Centrifugal pump pressure problem

Hi all!

I hope you can help me to find some answer. It is a real life problem. So we have a magnetic clutch centrifugal pump. 6 cubic meter/hour and 3 bar of pressure.

When we use it in full bypass - from the vessel back into the vessel - its operating pressure is 3 bars. Everything is alright. Vessels and hence bypass height is 4 meter tops. But when we change from full bypass to full carry pressure drops in 1 bar and flow rate maybe... 0,1 cubic meters/hour.

Pipes are clear, we checked and replaced the pumps head with the magnet and the impaller (not the electic motor) and it is still not working properly. Full bypass still 3 bars pressure, but full carry 1 bar and almost no flow rate.

Our mech engineers says it should have to work, because if there is pressure then there have to be flow.

So... any ideas?

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Thx all!