Is the pump selection appropriate?
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Dear group members: Our unit has purchased two chemical pumps (parameters: flow rate – 100 m3/h, power – 15 KW, head – 32 m, speed – 2900 r/min), which are to be used as circulation pumps in two lye spray towers. The motors associated with these pumps are both 15 KW in capacity. The height of the spray towers is 10 m and 17 m respectively, while the distance from the pump outlet to the farthest water outlet is 18 m and 25 m respectively. There are 3 90-degree elbows and 2 valves each. Recently, both pumps have experienced sudden shutdowns. The temperature gauge showed that the highest temperature on the surface of the motors was around 60°C, while the outdoor temperature was about 30°C. Is the pump selection appropriate? What are the problems? What is the reasonable head range for a pump?
Overheating of the motor’s surface may be caused by an excessive load. You can close the pump’s outlet valve to increase the outlet head and thereby reduce the load; then check whether the overheating persists. This can help rule out some of the possible causes of the problem
When the pump is operated, is it under overload or operating at a current higher than the rated value? If so, it means the pump motor is too small; as mentioned on the second floor, the opening of the pump’s outlet valve can be reduced to lower the pump’s flow rate and thus the motor load. If the pump shuts down due to low current, then it is necessary to check the pump’s inlet pipeline, determine the flow velocity in that pipeline, and inspect the pump’s inlet filter to see if there is any blockage. It is also important to consult the manufacturer to verify whether the pump’s net positive suction head matches the system’s net positive suction head
The overcurrent trip occurred because the head value selected for the pump was too high, while the actual head was lower; as a result, the actual flow rate of the pump exceeded the rated flow rate, and the motor load also surpassed the rated load. Since no flow meter is installed at the pump outlet, it is not yet possible to accurately determine the cause.
There is indeed such a situation: the actual head required is low, which results in a high flow rate at the pump outlet. The flow rate can be controlled by using the valve at the pump outlet, thereby reducing the load on the motor
Since it’s a tower that is only 10 or 17 meters tall, choose a height of 13–15 meters; a head of 20–22 meters will be sufficient. Close the outlet valve to about half its position, so that the pressure difference between the pump’s inlet and outlet is around 3 kilograms. Check to see if there is any overflow
This problem is likely caused by operational factors. Please follow the procedures below to see if they are suitable; this is for reference only. Preparations before startup: Close the outlet valve completely, open the inlet valve fully, and fill the pump with liquid to remove air. Start the liquid pump; once it is operating normally, the pressure will reach its maximum value. Using the motor’s rated current as a reference, gradually open the outlet valve until the current reaches the rated value. This process should be carried out slowly, by referring to the system pressure; once the system pressure is filled with liquid, the pressure reaches the desired level immediately. On this basis, adjust the fluid distribution across each branch. However, it should be noted that the rated current of the motor must not be too low or too high. If it is too small, gas entrapment may occur; if it is too large, the flow rate may be too high. It’s worth a try.
1. Is it unnecessary to include the word “pressure” in front of “when the system pressure is filled with liquid, the pressure immediately reaches the required level”?； 2. In “pressure immediately meets the requirement”, should the pressure referred to be approximately equal to the pressure generated by the actual head, or the pressure generated by the rated head? 3. Is the head value of the pump selected by our unit significantly too high?
For use in spray towers; a nozzle should be connected at the front end, and the head pressure selection is correct. It’s likely that the motor is too small; the motor current can be measured to check whether it’s exceeding the normal level. Closing the outlet valve is one method. Keep it for a long time or replace the motor?