For centrifugal pumps suffering from cavitation, when a frequency converter is installed to reduce the speed, on what basis do the flow rate and head decrease?
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There is a process circulation pump, with the medium being naphthalene and washing oil； The operating temperature is 270 degrees, with a positive pressure of about 60 KPa at the pump inlet. The pump’s rated flow rate is 180 m3/h, and its head is 80 m. The pump in actual operation suffers from severe cavitation; the actual flow rate is around 140 m3/h (it cannot be increased any further, with significant fluctuations in the pressure gauge and ammeter readings), and the pressure is 0.4 MPa (with a density of around 0.9). It is not possible to achieve this due to the limitations imposed by the device’s operational conditions, such as increasing the inlet pressure and height or reducing the temperature of the medium. An inverter is considered as a solution to reduce the speed and thus eliminate cavitation, but the flow rate and head must not be lower than the current values; otherwise, the process requirements cannot be met. If the rotational speed decreases, do the flow rate and head decrease in proportion as well? I’m wondering whether this reduction applies to the actual flow rate and head under current operation, or to the rated values specified in the pump’s design I would appreciate it if experts could give me some advice. Thank you. Note: The pump is designed for a flow rate of 180 and a head of 80 ； Actual maximum 140 head 40
The cause of cavitation should be identified; frequency conversion cannot solve the problem fundamentally. Increase the inlet pipe diameter and reduce the pump inlet diameter. Increase inbound traffic. . . . . . . . . . . . Inlet pressure. . . . . . .
The diameter of the pump inlet pipe is already sufficient; no additional enlargement is needed. The pump’s own inlet is 150 mm in diameter, while the pipe is 250 mm in diameter； Furthermore, the manufacturing process prevents pressure application at the inlet
Error: The pump’s own inlet is not 150mm, it is 125mm
Is it a problem with the process medium not matching the design, or is it an issue with the pump selection? Adding a variable frequency drive probably won’t help much, right? The flow rate is in direct proportion to the rotational speed: Q1/Q2 = n1/n2 ； The head is related to the speed in a quadratic manner: H1/H2 = (n1/n2)². The shaft power of the motor is related to the speed in a cubic manner: P1/P2 = (n1/n2)³. From these relationships, it can be determined that the formula for the motor speed is n = 60f/p, where n represents the synchronous speed of the motor, f is the supply frequency, and p is the number of pole pairs in the motor. This shows that the supply frequency f is proportional to the speed of the motor. Thus, the frequency also has an n-th power (n=123) proportional relationship with flow rate, head, and motor shaft power, as mentioned above.
The proportionality law mentioned in 5L applies to pumps in systems without back pressure; this means that the pipeline characteristic curve is a parabola starting from the origin. When the speed is changed, the flow rate and head change according to this proportionality law. Even though this law holds true, a decrease in speed still results in a reduction in actual flow rate and head. In other words, the flow rate and head at a certain operating speed n are proportional to those at a lower speed of n1. The rated values are only for reference during selection, as the pipeline conditions in actual use change many times over. However, all of this isn’t very useful. There is pressure at the pump inlet (though it’s relatively low); it’s possible to test whether the proportionality law still applies =)
Reducing the speed will lower the head, and the flow rate should also decrease, but what’s the use of that?
The last edit to this post was made by liuqun06092863 on 2015-12-19 at 12:14. I checked information on naphthalene and washing oil; at 270 degrees and at normal pressure, naphthalene has already boiled, and the boiling point of washing oil is also close to that value. It would be best to find out what state naphthalene and washing oil are in at 270 degrees and 60 KPA. It doesn’t seem like it’s a pump problem