A combined method of pumps control.

A combined method of pump control.
 
This method unites both methods when the overall flow is controlled by the number of working pumps and by the speed of one or all pumps.
 
This is the most flexible method which allows providing an algorithm for pump operation.
 
The overall characteristic of two pumps one of which is working with the reduced speed. 
 
Speed control and parallel operation
Pump station includes two same pumps.
One pump has VFD second pump can operate only with at constant speed (1450 rpm). 
Control station has an algorithm of supporting constant pressure (head) in a system.
Flow can change from Qmin to Qmax. 
How can this pumping system provide this algorithm?
When a system curve has position #1 one pump operates and VFD reduces it's speed to 1300 rpm  so that a pump's curve intersects system curve at point Qmin H.
Note that the duty point lays within the allowable operating range. 
 
If the system curve takes the position #2 (It means that the flow requirement has increased). VFD increases the speed up to 1380 rpm.
The maximum flow that can be provided by the single pump is Q3. Duty point is close to the right border of the allowable operating range and the pump operates at the nominal speed 1450 rpm. 
If flow increases further then to provide required flow the second pump
turns on. It operates at the nominal speed and the second pump operates with necessary speed to provide required flow, point #4.
The maximum flow can be provided by both pumps at nominal speed 1450 rpm, point #5.
To provide more flexible algorithm and exclude possible unstable operation during connection or disconnection of pumps recommended to use VFD for every pump.