14.9 Pwr feed pumpsets
The feed pump generated head requirement on a water reactor unit is in the region of 900 m. This is considerably lower than that necessary on a fossil-fired station and allows the use of a single-stage double-entry pump. Changing to the simpler double-entry design, eliminates the need for any balance drum to react against large impeller loadings.
This leads to an improvement in pump hydraulic efficiency compared with the equivalent multi-stage pump design, and a reduction in internal leakage losses.
The adoption of a 3 x 50% arrangement, with booster pumps driven in tandem to provide the necessary NPSH, is in line with overseas practice. The use of fixed-speed motors and fluid couplings is well within the experience of plant already operating on starting/standby duties on CEGB fossil-fired stations.
Double-entry feed pumps (Fig 4.65) can retain all the features of advanced class construction, i.e., a stiff shaft, 'cartridge' design, with vapour lock and thermal shock capability. The provision of mechanical seals is intended to minimise leakage in the running and shutdown modes without prejudicing pumpset reliability. The seals themselves are designed to have a minimum life of 15 000 h between overhaul. A 'cartridge' seal assembly reduces downtime for any seal maintenance and minimises installation type faults.
To prevent any axial shuttling of the shaft, which could damage the mechanical seals, a deliberate impeller wear-ring diameter differential is introduced to create a known unbalanced unidirectional thrust which is taken by an external thrust bearing. To cater for radial loadings acting on the pump, the casing designs can either include radial diffusers or are of the double volute type.