Flow rate

The flow rate (Q) of a centrifugal pump is the useful volume flow delivered by the pump via its outlet cross-section (see Pump discharge nozzle). Volume flow rates which are tapped upstream of the pump's outlet cross-section for other purposes (e.g. bypass) must be taken into account when calculating the pump's flow rate.

If the fluid handled is compressible, a conversion must be made based on the condition prevailing in the pump suction nozzle the arithmetic mean (Q_s+ Q_d)/2 may have to be applied.

The unit of the flow rate is m³/s; in centrifugal pump engineering the units m³/h and l/sare more common. Various measuring methods are used for flow rate measurement (see Flow velocity measurement). Various types of flow rate are distinguished in conjunction with the H/Q curve (see Characteristic curve). 

Flow rates and their significance

• Flow rate at BEP (Q_opt): flow rate at the best efficiency point at the rotational speed and for the fluid handled specified in the supply contract
• Nominal flow rate (Q_N): flow rate for which the pump has been designed
• Supply contract flow rate (Qcontract): the flow rate agreed in the supply contract (order confirmation) 
• Minimum flow rate (Q_min): minimum permissible flow rate at which the pump can be continuously operated without suffering any damage at the rotational speed and with the fluid specified in the supply contract
• Maximum flow rate (Q_max): maximum permissible flow rate at which the pump can be continuously operated without suffering any damage at the rotational speed and with the fluid specified in the supply contract 
• Peak flow rate (Q_peak): flow rate at the apex (relative maximum of H/Q curve) of an unstable H/Q curve (see Characteristic curve)
• Balancing flow  (Qbal) 
• Leakage flow (Q_Leak) 
• Suction-side volume flow rate (Q_s): volume flow entering the pump suction nozzle 
• Inlet volume flow rate (Q_e): volume flow entering the system's inlet cross-section 
• Discharge-side volume flow rate (Q_d): volume flow leaving the pump discharge nozzle
• Outlet volume flow rate (Q_a): volume flow leaving the system's outlet cross-section. See Fig. 2 Head

Occasionally, a so-called pump flow rate (Q/Q_opt) and a system flow rate (Q/Q_contract) are defined, where Q_contract is the flow rate specified in the supply contract.

A centrifugal pump's mass flow rate (ṁ) is: