Trouble with the frequency converter of centrifugal pumps tripping
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Our plant has a domestic water pump with a head of 57 meters and a flow rate of 25 cubic meters per hour; it pumps water to a domestic water tank located at a height of about 37 meters above the ground. The operation of this pump is controlled by a level gauge in the tank – it starts when the liquid level is low and stops when it is high. The pressure in the pipes when the pump stops is around 0.4 Mpa. In recent days, the frequency converter of this pump has been shutting down frequently and requiring restarts. The electrical technicians say that there is nothing wrong with the frequency converter, and they suspect that the pump is operating under overload conditions. During the frequency-changing trip, it was found that the current exceeded the rated value, and the pipeline pressure was around 0.4 Mpa, which is similar to the pressure at the time of automatic shutdown. What exactly caused the pump to operate under overload conditions?
If there are no other issues, it is likely that the check valve is leaking; after the pump is stopped, all the water in the pump’s outlet pipe flows back, and when the pump is started again, this sudden increase in flow causes an overcurrent. The correct procedure is to close the outlet valve completely and open the inlet valve fully; then, while monitoring the current, gradually open the outlet valve until the current reaches the rated value. If the check valve does not leak, it can be controlled automatically. For reference.
Please ask the experts to analyze the reasons
1. Did you not jump before, and only start jumping in the past few days? 2. With this control scheme, there is no need at all for an inverter (I’m not sure if what you mean by frequency conversion control is correct?) ), Inverters are suitable for applications that require continuous operation with flow rate or outlet pressure control. In your case, the pump is activated based on the water tank level, resulting in intermittent operation. 3. The head required for this pump has been set at a relatively high value. The water tank is 37 meters high; the maximum water level is estimated to be 39 meters. The pressure loss due to pipeline resistance is expected to be less than 5 meters. The minimum pressure of the tap water at the pump’s inlet is also estimated to be 0.15 Mpa (equivalent to a water column of 15 meters). Therefore, the required head = 39 + 5 – 15 = 29 meters. 4. The tripping has occurred these past few days; it is likely due to high water pressure as well as a high pump head, which results in a large flow rate when the pump starts, causing the current to exceed the limit and leading to tripping. 5. It is recommended to close the pump outlet valve to resolve the issue.
Previously, it would happen once or twice a year, but these past two days it’s happened more frequently; now it seems to have stopped on its own. We have two types of control: variable frequency and fixed frequency. Generally, variable frequency control is used; once a frequency is set, it remains unchanged. The pump’s inlet is located in a tank that is about four meters high. Based on your analysis, the pump should trip when it starts up; however, according to the reports from the personnel on site and the pipe pressure, it seems that the trip occurs after the pump has been running for some time.
It is recommended to use the power frequency directly, or increase the frequency of the inverter. For pumps, if variable frequency drive is used, the power rating of the pump should generally be selected one to two grades higher; the power rating of your pump might be too low.
Under normal circumstances, the pipeline pressure is 0.4 MPa when the pump is stopped, while the pump’s head is 57 meters; it is clear that the pump chosen is not appropriate (45 meters would be sufficient in reality). Given that it operates at a fixed frequency, it is recommended to increase the acceleration and deceleration times of the inverter slightly and observe the situation.
The resistance in the piping system has increased due to issues such as scale; it is recommended to adjust the parameters of the frequency converter and try extending the startup acceleration time
Our company has also encountered this issue where the entire frequency converter stops working, and it was not possible to determine the cause; we were unable to analyze what was going on at that time
Have the frequency conversion parameters changed? I didn’t jump around that much before, but now it happens frequently; I think someone has changed the frequency parameters