On the Prediction and Theory of the Temperature Increase of Low Pressure Last Stage Moving Blades During Low Volume Flow Conditions, and Limiting it Through Steam Extraction Methods

Abstract

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade. Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the last stage moving blade increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of last stage moving blades continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effect of geometry, mesh density, turbulence model and time dependency. The CFD results were verified against steam turbine data from a test facility. The proposed complimentary solutions to limit the temperature increase include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

Numerical investigation on flow instabilities in low-pressure steam turbine last stage under different low-load conditions

“…Further research by Sigg and Rice 12 showed that the system RI was influenced by the exhaust shape. Nonsynchronous unsteady aerodynamic excitations in the last stage were also found by Liu et al 13 and Tanuma et al 14 Besides the flow fluctuation, Beevers et al 15 and Mambro et al 16,17 found that in this region, due to lack of axial flow, heat was difficult to dissipate through convection, and the temperature of the flow increased abnormally, which caused the LSMB tip and the casing to be in high temperature.…”

Analysis of Geometric Errors of Throat Sizes of Last Stage Blades in a Mid-Size Steam Turbine

“…Baseload thermal power plants must operate under variable loads due to wind and solar energy share. Steam turbines are subject to severe operating conditions [5,6], and rotor last stage blades can operate in a compressor mode delivering energy to the working fluid in low volume flow conditions [7]. Concepts of a dynamic power plant and solutions for an operational flexibility enhancement appear more often in the literature [8][9][10][11][12].…”