What happens when the pipeline losses are greater than the head of the centrifugal pump?
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For example, the pipeline at the pump outlet is 200 meters long; the highest point is located 1200 meters away from the pump outlet, after which the fluid enters a container at atmospheric pressure. The pump has a flow rate of 30 m3/h and a head of 15 meters. At a distance of 120 meters, the pipe losses plus the static pressure drop equal the pump’s head. Can the pump then push the liquid out? What phenomenon will it be?
It’s probably due to low flow rate. According to the pump’s flow-rate vs. head curve, as the flow rate decreases, the head increases
Theoretically, it can’t be sent out. In fact, the current will increase, the power will also increase, the head will rise, and it may be possible to pump liquid. But the pump wears out quickly when overloaded
As the flow rate decreases, the pressure loss in the pipeline also decreases, so a certain amount of fluid will always be delivered. Assuming that the actual flow rate is too low, below the minimum flow requirement of the pump, the pump will be damaged. First, the temperature of the fluid inside the pump rises continuously, then the pump becomes vacuumed, and finally vibrations cause the seals to be damaged or the shaft to become seized.
Is the pump’s speed a fixed value? Is the low flow rate due to a slower rotation speed? Is the motor automatically reducing its speed?
It depends on how what you call resistance loss is calculated; as long as there is flow, no matter how slow it is, fluid will always flow in. Inflow is impossible only when the head is less than or equal to the static pressure. The pump is running at high load under no-load conditions.
There are many conceptual errors in the question; first read the book to understand the basic concepts before asking questions.
When the outlet valve is closed, isn’t that the limit of the outlet resistance? The flow is minimal at this time. Export resistance increases, flow decreases → 0
Thank you all for your responses. To summarize: if a flow rate of 30 m3/h is desired, the pipe losses in this case are greater than 15 m; therefore, the pump with a capacity of 30 m3/h and a head of 15 m cannot meet the requirements. As a result, the flow rate decreases while the pump’s load increases. If the flow rate falls below the pump’s minimum required value, the pump will be damaged very quickly. Right?
Choosing a too low head will result in a lower flow rate and reduced pressure losses in the pipeline; in other words, the pressure losses in the pipeline are in dynamic equilibrium with the head. The extreme case is when the flow rate drops to zero; at that point it’s the same as having the outlet valve closed. At this point, the fluid inside the pump circulates internally, causing its temperature to rise.