Les pompes alimentaires multicellulaires du palier 700 MW et au-delà

Multi-stage feed pumps for above 700 MW

¹ BERGERON S. A., Paris
² ALSTHOM-ATLANTIQUE-RATEAU, La Courneuve

Abstract

Introduction Electricité de France have always attached considerable importance to improving power plant equipment efficiency, as a result of which full-discharge feed pump outputs have shown a spectacular increase over the last ten years (see Table 1). With a view to further improvement of the availability of such equipment, EDF have now set themselves the two following additional standards : a) feed pumps must be able to operate for one minute under vaporizing conditions. b) Internal pump assemblies must be designed for rapid replacement, i.e. in a few hours. General feed pump design features Basic features Each output range is catered for by a single full-discharge feed pump set comprising the pump and its turbine drive, reduction gear and auxiliary feed pump on a common shaft assembly. Number of stages The one-minute vaporization requirement implies a rigid shaft, and therefore, no more than three stages, not exceeding 650 m. Hydraulic design Hydraulic characteristics Discharge/head curve Pump geometry must be designed for a stable, continuous downward discharge/head characteristic. Efficiency Since efficiency falls off with specific speed, excessive stage heads are undesirable. Efficiency also decreases if radial clearances are widened with a view to minimal risk of rotor/stator contact under vaporizing conditions (see Fig. 2). Cavitation problems The risk of erosion can be eliminated by determining the critical NPSH by visual observation of cavitation figures on a scale model. There is no general relationship between "industrial" (NPSH)c and critical erosion (NPSH)f' (see Figs. 3 and 4). Fluctuating pressure Clearance geometry between the impeller and diffuser requires very careful determination to reduce pressure fluctuations to a minimum, especially at low discharge. Hydraulic thrust Calculated data have been checked by model tests and corrected to allow for such factors as the effect of leakage or various radial and axial clearances, etc. Mechanical design General Even temperature distribution and mechanical stability of the impeller are most important design requirements. The active pump components are designed as a barrel assembly with mechanical shaft seals and hydrodynamic bearings with pivoting segments (see Fig. 5). Special features of pumps for above 700 MW Sub-critical rigid impeller assembly. Axial balance ensured by a barrel assembly and double-acting segment thrust bearing. Mechanical seals tested on a special EDF-designed rig (see Figs. 6 and 7). Special self-sealing boltless head cover fitting arrangement, allowing rapid dismantling of the internal pump component assembly (see Figs. 8 and 9). Conclusion The first pumps of this type were commissioned at the beginning of 1977. They have successfully passed one-minute vaporizing tests on site and proved satisfactory in subsequent service.