of Header pullout to tube welding Manual GTAW process Tube to tube welding Automatic pulsed GTAW process. Tube bundle activities are carried out in separate nuclear clean hall conditions as per class-1 component requirements of PFBR to ensure the quality.
CHALLENGES IN HEADER PULLOUT TO TUBE WELDING
SERPENTINE M TUBE WELDING AND FABRICATION PREHEATING TUBE TO TUBE WELDING HELIUM LEAK TEST FOR M TUBE
POST WELD HEAT TREATMENT (PWHT) AND CHALLENGES: Due to complex constructional features, the heat treatment is not straight forward. The PWHT of individual tube to tube weld joint is carried out by electrical resistance method using metallic split cartridge. Enormous nos. of trials were conducted to establish the procedure for local PWHT of header pullout to tube weld joints using metallic split cartridge. The temperature control was extremely difficult due to asymmetric shape and non-uniform mass of pullouts. Hence, it was decided to carry out PWHT of header pullout to tube weld joints along with PWHT of 12mm thick header weld joints..
After fabrication of complete cylindrical header, PWHT is carried out at 760±10 o C for 2 hours for 12mm thick longitudinal & circumferential weld joints. Then, welding of middle row header pullout to tube joint is carried out and complete header assembly is again heat treated at 760±10 o C for 1 hour soaking time. Subsequently, inner and outer row header pullout to tube welding is carried out and complete header is again subjected to heat treatment for another 1 hour soaking time. 12mm thick weld joints undergoes heat treatment for total 4 hours. Middle row header pullout to tube weld joint undergoes heat treatment for total 2 hours soaking time. Outer & inner row header pullout to tube weld joint undergoes heat treatment for 1 hours soaking time.
Final integration of M tubes with header bend tubes
Testing & Surface Treatment The tube side is subjected to pneumatic test at 8 bars gauge pressure. Neither drop in pressure nor leakage/deformation is acceptable After pneumatic test, surface treatment is carried out which involves degreasing, pickling, passivation by circulation of solutions. Subsequently, tube side is subjected to Helium leak test Global leak rate shall not be more than 6.66X10-9 Pa-m 3 /s Local leak rate shall not be more than 2.66X10-9 Pa-m 3 /s
CONCLUSION: The design, manufacture and construction of components should employ proven techniques and it should be possible to conduct analysis of the design as may be necessary for the purpose of demonstrating adequate integrity at any specified time throughout the plant life. The important fabrication rules are use of high standard of materials, use of high quality welding during all the stages of manufacture supported by a quality assurance program which ensures full approval of procedures and provides verification of compliance with the procedures & practices. Very high standard quality control and quality assurance during design, material procurement, forming, welding, fabrication, handling and testing has given confidence on trouble free service from Sodium to Air Heat Exchangers for the design service life of 40 years.
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