Can a centrifugal pump operate at any speed?

Of course not; the speed range is related to the critical speed

For variable-frequency water pumps, as the frequency decreases, the rotational speed also drops, and the performance curve of the pump shifts downward. The manuals for such pumps provide performance curves measured at different frequencies; generally, the performance drops by half at 30 Hz. Variable frequency technology is intended to save energy, but at low frequencies it often fails to meet the required performance standards. Manufacturers usually recommend maintaining a rotational speed of over 50% of the rated value. At the same time, variable-frequency water pumps mostly use V/f control; at low frequencies, the motor torque also decreases significantly, though I’m not very familiar with electrical aspects. . . . . . .

We use pumps by referring to their performance parameters, rather than the speed; the speed is determined based on the requirements of those performance parameters. To reduce the specifications of hydraulic models, many common chemical process pumps can operate at four different speed levels: with 50 Hz 4-pole motors (1450 RPM) and 2-pole motors (2900 RPM), as well as with 60 Hz 4-pole motors (1750 RPM) and 2-pole motors (3550 RPM). This allows for different performance parameters to be achieved; variable frequency control as well as the ability to use any intermediate speed are available, but the desired speed is determined based on the required performance characteristics. If the speed exceeds the aforementioned range by too much, the performance parameters will deviate significantly from the original hydraulic design, making it unreasonable and uneconomical; in addition, it is also constrained by the critical speed mentioned regarding excessive foam formation.

Once the hydraulic model of the pump is established, it achieves the highest design efficiency at the rated speed. As the speed decreases, the efficiency gradually drops. It’s not that any speed can be used; rather, changing the speed also changes the performance curve.

Of course not; it’s necessary to follow the manufacturer’s standards.

Theoretically, it is possible, but at speeds that do not meet the requirements, this may lead to reduced efficiency, increased energy consumption, inadequate flow rates and outlet pressures. In the case of self-priming seals, low speeds can also cause seal failures. Therefore, the use of a pump actually requires a specific speed.