Replacement and repair of nozzle stub of concentrated downcomer in steam drum

For the problems existing in the replacement and repair of nozzle stub of concentrated downcomer in steam drum, such as displacement of the downcomer, high cold crack sensitivity of welded joints and deviation of the maximum temperature from the weld during post weld heat treatment(PWHT), the corresponding technical schemes were formulated through theoretical calculation and analysis. The results showed that taking cylinder of steam drum as the load-bearing body and adopting the well-shaped suspension and fixing device could effectively prevent the excessive displacement of the downcomer in the process of cutting and repairing nozzle stub, and avoid staggering and folding of weld. Using asymmetric K-groove, reasonably arranging welding sequence of fillet joint and butt joint, increasing the preheating temperature and interlayer temperature to 180^\circ \mathrmC, and carrying out intermediate eliminating hydrogen heat treatment, the cold crack and other defects of BHW 35 steel joint under the condition of large thick wall and high restraint were effectively avoided. The reasonable temperature field of PWHT was obtained with the main heating of the stub fillet weld and the auxiliary heating in the upper area of the steam drum corresponding to the downcomer pipe hole. After repairing, all inspections and tests were qualified, among which the hardness of the drum cylinder had no significant change from that before repair and the maximum residual stress measured in the repair area was 133 MPa.