Hello, you must restrict the flow by closing a valve (e.g. globe valve) immediately after the centrifugal pump outlet. Thus, you can measure the fluid pressure at the pump outlet and the flow rate (with a Pitot tube, for example) to build the H vs. Q diagram at several operating points. The electric motor must be controlled to maintain constant speeds.

Thank you very much dear Alexandre Schalch Mendes but why we do not put that valve at the inlet of the pump to control the inlet flow rate of the pump? This is what makes me confused!

Thanks

Dear @Ahmed S. Mansour, if you install the control valve at the inlet of the pump, the restriction of the flow will reduce even more the inlet pressure, what will generate the cavitation phenomen in the pump rotor.

Install a control valve at the discharge (outlet) zone.

Thank you dear Alexandre Schalch Mendes ! Now I get it

Thank you dear Barhm Abdullah Mohamad .