9.5 Series pump operation
Series pump operation
Series pump operation is utilized when high delivery heads are required. The series connection of pumps is especially applicable for high delivery heads in combination with comparatively small flows. By dividing up the delivery head between more than one pump, a higher specific speed (nq) is maintained for each individual pump, thereby increasing efficiency.
For more information on specific speed (nq) see section 3.1.7 Pump specific speed >>>
Figure 9.5a Maximum efficiency for centrifugal pumps at various specific speeds.
Mufti-stage pumps are compact, integral forms of series connection and can be considered as several (No. of stage per pump) pumps operating in series but with only one casing. Each stage adding energy (head) to the fluid.
Systems incorporating a number of pumps, where the pumps are situated in close proximity to one another are simply dealt with by calculating a common performance curve for the complete pump package. The common pump curve is then compared with the system curve. For series connected pumps the resultant Q-H curve is obtained by adding the actual delivery heads for each value of volume flow (figure 9.5b).
Figure 9.5b Resultant Q-H curve for series connected pumps.
The resultant efficiency at various flows is calculated from the relationship.
When two or more similar pumps are connected and operates in series the same efficiency curve applies for the “package” as for each separate pump.
Figure 9.5c Pump nominal duty points when series connected.
The same flow passes through both pumps. The inlet pressure for pump 2 is the pump outlet pressure of pump 1. The total amount of energy added to the fluid, will be the sum of the total head of both pumps.
The pump pressure rating and shaft seals can thus be affected. The axial forces, which are dependent on the differences between the inlet and atmospheric pressure, will also be different for the two pumps.