The parameters of centrifugal pumps are basically the same, so why is the flow rate different? Please analyze it!

The parameters differ so much – and you still say they’re basically the same????

These two pumps merely have the same power and speed of the driving motor, while their parameters such as head and flow rate differ significantly.

The specific speed of a centrifugal pump is independent of the properties of the liquid being transported, and is closely related to the shape of the impeller and the shape of the pump’s performance curve. Pumps with a higher specific speed correspond to a larger flow rate and a smaller head at the point of highest efficiency ； Pumps with a low specific speed, on the other hand, are suitable for lower flow rates and higher head pressures. When the outlet diameter of the pumps is the same, if the flow rates of the two pumps are similar, the pump with a lower specific speed will have a lower head and higher shaft power consumption. Generally, in centrifugal pumps with a lower specific speed, the impeller has a narrow outlet width and a large outer diameter, and the flow channel formed by the blades is narrow and long. If the specific speed is high, the impeller outlet is wide and the outer diameter is small, then the flow channel is short and wide.

If you look at the calculations for selecting the pump, you’ll understand why. Besides, the data you provided aren’t even roughly the same; the differences are too large

Flow rate, head, rotational speed, power requirement, impeller diameter: Pump 1: 1800 m3/h, 37 m, 1480 r/min, 315 kW, 388 mm; Pump 2: 1400 m3/h, 58 m, 1480 r/min, 315 kW, 440 mm. Please pay close attention – what the original poster is asking is why the power is the same while the flow rates differ. As can be seen from the curve of centrifugal pumps, it is parabolic in shape; flow rate is inversely proportional to head, while head is directly proportional to the impeller diameter. Thus we can understand why these parameters vary. Even with the same power, we can determine this by comparing the curves carefully, or by applying the law of conservation of energy.

I don’t understand how what was said on floor 4 relates to the comparison of the rotational speeds between these two pumps Factories rarely deal with specific speed; it is only used during the design phase

Thank you for all of your analyses and explanations! I learned it!

Haha, I’ll explain it in another way: the criteria used by the poster on floor 1 are power and rotational speed, with rotational speed being the prerequisite and power being the result. 2 Regarding the definition of power, under similar conditions of structure, efficiency, and density, different combinations of flow rate and head can yield the same power, which is one of the reasons for the differences. The author’s sudden-turn questions are another example: the combination of 1800 and 37, and the combination of 1400 and 58. Put simply: within a certain period of time, when a child runs up a tall building and an adult goes down, their power output is the same. In question 3, the difference in the diameter of the impeller is one of the key reasons for the differences in performance; imagine that as the speed at the edge of the impeller increases, the centrifugal force also increases, resulting in a significant rise in head pressure.

According to the cutting law, head is proportional to the impeller diameter； The flow rate is proportional to the impeller width; the wider the impeller, the greater the flow rate

Use this calculation formula to find out why: shaft power = density × gravitational acceleration × flow rate × head / 1000 × efficiency, where density is in kg/m3, flow rate is in m3/s, head is in meters, and power is in kw

Please take another look at this material: Centrifugal Pump Characteristic Curves and Their Applications (with illustrations). Open this link to download it: http://bbs.hcbbs.com/post.php?action=newthread&fid=286&extra=page%3D1

Hah, this MM is right. In fact, for pumps of the same specification, with different impeller outer diameters, P is only related to Q, H, and efficiency; even if P isn’t exactly the same under two different operating conditions, the motor may still be selected for the same gear^_^

All these parameters are normal. When the motor power is the same, a higher flow rate results in a lower head pressure. For higher head, the impeller diameter needs to be larger, while the flow rate needs to be smaller.

Just calculate whether the output power is the same to find out.

OP, calculate it using the formula provided above, hehe.