12.3 Vertical pumps
12.3.1 Vertical metal-casing pumps
Volute casing designs have been used by the CEGB for pumping duties up to around 8 mVs. On low head coastal applications, standard gear-driven units have been installed, while on the higher head inland cooling tower applications, it has been possible to use a direct motor drive while still retaining the same equivalent specific speed as the equivalent gear-driven units.
Alternative bowl pump designs have been used extensively on overseas applications. With this design, the pump forms part of the piping and its intake is from a relatively unsophisticated suction chamber; it generally has a smaller civil engineering cost than the equivalent concrete volute design. There is no complicated volute to construct, and as the pump has a smaller overall diameter than the concrete volute, the pumphouse floor area is smaller. On the debit side, the bowl pump arrangement has a much higher mechanical pumping equipment cost.
Maintenance of the bowl pump is also difficult and the crane normally has to be sized to take the total pump weight for installation and overhaul. As the pump is withdrawn vertically for major overhaul work, a suction isolating valve is not required. This however leads to a relatively long pump to facilitate vertical isolation. One major technical disadvantage of the bowl pump is the need for submerged shaft bearing bushes. These are not readily accessible and can have a relatively short life in silt-laden water.
Casting problems have tended to limit the size of both the volute and bowl pump designs. The standard use of cast iron necessitates the inclusion of substantial allowances on coastal units to allow for corrosion/erosion effects over the life of the station.