Pump flow rate

Depending on the interaction between a selected pump, its performance curve and a specific installation situation, the so-called operating point is determined at the intersection on the Q/H diagram. This results in a specific pumping height and flow rate. During further design calculations, it must be verified whether specific velocities are maintained in the pressure pipe. 

• At least 0.7 m/s to prevent sedimentation 
• Maximum 2.3 m/s to avoid high friction losses 

Furthermore, the Qp at the operating point must be greater than the flow rate at the inlet Qzu. If these requirements are not met, either the lifting station or the pressure pipe must be adjusted – until the required limit values are met. 

The pump performance curve – usually presented as a graph or a table – indicates the pumping capacity Q, also known as the volumetric flow rate, delivered at a specific pumping height H.

The system characteristic curve describes the relationship between the flow rate Q and the pumping capacity H for a specific pipeline. Two factors must be taken into account: 

• The geodetic head H_geo, which is determined by the difference in elevation between the base of the collection tank and the base of the backwater loop and is always constant
• The head loss H_v, which is determined by the pipe dimensions and fittings and increases continuously as the flow rate rises 

The operating point is now the point of intersection between the system characteristic curve and the pump characteristic curve.