The Structure And Durability Of Assembled Nozzleplates For Steam Turbines
Nozzle plates are key parts of steam turbine for power plants. Hot steam in generators flows into blades assembly to rotate steam turbines. Before high pressured steam gets into the blade assemblies, the direction of flow for steam air is controlled in order to maximize the efficiency of rotation of a steam turbine. Nozzle plates take this important role in steam turbines. These nozzle plates, composed of a lot of turbine blades and rings, currently are manufactured in modern industries by welding the blades with outer rings and inner rings of plates. Welding is a historical joining process and used for long periods of time in metal working industries because metal parts are melted and securely fastened together. However, it causes several problems for joining metal parts. First of all, the pitches between the welded neighboring turbine blades are not evenly spaced because the blades are moved or twisted while they are being under welded. These are led by heat deformation in the process of high temperature to melt the parental metals and welding rods. It is not easy to disassemble when some damaged blades are substituted with new ones as well. This brings about high cost of maintenance fee for nozzle plates. In addition, welding a lot of turbine blades with a plate takes much manufacturing lead time, which increases manufacturing cost. Therefore, this research suggests assembled nozzle plates which get rid of the problems described above. The fundamental premise for assembled nozzle plates is structural safety and durability for the new design. This paper technologically describes the way of structural design which is mechanically safe under being operated. The stress analysis helps to make guidelines for assuring structural safety. Modeling the nozzle plate with a 3 Dimensional CAD modeler is utilized for stress analysis. The working conditions of nozzle plates are under hot temperature of 374 degree Celsius and one Mega Pascal pressure with 3,600 rpm. Once the safety is proved with stress analysis, then a durability test is carried on for a next step. Fatigue analysis is proceeded to get fatigue life, fatigue safety index, and fatigue failure of a nozzle plate. The modeling the nozzle plate, the stress and fatigue analysis are carried with CATIA and Nastran, which are commercial CAE software. The stress level of the nozzle plate is found under safety limit of yield strength 373.696MPa of stainless steel 4140. The displacement from the test is within tolerance limit of nozzle plates. The fatigue life is calculated above 1.25×10^10 cycles at the worst part of the nozzle plate. This paper concludes that the suggested model of the assembled nozzle plate of stainless steel 4140 is safe in terms of structural analysis and durability as it is under operation in a commercial steam turbine.
Index Terms - Durability, Nozzle plate, Steam turbine, Structural analysis.