# 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 (Qs+ Qd)/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 (Qopt): flow rate at the best efficiency point at the rotational speed and for the fluid handled specified in the supply contract
• Nominal flow rate (QN): 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 (Qmin): 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 (Qmax): 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 (Qpeak): 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 (Qleak)
• Suction-side volume flow rate (Qs): volume flow entering the pump suction nozzle
• Inlet volume flow rate (Qe): volume flow entering the system's inlet cross-section
• Discharge-side volume flow rate (Qd): volume flow leaving the pump discharge nozzle
• Outlet volume flow rate (Qa): volume flow leaving the system's outlet cross-section. See Fig. 2 Head

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

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