Regarding the very end of the pump’s performance curve
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Suppose I have a pump on a test bench for testing; starting from the lowest flow rate, I gradually increase the flow rate and plot a performance curve. Generally, this flow-pressure curve does not extend indefinitely. Now I want to continue increasing the flow rate; so what will be the effects on flow rate and pressure? Well, asynchronous motor drive. a. Will the flow continue to increase and the pressure continue to decrease, until the pressure reaches zero? b. Will there be severe fluctuations in flow rate and pressure? c. Other cases.
There should be severe fluctuations; as the flow rate increases, the NPSHr of the pump rises, and cavitation may eventually occur.
Oh, I forgot to mention two assumptions: a) the inlet pressure can meet the requirements for cavitation margin； b. The power of the motor is approximately infinite.
This proposition is good. In a piping system, as the flow rate increases, the pressure certainly decreases. However, the pressure cannot drop to zero, because the size of the pipes determines the maximum flow rate of the medium being transported. Moreover, if the pressure reached zero, the check valves would not be able to open, making it impossible to transport the fluid. In an ideal situation, for example, if the pump outlet is directly connected to the atmosphere, then the outlet pressure should be zero, and the corresponding flow rate would be the pump’s maximum flow rate (i.e., the intersection point with the flow rate axis). This is just my personal opinion; I welcome experts to provide correct explanations
Even if the outlet is open, it’s just that the liquid loses weight and pressure, and the pressure will drop to 0; but I don’t think the total head can possibly drop to 0 as well. Because the volute of the pump itself, along with the inlet and outlet pipes, also represents a head loss resistance. Therefore, on the performance curve, the head cannot drop to 0 either.
According to the poster’s assumption, without taking into account the motor power and net positive suction head, the flow-rate-head curve will continue upward until it reaches the maximum flow rate that the pump’s performance allows. However, according to the principles for selecting pumps, there is a \"preferred operating range\" and an \"allowed operating range\" for such pumps; the allowed operating range is determined mainly by two factors: the pump’s flow rate and vibration levels. When the flow rate is too high or too low, causing the pump’s vibration to exceed acceptable levels, operating under such conditions becomes meaningless.
When the valve is fully open, it reaches the end of the performance curve; it’s impossible to increase the flow rate any further, as the diameter of the pipe limits the flow, unless one can increase the motor speed indefinitely.
It’s like the STONE WALL of centrifugal compressors
Actually, this is what I want to say. Due to the limited speed of the pump, its flow rate cannot increase indefinitely; in other words, once a certain flow rate is reached, it will no longer increase. The question is, what is the pressure corresponding to this flow rate?
How should this be answered? The pressure varies depending on the different specifications and models; the pressure corresponding to the maximum flow rate is the outlet pressure of the pump at that time.
At a certain speed, the pump achieves its maximum flow rate, and the pressure at the pump outlet approaches zero; however, it is impossible to reach zero, as true zero pressure implies no flow rate.