Does the current increase when the water pump valve is closed? ? ?
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In my factory, there is a pump of the KQL 125/185-30/2T model produced by Kaiquan. The nameplate indicates a head of 50 M and a power rating of 30 KW; the motor used is one with a frequency of 60 Hz and a power of 30 KW. It is not known whether the impeller has been processed in any way. The power grid frequency is 60HZ. When the water pump is turned on and the pressure at the outlet is 2 bar, the current is 64A; when the outlet pressure rises to 3 bar with the valve closed, the current is 67A; and when the valve is further closed to achieve a pressure of 5 bar, the current is 71A. The current at startup with the valve fully closed is 39A. According to the typical characteristic curve of a water pump, as the valve is closed, the flow rate decreases and the current should also decrease. Why does it increase instead? What is the characteristic curve of the water pump in this situation? Are there any ways to reduce the motor current? Also, the pressure and flow rate do not meet the requirements. Why is that? The designed operating point is a head of 50 M and a flow rate of 140 M3/h; the ammeter should be fine, as it has been tested using a stopwatch. By adding a orifice plate in front of the pump flange, the current is reduced. The water from the pump is drawn up from the tank; the diameter of the main pipe is DN150, and there is a valve at the pump’s outlet. The water is supplied to the branch pipes and then collects in the return main pipe. I earnestly ask the experts to analyze it. This post was last edited by Munchausen on 2009-3-27 14:08]
I’ve never heard of such a situation as you described before! The possible reasons for analysis are as follows: 1. There is a problem with the ammeter ； 2. There is a problem with the arrangement of the impeller blades ； 3. The problem of a rapid drop in efficiency when operating at low flow rates ； Another issue is that the flow rate isn’t sufficient; what flow rate does your pump require? The design point for this pump should be at a flow rate of 200 m3/h ； Head is 50m at 60HZ! :)
Is there something wrong with your ammeter? Shouldn’t the starting current be very high when the valve is closed? (Note: you are referring to the starting current.) Flow rate is inversely proportional to head pressure; when the valve is closed, the flow rate decreases, but the head pressure increases, and as a result, more energy is consumed at the valve. The situation you mentioned is related to the ammeter (it might be a small one); so what is the diameter of the outlet pipe? If you choose a larger outlet diameter, that is, when the valve is closed, not only does the flow rate change very little but the head pressure also increases; as a result, the actual output power of the motor rises, and the current increases as well! (Guess):$
In the projects we work on, it often happens that both the pumps and the processing systems are constructed in accordance with the design specifications; the pump head calculated based on those specs is also appropriate. However, the current drawn by the pump exceeds the normal operating level, which often leads to the motor burning out. In some wells, the motors have been replaced several times yet the problem persists. It seems that excessive resistance is the cause of the increased pump current
There is probably almost no relationship with the ammeter; I think it’s because the outlet pipe remains constant, and when the valve is closed, not only does the flow rate decrease, but the flow velocity also drops even though the outlet pipe doesn’t change； Conversely, when the water demand of the equipment remains unchanged, the flow rate decreases, and the flow velocity increases accordingly; as a result, the actual output power of the motor rises, which in turn causes the current to increase. So Xu Tong as a whole needs to be synchronized, (just an analysis, hehe)
The outlet diameter after the pump is DN150; there is an outlet valve behind the water pump. After supplying water to several small branch pipes, it flows back to the main pipeline reservoir. After startup with the valve fully closed, the current is 39A; it is possible that the low-load protection has tripped. By adding an orifice plate at the inlet flange, the current is reduced. The current decreases when the outlet valve is opened wider.
The required flow rate is 140 M3/h, and the required head is 40–50. The speed indicated on the pump’s nameplate is 2950; it’s not clear whether the impeller has been modified in any way. In a situation like this, what does the characteristic curve look like?
What the original poster described is normal. In the first half of the power curve of certain centrifugal pumps with lower head, the curve is essentially linear, rising slowly; once a relatively high flow rate is reached, it gradually levels off and begins to decline, meaning that the second half of the curve has an arch-shaped pattern. The specific reason for this is that as the flow rate increases, the head of the pump decreases; in centrifugal pumps with high flow rates but low head, the drop in head is relatively large, whereas the decline in pump efficiency is not as rapid, which leads to the aforementioned phenomenon.
Although reducing the valve opening lowers the flow rate, the resistance increases significantly, resulting in greater energy loss
1 Is there a problem with the ammeter? 2 Is the pressure at the pump inlet too high?
If the current is too high, it is possible to consider modifying the sizes of the inlet and outlet pipes of the pump, taking into account both the efficiency characteristics of the pump and those of the piping system; for example, enlarging the pump’s inlet pipe.
Has anyone encountered a situation where, when the valve is closed, the current actually increases?
Has the flow rate not decreased while the pressure has increased? Otherwise it’s really hard to understand; is the pump’s rotation direction correct?
Has the poster confused the starting current with the normal operating current?
According to the poster, this situation does not correspond to normal conditions; it is likely a problem with the design of the pump’s efficiency curve. The impeller of the pump needs to be modified, and then performance tests must be conducted again to determine whether the requirements are met. No, keep using it as long as the motor is fine; otherwise, replace the pump.
Is there a problem with the matching of the inlet and outlet pipelines?
The installation location of the pressure gauge is before or after the outlet valve; if it is after the outlet valve, this phenomenon is easy to explain. Let’s talk about the installation location of the pressure gauge.