14.6 Pump layout and drive - part 1
The increasing head per stage in the high head boiler feed pumps has resulted in the need for an NPSH well in excess of that available, even from high level de-aerators. This can be provided by slow speed booster pumps, driven either separately or in tandem with the pressure stage pump.
Separately-driven booster pumps necessitate the use of a common discharge manifold to ensure that at all times the pressure stage pumps, with their independent prime mover, have sufficient NPSH to prevent vapour locking in the event of failure of one of the working booster pumps. This arrangement (Fig 4.62) also requires any stationary booster pumps to be protected with non-return valves, and the pressure stage pump leak-off valves to be closed when the pump is standing to prevent recirculation back to the de-aerator.
Tandem sets (Fig 4.63) require less complex pipework arrangements, less complex feed systems and take up less turbine hall space. Being dependent on a single drive, they are also more reliable than the arrangement with separately-driven booster and pressure stage pumps.
For part-load operation, a fixed-speed booster pump is preferred as it enables the NPSH to be maintained as the load is reduced. Where pumps are to be capable of operation well in excess of the rated duty, variable-speed booster pump would provide better cover for the pressure stage pump, although this may require some increase in the NPSH available to the booster pump itself.
The choice of unit often dictates the basic type of drive to be employed. On 660 MW units, because of start-up problems, motor drives (even when supplied from an 11 kV supply) are unacceptable on technical grounds for pump trains substantially greater than 60% MCR duty. Thus any 100% pump group has to be driven by a steam turbine.
In assessing the number and capacity of pumps to be employed, normal CEGB practice on fossil-fired units has been to ensure that failure of a single pump train must not impair the start-up of a unit or affect output. This has led to the installation on most stations of a 100% turbine-driven set, plus 2 x 50% motor-driven sets.