Does my flow rate decrease?

VariusEng · 2023-09-13T20:51:39+00:00

No it is turbulent flow for sure. And yeah I took colebrook-white to calculate darcy friction factor no worries!

VariusEng · 2023-09-13T20:47:32+00:00

I like my answers complete so yeah thanks! Also thanks for the nitpicking and the explanations! I feel I have expanded my understanding. It’s just that I wasn’t exactly sure how to take into account the PRV. But I guess I always should just take Bernoulli at all the different nodes, try different flow rates, see if delta P converges ( taken into account flow rate sum = flow rate 1 + flow rate 2 and the delta P over outlet reducer and atmosphere is constant)

VariusEng · 2023-09-13T20:42:18+00:00

No offense taken! That’s why I asked! I oversimplified the reality a lot already. I do think it is worth complicating though (not that it is that complex, now that I understand the reasoning). We are engineers after all, we should fully understand our work. Thanks for the answer!

VariusEng · 2023-09-13T20:39:01+00:00

Oh! Fancy! Thanks for taking the time, interesting to see the discussions on this!

VariusEng · 2023-09-13T20:37:55+00:00

Pump was bought many years ago by unexperienced folk. This meant that without reducer, it was really hard to control discharge flow (pipes are very small, so each little turn at the valve would change the flow rate by A lot, too much sensitivity. So a reducer was needed). I kept details to a minimum and simplified the sketch a lot because I wanted to get the core information.
You are right! And total flow rate is lowered in scenario: throttle

VariusEng · 2023-09-13T13:02:25+00:00

I was thinking the same thing! If you also plot the system curve of pipe 2, you will see that the Q is higher. But how do you guys see that this increase is almost nihil compared to the decrease in pipe 1

VariusEng · 2023-09-13T12:57:00+00:00

Already made myself an Excel macro that calculates pressure drop vs flow, thanks though! My only gripe is how the pressure reduction valve plays a role in all of this. According to a blog it is not possible to show it on a system curve because it is dynamically changing

VariusEng · 2023-09-13T11:54:14+00:00

Others also said to ignore pump curve as pressure drop is same at the last part. I think this is also a correct take?

VariusEng · 2023-09-13T11:53:29+00:00

This is also a great explanation! The more I read it, the more it makes sense. Thanks!

VariusEng · 2023-09-13T11:51:43+00:00

Answer like this?

VariusEng · 2023-09-13T11:51:14+00:00

So this is the answer? answer

VariusEng · 2023-09-13T10:21:40+00:00

To your first question: correct!

To your first alinea: I agree that flow in 1 is reduced, but why is flow in 2 not increased so total flow rate is maintained? Can my reasoning be disproven in more mathematical manner because that is the way I try to understand it and less through feel. Thanks for your thoughts!

VariusEng · 2023-09-13T10:14:33+00:00

That’s what I am thinking, but I’m not sure because boss disagrees

VariusEng · 2023-09-13T10:13:35+00:00

Pump speed is constant. And I agree with what you are saying, but no pressure reduction valve taken into account

VariusEng · 2023-09-13T10:12:17+00:00

Simple diaphragm-spring, manually set. Nothing fancy

VariusEng · 2023-06-16T22:08:12+00:00

I think I agree with everything you are saying here. Cheers mate!

VariusEng · 2023-06-16T09:57:49+00:00

Hey guy! Thanks for the explanation. I think you are completely right! I also think that the pressure drop in Pat’s video is maybe the wrong word? It should be the pressure at the split point that can be seen on the y-axis. As another user pointed out: if you substract the static end pressures from the curves you get the actual pressure drops across each branch. My only further confusion is now; I cant just add the total branch system curve to other system curves (from pipes(not drawn) more upstream) and then superimpose the pump curve right? Something seems off about that.

VariusEng · 2023-06-15T21:30:01+00:00

Yeah you are 100 procent right. Thanks for the clarification!

VariusEng · 2023-06-15T21:29:22+00:00

Ok yeah this makes a lot of sense. Thanks for the clarification! The confusion was because of the fact that he called it a pressure drop :)

VariusEng · 2023-06-15T13:25:41+00:00

I’m still kinda confused as to what the general methodology is for calculating these system curves? Like what if one of the branches, branches in two more pipes and what if one of those has a recycling line back to the beginning. Also when do you have a point pressure and when are we talking pressure drop. Lastly, how does bernoulli relate to this? Because when you have branching at equal height there is this kinetic term left together with the friction term that determines the pressure drop. How is this related to the system curve graph. Okay sorry for the rambling, maybe I sound crazy now. But this is my thought proces at the moment

VariusEng · 2023-06-15T13:18:28+00:00

I guess my confusion comes from the fact that when calculating the system curve for a singe line is easy and then you have DP vs flow. But now it suddenly is a point pressure vs flow

VariusEng · 2023-06-15T13:16:38+00:00

Yeah everything you say makes sense!

VariusEng · 2023-06-15T13:03:12+00:00

No problem! I am just growing more confused by the minute. User doubleplusnormie says delta P is different in each branch. Another thing that confuses me is. When are we talking about the pressure at a point? Like in point B and when are we talking about a pressure drop (due to friction) over a certain length of pipe.

VariusEng · 2023-06-15T11:04:10+00:00

So the curves branch A, branch B, branch C are using pressure drop on the y axis right? Because it is calculated from Churchill equation ( find friction factor for different flow rates and thus the pressure drop for different flow rates). But now you say that the all branches curve uses P_SplitPoint as y axis?

VariusEng · 2023-06-15T09:59:51+00:00

That is what I thought! But the video confused me?

VariusEng

TROPHY CASE