Centrifugal pump shuts down due to overload
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One of the pumps in our facility is used for sending fluid to the flare stack; when the outlet valve is turned two full turns, the current level becomes too high and the pump stops working. Sometimes it stops operating on its own due to overload. I don’t know the reason! I need help from fellow sea lovers! I consulted the manufacturer, who said there are two reasons for the overload. 1. The specific gravity of the medium increases. 2. If the outlet pipeline of the centrifugal pump is empty or the pressure is very low, starting the pump will cause it to shut down due to overload. The first reason is that the medium in the torch tank is quite diverse, containing various types of substances; it’s not clear whether there are any issues with the selection process. The second reason is unclear; the pipeline to the downstream area is quite long, so it might be empty or have low pressure! I hope someone who understands this can explain why the outlet pipeline is empty or has very low pressure, causing the pump to overload and shut down!
Generally, the export pipeline is empty or at very low pressure; first, a bypass line is used to build pressure, and only after the pressure reaches normal levels is the outlet valve opened.
It should have little to do with the density of the medium; the problem might lie in the outlet control. At the start of pumping, the opening degree of the outlet valve can be adjusted based on pressure, with the pressure ideally not falling below 30% of the rated head
If the density of the medium being transported is too high, the shaft power will increase; if the motor chosen is too conservative in terms of its specifications, this can lead to overload.
It is best to wait until the medium in the pump outlet pipe has filled up by gravity before starting the pump; the opening degree of the pump outlet valve should be adjusted based on the motor current.
The manufacturer’s explanation is correct. The current solution is to replace the motor after verifying with the pump manufacturer. The emphasis on verifying this with the pump manufacturer is mainly to ensure the maximum stress that the pump shaft can withstand, especially when the density of the medium increases. As for why these two situations you mentioned occur: 1. Overloading happens when the density increases ； 2. Overload occurs when the back pressure of the pipeline decreases. The reasons are as follows: Looking at the performance curve of the pump: 1. Starting from zero flow rate, as the flow rate increases, the outlet pressure of the pump begins to decrease while its efficiency starts to increase (if vibration is measured, it will be observed that the vibration levels decrease gradually; this can be referenced using the diagrams in API 610), and the shaft power of the pump also continues to increase ； 2. After the flow rate increases to a certain level, the efficiency reaches its maximum, which is what is known as the BEP (Best Efficiency Point); at this point, vibration is also at its minimum ； 3. The flow rate continues to increase; before and after passing the BEP, the pressure keeps dropping, and the shaft power of the pump also continues to rise. However, the efficiency of the pump begins to decline gradually, and its vibration levels start to increase step by step. In a process system, flow rate is coupled with pressure; in other words, the flow rate depends on the outlet pressure. And a certain flow rate and pressure correspond to a shaft power. Overload occurs when the corresponding shaft power exceeds the upper limit of the motor’s output power. More or less.
If the pipeline is empty, it means there is no pressure at the pump outlet; in other words, the pressure at the pump outlet is very low while the flow rate is high, so an overcurrent condition is normal in this case. The regulation of the pump can be controlled based on the performance curve, the head value indicated on the nameplate, or the pump’s current. Additionally, when starting a centrifugal pump, it is necessary to close the outlet valve first, and then gradually open it to reach the rated operating conditions. If the rated operating conditions do not meet the usage requirements, it is necessary to check whether there is an issue with the selection
It is the pump for the torch separator tank; under normal circumstances, it should start and stop automatically based on the liquid level. Manual operation is no longer possible~~~~~ So, the solution is to replace the motor. For similar operating conditions, we always specify this to the manufacturer in the technical clarifications during the procurement stage: a、intermittent operation ； b、Pump start with the outlet valve fully open. When the manufacturer selects a motor, it takes the entire performance curve into account, so the situation described by the original poster will not occur.
Regarding the manufacturer’s response that an increased density may cause the motor to overload, this is correct； However, I have a different opinion on the second point. When the pipeline downstream of the valve is empty and the valve is opened, the instantaneous flow rate through the valve will be very high. This should not cause overload of the motor, as the valve already limits the flow rate. Instead, it will lead to insufficient fluid in the pump’s inlet pipeline, causing the pump to run out of fluid and resulting in a sudden drop in flow rate and pressure. Therefore, I don’t think the overload is caused by an empty pipeline; it is still recommended to check whether the motor’s power is sufficient
The last edit to this post was made by xj1983220 on 2015-8-23 at 20:16. The power of the pump shaft is given by P=ρghQ/pump efficiency. First of all, the power factor for this pump is definitely too low; in other words, the motor power selected is too small； Secondly, as the specific gravity, or ρ, increases, the shaft power certainly increases as well, which makes overload more likely to occur ； Third, you said that the outlet pipeline is empty, with no fluid and no pressure; in other words, the back pressure is low. It’s easy to understand why the pump tends to trip when started ； Is it mainly because there’s no flow meter on the outlet pipeline of this pump? It would be convenient if there were a flow meter; the outlet pressure of a centrifugal pump is controlled by the head, and the head is influenced by the flow rate ； In other words, when there is little change in the medium, the opening degree of the outlet valve remains unchanged, and the pressure at the pump outlet also changes little. However, when the back pressure is low, the pressure difference before and after the outlet valve increases, resulting in an increased flow rate of the medium. Thus, with the same valve opening degree, the flow volume increases; although the head pressure decreases slightly, the decrease is minimal ； An increase in traffic leads to an increase in load, resulting in overload and overcurrent. Therefore, when the back pressure is low, it is recommended to set the outlet valve opening to a smaller value. 2 suggestions: 1. It is recommended to replace the motor with one that has a higher power rating ； 2. If you don’t replace the motor, it is recommended to replace the control column (one with an ammeter). When operating this pump, connect it to a power supply to measure the real-time current, which will help you control the opening degree of the outlet valve and keep track of it ； It’s much more convenient with an ammeter ；
1. Blockage, 2. Excessive flow, 3. Jammed rotating parts, 4. Motor failure