14.3 Advanced class feed pump construction
The pump casing consists of a forged steel barrel to which are welded the suction and discharge branches. Extensive areas have stainless steel cladding to give protection against erosion. Supporting feet are arranged at the centreline to give positive restraint to movement upwards and downwards and yet allowing for free thermal expansion. Sliding keys control thermal movement and maintain alignment with the drive.
The pumps have all their bearings, both thrust and journal, in housings supported off the pump casing or internal cartridge. This makes the pump more immune to the effects of pipework loads and minimises the effects of shaft misalignment.
The 'cartridge' inner pump assembly, complete with integral bearings, is designed so that it can readily be transported to a suitable maintenance area as a package unit. The cartridge components are pre-assembled on a manufacturing jig so that no subsequent adjustment is necessary to ensure component concentricity, and minimal adjustment is required to achieve the necessary degree of coupling alignment after the cartridge is installed in the pump barrel.
On insertion into the barrel, the suction and discharge ends are slid apart and located at their respective ends of the barrel. Three joints are made during this operation; the suction ring section and main cover joints. Although different designs of pump have been adopted (Figs 4.56 and 4.57), cartridge replacement follows the same strict methodical procedure.
To ensure rapid cartridge changeover, the normally heavy externally-bolted discharge cover arrangement has been replaced by an internal self-sealing high pressure joint system. Rapid replacement of the cartridge necessitates the use of specially designed withdrawal gear as illustrated in Fig 4.58.
The impellers are of high quality precision-cast stainless steel, with the castings subject to extensive radiographic inspection and stringent dimensional control to ensure repeatable cavitation performance. The first stage impellers are designed to have a cavitation life of 45 000 hours when operated in the range 80-100<Vo of the rated now.
Diffusers can be either axial or radial; both arrangements have been successfully used on advanced class feed pumps. The inclusion of axial diffusers avoids the generation of large pressure pulses between the impeller and diffuser blades, which on these high head per stage machines could constitute a fatigue hazard. Alternatively, radial diffusers result in better head/flow stability characteristics. They also give a slightly higher peak efficiency than the axial design, although this can be masked by the effects of gland leakage losses.
Forced cooling of feed pumps has been provided on stations where there is not full standby capacity, and where power generation is lost following the outage of the main 100% feed pump. Forced cooling reduces pump downtime and enables the machine to be returned to service and full generation to be restored, as soon as possible.