Adding to some of the comments and directly to your questions:
1. You've stated one rated duty - are there other duty points on your application? Normally you'd need this sort of arrangement when there are very low flows and you have to get them with the same pump. If this were the case and...
Recirculation acts as a hydraulic excitation force into the fluid adding energy in order to achieve a stable Q-H curve. There are articles and discussions on the internet about it.
You should to get certain amount of recirculation at the inlet of the impeller whilst at part-load conditions - as...
The absorbed power of the pumps you are offering will fall then in between 11-50 kW: +15% tolerance - from what you've written the VDMA rules state.
API-676 Table 8.3.6 highlights a tolerance of +4% for the rated power - see from below link...
Ideally you would need to check with Saudi if they are willing to accept the efficiency levels that come with the VDMA tolerances in power that you mention, i.e. a maximum of %25 to values inferior of 5 kW
The rated capacity tolerance that you say is contained in API-676 is 15% maximum - does...
At and around BEP the flow is symmetric through impeller walls and volute channels. As you most likely know, at part-load conditions you get separation of the flow and recirculation both at discharge and inlet of the impeller. It might be that the CFD model that you are using is not picking up...
It seems as if the operating range shown in the curves you've posted set the limits to the minimum and maximum flows for which the pump can operate safely. So, like you said, I would go for the maximum reading of the operating range to determine the maximum possible flow.
You'll need also to...
There is normally a percentage value further to the BEP flow until where the pump can run safely.
What is the application that you are having? Are you able to share rated head, flow, speed? If you can post a pump curve it would also help.
Normally with a centrifugal pump efficiency tends to drop towards part-load (lower flows of BEP) and also at overload (higher flows of BEP).
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