14.10 Future trends
The CEGB has placed great emphasis in recent years on design to achieve maximum availability. This has led to the provision of robust advanced class pumps with 100% standby capacity and substantial spares provisions. The alternative overseas policy, by comparison, has tended towards low pump costs and a premium on high hydraulic efficiency.
This has necessitated very tight internal running clearances and resulted in pumps having less margin against rubbing and seizure than the advanced class design, and an inability to withstand vapour lock or thermal transient conditions.
A comparison of operating experience has demonstrated the effectiveness of providing rugged reliable feed pumps and highlighted the strong financial incentives of installing spare capacity to maximise feed pump availability. It is now evident that overseas ultilities are moving towards CEGB practice.
When determining standby requirements, the current trend is to provide 3 X 50% pumpsets. This arrangement meets the basic objectives listed in the introduction to this section and achieves them on larger fossil-fired plant without any excessive extrapolations of existing technology, by taking maximum advantage of UK manufacturer's international experience to produce reliable plant.
A 3 x 50% configuration with quick-start variable-speed motor-driven units represents a low capital and through life cost option for a practical plant arrangement. It also provides a simple layout with complete interchangeability. The advantages of this form of drive compared with the fixed speed motor/fluid coupling package or the turbine drive options, are fully discussed in Chapter 2.
In assessing feed pump designs for any new generation of larger fossil-fired units, it is envisaged that feed pumps incorporating advanced class concepts will be continued, but with a limitation on impeller speed to minimise risks of impeller erosion during off-duty operating conditions. A likely economic solution is a three-stage machine with booster pump, resulting in a speed where both condensing steam turbines and inverter-fed variable-speed motor drives become viable options. By reverting to a three-stage design (Fig 4.66) and decreasing the head per stage, there are further benefits to be gained in terms of a relaxation of pumpset NPSH requirements. This can lead to a drop in the necessary height of the de-aerator with consequent reductions in station civil costs.
The development of lightweight hydraulic ten-sioning gear for high tensile bolting permits the re-introduction of bolted cover joints in place of the self-sealing joint arrangements, without significantly affecting the time to change cartridges. An additional benefit from the production viewpoint is that the very tight control of axial tolerance build-up on all components relating to the self-seal joint, which is necessary to achieve complete interchangeability of cartridges and barrels, is relaxed. There are also major cost savings to be obtained from the simpler bolted discharge cover incorporating spiral-wound sealing gaskets.