| dc.description.abstract | In the last few years, the installed capacity and production of electricity from all the renewable energies have significantly risen, supported by an ever-increasing spread of new energy policies all around the world. Because of that, thermoelectric power plants suffer of high variability: the continuous starts & stops per day of the steam circuit, instead of the reliability during continuous operation, become the main requirement for typical boiler feedwater pump. The most critical item of the steam circuit is the Boiler Feedwater Pump (BFWP): it is typically a multistage, between-bearing, single-casing ring section or double-casing barrel pump, designed for power rating up to tens of megawatts and water temperatures in the range of 150 °C (302 °F) and 210 °C (410 °F). The most critical condition that the pump can face during its life is the so-called “Cold Start-up”: there can be extreme and exceptional conditions where the pump is required to promptly start-up from a cold state (site temperature, say 20/30 °C, 68 °F/86 °F) with the water at its maximum temperature (210 °C, 410 °F). Those conditions are extremely unfavorable for the pump: transient and alternating thermo-structural stress can lead, on one side, to the complete closure of the running clearances of the rotor and, on the other side, to fatigue stress of the tie-rods. The running clearances of the pump, as for all turbomachines, are required to be as tighter as possible to reduce the power consumption, and this is of primary importance for a high-power pump. But, on the contrary, the tighter the running clearance, the more the risk of rotor seizure during the cold start-up. When the water at 210 °C (410 °F) runs over the pump at stand-still, the thin-walled components, such as the impellers and the rotating wear rings, expand more rapidly than the thicker-walled components, such as the stator that holds the statoric wear ring. The numerical approach of the finite element analysis (FEA) is used to find an optimal compromise between two conflicting needs: having a high efficiency pump (with the running clearances as tighter as possible) without the risk of pump seizure during the cold start-up, through the fulfillment of a defined criterion. The continuous starts & stops cause also the pump tie-rods to be subjected to high-cycle fatigue, increased by the transitory thermal Copyright© 2021 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station load. Thus, the finite element analysis also verifies that the minimum design life of 30 years for the tie-rods is guaranteed. The subjects of this paper are a barrel pump (BB5, as per API 610 designation), 6 stages with maximum power 15 MW and a ring section pump (BB4, as per API 610 designation), 8 stages with maximum power 4.5 MW running with the maximum water temperature of 210 °C (410 °F). The former is the typical Boiler Feedwater Pump used for coal-to-fire power plants in configuration 3x50%, the second for a combined cycle power plant in configuration 2x100%. Outcomes of the simulation of the cold start-up are the axial deformation and displacement of the rotor, the radial deformation in close proximity to the running clearances and the stress of the tie-rods. | |
