Within what range can the flow rate and head of a centrifugal pump vary while keeping the current within acceptable limits?

The upper limit should not exceed the set value, and the lower limit should not cause the pump body to overheat; you can decide it as you like: lol. To put it more precisely, the pressure and flow rate are adjusted using the pump’s outlet valve. When opening this valve, the rated current should be used as a reference point. After the pump is started, adjustments are made such that once the pressure reaches its maximum value, the flow rate to the system is increased gradually by opening the outlet valve more widely. As the outlet valve opens, the pressure will decrease, as the pressure in the system is not necessarily at the outlet pressure level. It is important to ensure that the pressure does not drop below the rated value; wait until the system pressure starts to rise again before continuing to open the outlet valve further. As the outlet valve opens more, the current also increases gradually. When the current reaches the rated value, that point represents the pump’s maximum flow rate and maximum load, and this is the maximum opening degree for the outlet valve.

There is no standard for this; when calculating, care should be taken to ensure that the resistance is not significantly different from the head

There are no relevant standards; it mainly depends on the performance curve of the pump at the time of manufacture. You can specify what performance requirements the manufacturer should meet, API-610

There is no standard for this; to avoid exceeding the current limit, it is necessary to reduce power consumption. Generally, there are two options: sacrificing flow rate or head pressure

Refer to the pump’s performance curve diagram; for operation, see the operation manual