Pump selection dilemma – one pump for two operating conditions
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At present, we need to select such a centrifugal pump (magnetic drive pump) equipped with an inverter-driven motor, one that can accommodate the following two operating conditions (or two operating points): 1) When the flow rate is 57.6 m3/h, the head required is at least 20 m； 2) At a flow rate of 24 m3/h, the head requirement is at least 5.5 m. These two operating points were calculated based on the current layout of our piping system, at two different flow rates, to determine the pump head required to overcome the resistance losses throughout the piping system; this helps us decide which pump would be the best choice. The device is rather special; it requires a pump, with frequency conversion being used to achieve this. Can everyone do this? Is this pump good to use?
Sure, the flow rate is proportional to the rotational speed, while the head is proportional to the square of the rotational speed. With a flow rate difference of about 2 times, the head difference is around 4 times – which is completely fine
You can’t get out of this curve; you can only change the frequency
Thank you. Are there any precautions to keep in mind when using this pump? There are no issues regarding lifespan or anything like that, right?
These single-stage cantilever pumps all feature rigid shafts, with relatively high first-order critical speeds; therefore, the speed does not approach the critical value even after frequency conversion. Just operate it normally
In the case of magnetic pumps, the cantilever is shorter, as the coupler used in such pumps needs to be selected according to the first method. The power transmitted through magnetic means is relatively high; if a metal isolation sleeve is used, attention must be paid to the heat generated by eddy currents. At low speeds, it is important to determine whether the internal circulation can remove this heat – if it can, then there is no problem with the pump. If the heat cannot be removed, it will cause the medium’s temperature to rise, and high temperatures can lead to the vaporization of the medium.
After the speed is reduced, the eddy current heating of the isolation sleeve also decreases; under normal circumstances, there will be no issue with heat not being dissipated. Unless there is a problem with the design of the internal cooling circuit.
Magnetic pumps are not suitable for operation at variable frequencies due to the presence of magnetic eddy currents
It is not recommended to use magnetic pumps, as the medium being transported needs to lubricate the bearings. I’m not sure about the frequency of adjustment for your two operating conditions – is it done manually or through automatic timing-based adjustments?