STEAM TURBINE DIAPHRAGMS RE-DESIGN AND MANUFACTURING CHALLENGES

This paper illustrates a recent project to resolve the issue of unavailable major spare parts for one of the heavy duty steam turbines utilizing reverse engineering principles in order to remanufacture the critical components. Steam Turbines are critical prime movers within the company's various hydrocarbon and refinery plants. Single stage turbines are used for light duty applications while multi stage steam turbines are used to run large gas compressors or pumps. Many of the steam turbines have been running for more than 40 years with well-established designs and technology. For the large multistage special purpose steam turbines, the spare parts such as casing, rotor, shaft and diaphragms are not shelf items and usually will take over 12 months to manufacture and supply by out of kingdom manufacturers which compromises the plant productivity and reliable operations, not to mention the high costs involved. Mechanical Services Shops Department, a machinery services organization within the company, was approached by one of the operating plants to overhaul a 6-stage steam turbine due to low performance and as part of a planned plant T&I shutdown. Upon dismantling, excessive pitting, erosion and micro-cracking was found on all 5 stages diaphragms, nozzle ring and bucketing diaphragm which led to the decision of replacing all diaphragms. Due to the unavailability of spare parts and long lead time, it was decided to reverse engineer these critical spare parts locally. Material of construction analysis were performed in order to understand how the original parts were manufactured. FEA study was conducted on various designs alternatives and materials to investigate how different types of materials are affecting the diaphragm ability to handle thermal and mechanical stresses and deformation. Accordingly, materials and design were selected based on the study outcome which showed 48% Improvement in stress load capability and 27% greater resistance to deformation due to thermal expansion compared to the original design. 3D scanning of the old parts was performed in order to create 3D models which are a digital replica of the existing parts. These models have been modified in order to compensate for the material lost due to the excessive erosion. Detailed manufacturing drawings were developed and manufacturing method of the new parts was changed from fabrication and welding of different components to a more modern method of 5 axis CNC machining from one single billet of forged metal.