3.2.17 Double suction pump

Double suction pump

Double suction pump design was historically the predominant pump type for applications where pipe connections exceeded 200 mm. The classic design with double bearings, two stuffing boxes and axially split pump casing with the pipe connections in the lower half, se figure 3.217a, is now only used for special applications and for large volume flows. In figure 3.217a the pump type with double bearing design and in 2.217b with a single bearing.

The specific speed per pump impeller half for double suction impeller type pumps is less than that of similar single suction pumps and this results in a corresponding flattening of the Q-H curve. In borderline cases the NPSH value can be improved which means that higher speeds can be used. In special instances where it is desirable to maintain as near pulsation free flow as possible, as for example fan pumps for paper machines, double suction pump impellers with off set blades on both sides of the impeller are used. Sometimes the location of the pump connection flanges can make the use of this pump type advantageous. The double bearings and the axial splitting of the pump casing has in recent years been gradually replaced by single bearings with radially split pump casings, see figure 3.217b, the basic reason being that only one shaft seal is necessary and that the manufacturing costs are lower.

Double suction with bearings assembled on each side are very robust and last longer than single suction pumps. The double suction design also eliminates the axial forces on the impeller when one side effectively cancels out the axial thrust of the other side, see Figure 3.217c, which allows for higher flows compared to a single suction impeller. Flows on both sides are however not always perfectly even why inboard and outboard bearings are needed to absorb the load which is not balanced out.

Double suction impeller as in Figure 3.217c consists of two back-to-back impellers, from which the water flow into a volute. In general one double suction pump works just like two same diameter end suction pumps in parallel, that is, volume flow can be twice that of an end suction pump with the impeller dimensions.

The inlet (suction) and outlet (pressure) flanges of double suction pump are in the same direction, perpendicular to the pump shaft, which might be good from a layout and installation point.

A double suction pump can be twice the cost of a comparable single suction design. Initial purchasing cost is an important factor when choosing a pump, in addition, single suction pumps are generally easier and less expensive to maintain. However, double suction pumps last longer and are more reliable which should be taken into account both from an economical LCC perspective but also from a reliability point of view, especially for applications with large impact on the over all process. A factory forced to close it’s operations for a few hours because of a pump failure can be money well spent for a more expensive but reliable pump. The pump type is mainly used in waterworks, pump stations, power stations, chemical industry, mining, fire fighting and shipping. It is typically recommended for high flows, starting from 500 m³/h (+2,000 GPM) and pipe connections exceeding 200 mm (8 inch).