Pump operating point or duty point is the intersection of pump characteristic and piping system characteristic.

The pump will always operate where the system and pump curves intersect.

At that point, there is a balance between what the system demands and what the pump can deliver.

Operating point of a pump

If duty point was defined on the Q-H curve, it is possible to define other pump parameters Power,

Efficiency using pump characteristics.   

Operating point of a pump

How does the operating point move along the pump curve during changes of a system curve?

Throttle control of a pump
Duty point at throttle control of a pump

Depending on the valve position the system curve changes and duty point moves along the pump curve.

Pump allowable operating range is the range on the Q-H curve within which operating point must be to provide effective and reliable pump operation.


Allowable pump operating range is defined by a pump manufacturer for every pump model. But usually, the allowable operating range for centrifugal pumps is defined as 0.7 - 1.1 x Qbep.


Pump operating range is shown on the Q-H characteristic by the Qmin and Qmax lines.

Only if the operating point is placed in the operating range the pump manufacturer guarantees effective and trouble free operation of a pump.

Operatig range of a pump

Overloading of a pump means that a pump operates at the duty point which lays further than the right border Qmax of allowable operating range.

It is called overloading of a pump.

Allowable operating range of a pump

What happens if the pump operating point is beyond the allowable operating range? 

It is obvious that the pump will operate with low efficiency which causes extra power consumption.


But the fact that the operation beyond the allowable operating range will reduce the pump reliability is not obvious for many pump users.


The diagram below illustrates how the pump reliability depends on the position of the pump operating point.


The BEP, or best efficiency point, is the point at which the pump operates at peak efficiency. At the BEP, the flow enters and leaves the pump with a minimum amount of flow separation, turbulence, and other losses.


Pump failure is often associated with the low quality of a pump in the conscious of many pump users.

But in reality, this is the consequence of deeper problems.