15 Miscellaneous pumps
15.1 Service water pumps
In every power station, there are a large number of pumps supplying service water to main plant items and as part of auxiliary systems, e.g., water treatment plant. These pumps can be classified under two separate categories, pipeline-mounted pumps and sump pumps.
Pipeline-mounted pumps up to around 0.1 mVs and 160 m head can be supplied as either horizontal end suction or vertical in-line designs. In both instances interchangeability is important and pumps are manufactured to standard dimensions. End suction designs are covered in BS5257 . These pumps should include a back pull-out feature which greatly assists maintenance activities with no disturbance of pipework during dismantling. The use of spacer couplings eliminates the need for removal of the motor during work on the pump.
Vertical in-line pumps are detailed in BS4082 . All pumpsets in this range are close-coupled, with the pump impeller mounted directly onto an extended motor shaft (Fig 4.67). This arrangement requires a special motor with a non-standard shaft and stronger bearings to carry the pump hydraulic forces. Although there is an initial cost penalty compared with the equivalent end suction designs, the vertical pumpsets take up less floor space and experience indicates they are more reliable and require significantly less maintenance. This is attributed to several factors, including the elimination of coupling alignment problems which can result in a reduced seal life.
Larger pipeline pumps are of the horizontal split casing type as described in Section 12 of this chapter.
In the past, sump pumps have been of a design in which the pump casing was suspended into the sump from columns, the impeller being driven from a high level motor through a long multi-bearing shaft system. This design caused severe problems when pump failure occurred, particularly that of dismantling, which is difficult and often very complicated.
Increasing use is now being made of submersible sump pumps (Fig 4.68), particularly for low and medium power outputs. Ease of withdrawal and replacement are good and reliability excellent, considering the arduous conditions often associated with sump drainage work.