Measures to reduce the flow rate used by centrifugal pumps below the designed value
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This post was last edited by yuchenchf on 2012-1-10 at 13:19. Our company has several centrifugal pumps with a rated flow rate of 200 M3/H, but in actual use the flow rate is much lower than this rated value; the outlet valve is only opened to 1/3 of its full position. It causes significant damage to the pump. To ensure its safe operation, a return pipe has been installed, but this results in considerable waste for the pump. It is planned to use an inverter to address this issue. I would like to ask the experts how to proceed with the design? How to achieve actual automatic control?
The rated flow rate is 200 M3/H, but in actual use, the flow rate is much lower than this rated value. Other issues can be considered; could it be a problem with the pump itself? Take one apart and have a look!
Insufficient incoming materials? Equipment failure? If it is the former, considering adding a frequency conversion control device is an option.
Is the selection inappropriate? Is it also due to the transformation?
A frequency converter can be added to the motor to change its speed
The low flow rate during use is due to process modifications; now, no such large amount of feed is required. As a result, the flow rate of the pump became too high. Modifications were carried out by adding variable frequency control, but it is desired to convert it into an automatically controlled system. I hope experts can advise me on how to proceed with the design
First, use the similarity law to calculate whether the new flow rate and head can meet the requirements
Generally, it’s sufficient to use a direct frequency converter; there’s no need to reduce the speed significantly, and the original motor can be used.
This depends on the flow rate required by the user, as variable frequency voltage reduction can be used; this will result in a decrease in both flow rate and head pressure. If the head pressure needs to remain unchanged while the flow rate needs to be reduced significantly, this will cause the pump to operate outside its efficient range, thereby reducing its service life. It will also lead to imbalance in the pump’s components, making parts such as bearings more prone to damage； At the same time, since you previously used a motor that was not variable-frequency, if the frequency range is very wide, the motor will also overheat and get damaged ； However, if the building owner requires a small range of flow variation, it is entirely possible to use an inverter for speed control in order to adjust the flow rate!
If the flow rate is low over a short period of time, 1) a bypass return pipe can be added, or 2) an inverter can be installed in front of the motor to adjust the voltage and frequency thereby controlling the motor’s speed. For centrifugal pumps, there is a linear relationship between flow rate and speed: Q1/Q2 = n1/n2; whereas there is a quadratic relationship between pressure and speed: H1/H2 = (n1/n2)^2. If the design is not appropriate and a pump designed for higher flow rates is used for low-flow conditions, it is best to replace it with another pump
If the flow rate is low over a short period of time, 1) a bypass return pipe can be added, or 2) an inverter can be installed in front of the motor to adjust the voltage and frequency thereby controlling the motor’s speed. For centrifugal pumps, there is a linear relationship between flow rate and speed: Q1/Q2 = n1/n2; whereas there is a quadratic relationship between pressure and speed: H1/H2 = (n1/n2)^2. If the design is not appropriate and a pump designed for higher flow rates is used for low-flow conditions, it is best to replace it with another pump