Test Protocol for Accelerated Creep and Transient Thermo-Mechanical Testing of Super Critical Steam Turbine Rotor

Rotor constitutes one of the critical components of steam turbine. Steam turbine rotor undergoes severe thermal transients during start up and shut down. When steam turbine starts up, temperature of rotor surface gradually raises with the incoming steam temperature. During this process temperature rise of central part lags behind that of rotor surface, resulting in extreme compressive stresses developed within the rotor. Similarly the rotor surface is subjected to tensile stresses during the cooling down in the shutdown process. After a certain number of cycles, the surface or core of rotor may produce cracks due to Low Cycle Fatigue (LCF) phenomena. For a Steam turbine unit which works under very high pressure and temperature, the working environment of rotor is severe. It is required to test the rotor in rotating condition in a high temperature spin rest rig before putting into actual operation. To test the turbine rotor in a high temperature test rig, it is required to simulate the radial temperature gradient in order to achieve the required thermal stress in the rotor. No standard test protocol exists for the accelerated LCF testing for the rotor. A test protocol is developed for Accelerated Creep and Transient Thermo-mechanical testing of steam turbine rotor at high temperature taking clue from the existing ASME standard. This paper presents a specially formulated accelerated LCF test protocol, developed with inspiration from ASME standard [1] and creep parameters derived based on Larsen Miller Parameter (LMP) [2]. The test protocol arrives at temperature difference between rotor surface and bulk mean temperature in the solid rotor considering equivalent cold starts obtained from actual cold, warm and hot starts. It is considered that this simulates damage as envisaged in the life time of actual rotor but within a short/feasible time duration. Test protocol aims to arrive at a temperature difference between test rotor surface and bulk mean temperature during transient thermal loadings. On arriving at steady-state temperature the test rotor undergoes creep phenomena. The principal aim of test protocol is to arrive at a suitable test cycle consisting of transient thermal loading, steady thermal loading followed by transient cooling. This paper describes a test protocol for given number of cycles and operating temperature and equivalent cold starts such that the accelerated testing is completed within a reasonable time frame. This requirement is to address the need for validation of rotor for high temperature super critical steam turbine for higher efficiency.