Pump outlet pressure issue
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I hope the moderator and those in charge will forgive me for this post. I’ve posted similar posts before, but the issue is still a bit different this time. I’m not majoring in this field; I’ve only learned a little about fluid dynamics. I just want to ask everyone more questions. I want to clarify a question: regarding a pump, what’s the difference between one with a thin outlet pipe and one with a thick outlet pipe? It is the same pump, with the same flow rate and the same output head. As we all know, in narrow pipes the flow velocity is high and the pressure is also high; in wide pipes, the flow velocity is low and the pressure is low. With a high flow velocity comes a high pressure – doesn’t this violate the conservation principle of Bernoulli’s equation? Is it because of the different pipe diameters? Since this is not considered the same operating condition, does the Bernoulli equation no longer apply? There is also the high flow rate, which is easy to explain. Flow rate = flow speed × cross-sectional area; therefore, it’s not difficult to understand why a smaller pipe diameter results in a higher flow rate. But how can high pressure occur in a pipe with a small diameter? Can it be explained using the middle school formula for pressure = force × area over which the force is applied? It doesn’t satisfy Bernoulli’s principle?
As long as it is outlet flow restriction, for the same flow rate, the pressure at the pump outlet remains the same; however, due to different pump outlet diameters, the pressure drop in the pipes varies, which results in different pressures within the pipes.
The original poster has a misconception; the energy of the same pump is not necessarily conserved. When the outlet conditions vary, the pump operates under different conditions, resulting in different power levels and thus different amounts of work done.
Isn’t closing the pump’s outlet valve and having thin pipes the same situation?
Thank you for the explanation; it’s very easy to understand. The original poster isn’t majoring in hydraulics; they’re studying something related to power plants, so their knowledge in this area is limited. That’s why they ask questions when they encounter things they don’t understand. Thank you for your patient answers, expert: lol
If we install pipes of different diameters at the pump’s outlet, the pump’s flow rate remains the same, but the flow velocity and head pressure change, right? Why is the pressure different due to the change in diameter? Is this the characteristic curve of the pump? Or should it be explained simply by the compressed area? (Pressure = Force / Area)
Your question is a bit confusing; you might not have clearly distinguished the concepts. First of all, it should be noted that the claim that your small tube pressure is high while the large tube pressure is low is unfounded, and no prerequisites are mentioned either. Regarding the pipe diameter issue you mentioned, a basic premise needs to be assumed. It means the data volume is the same, right? Is the inlet pressure the same? Is it the static pressure that is the same, or the total pressure?
You mentioned before that when the pipe diameters are different, the power requirements of the pump also vary, and it’s not possible to use Bernoulli’s principle for comparison. But in the situation I mentioned earlier, where a pump’s outlet pipe is connected to a control valve, closing the control valve reduces the flow rate while increasing the pressure. According to the pump’s performance curve, as the flow rate decreases, the power also decreases. Why can Bernoulli’s principle be used here to determine the changes in flow velocity and pressure? I can’t figure it out. Here, the Bernoulli equation is used to determine the changes in pressure due to force, but actually, before and after adjusting the valve with a switch, does the power of the pump also change?
What you mean by reducing the valve opening is in line with Bernoulli’s equation: when the valve is closed more, the flow rate decreases, and the pressure increases. However, the change in pipe diameter cannot be used with Bernoulli’s equation to determine this pressure change. Why? According to you, all of these should satisfy Bernoulli’s equation, right?
Actually, I just want to know that for the same pump, when connected to a thin pipe or a thick pipe, which one will produce higher pressure and which one will have a faster flow rate. Why? It must be the high pressure in the thin tube; could it be that the flow velocity in the thin tube is slower than that in the thick tube? The flow velocity in thin tubes should be faster than that in thick tubes, that’s what I remember.
Actually, I just want to know that for the same pump, when connected to a thin pipe or a thick pipe, which one will produce higher pressure and which one will have a faster flow rate. Why? It must be the high pressure in the thin tube; could it be that the flow velocity in the thin tube is slower than that in the thick tube? The flow velocity in thin tubes should be faster than that in thick tubes, that’s what I remember. Please, experts, help me figure this out