I think the problem has deviated from the original question, which is: what is the result pressure in the combined pipe considering its end closed? So the system is isolated from any external effects. Can you answer this question please?

@@ELHAOUARIAbdelhalim If there are no valves, nothing is preventing the pressurized tanks from depressurisation. so it's safe to assume that both of them have a tank on their pipeline. the whole segment afterwards, up until the closed (in this case) pipe, is full with air at a certain pressure. what is that pressure? it's the same exact problem as before, just that now that pressure is not automatically 1 bar since it does not go in the atmosphere but could be anything depending on how you filled this pipe. If you only filled it with air from the top tank, it would be the top pressure. if you filled it with atmospheric air, it would be still 1 bar. if you vacuumed it, would less.

This is what I really don't get in this problem. If you want to stay "inside the box" of this odd though-experiment, then A is simply going to back-flow into B as well as exit on the open end. After all, there is no pressure drop in the pipes and the source ALWAYS provides 15 bar, those 15 bar are there all the way until the exit. B can only be at 15 bar too without any additional devices in-between.

@@leocurious9919that's what I thought to. Although I thought a little less than 15 bar since you will drop some pressure with the flow going to the 10 bar side.

@@Jonas_Aa Just like a source that always provides 15 bar, I would assume that there is no pressure loss. The diameter etc. is not given, so it could really be anything from 0 to 15 bar. It could be a capillary tube for all I know.

@@leocurious9919 The pipes are actually carbon nanotubes.

Actually weirdness happens - I worked on a 30 bar pure nitrogen system where we detected a leak because atmospheric oxygen was contaminating the nitrogen - yes, one atmosphere air was leaking back into a 31 bar nitrogen system… And tightening the leaking tube fitting eliminated the ppm level contamination.

Do you remember asking this? Boyle's law, Charles' Law, coleslaw - that was the answer...

@@ProcesswithPat I do remember the moment fairly clearly. But I think that "joke" was erased from my memory haha. Maybe I was too stressed at the time to appreciate cabbage based humour

@@ProcesswithPat ah, cole's law

​@@ProcesswithPat2

Is it valid to reason about systems like the one described above that unless you know "the load," you cannot know the behavior of the whole system? As in electrical engineering, you don't speak of current until there's a force pushing it (i.e. voltage difference), and even adding a seemingly minor element affects the characteristics of the whole system.

Similarly, temperature is a construct...

And Reynolds ;-) Though we've yet to see that properly in electrical engineering!

@@philipoakley5498 I'm not that deep in electrical engineering. I'm more the mechanical guy that has kept an interest for the electronics... rare in my branch, but helps a lot.

@@jackmclane1826 for fluid flows you can get sidewall effects and turbulent flow so there are some differences between plumbing flows and electrical flows. Electric just depends on potential (pressure) difference. It's all about how the flow 'valves' are constrained adjusted;-)

@@philipoakley5498 But you also get eddy currents that have a similar effect to turbulent flows in fluids. They can pretty much push the flow of current more to the inside of the conductor, see 'skin effect' for more info. This can pretty much work as an additional impedance/resistance, creating voltage loss across the wire, just like you can get pressure loss from transfering fluids over ditances. Although I'm not saying the two are the same. It's interesting to see that similiar effects arise in both cases.

@@vojtechsmetana5261 Do watch out for some of these analogies, as they fall prey to the fallacy of pushing analogies too far. Things like skin effect have a strong frequency effect. And electrical flow has to be circular, which isn't the case in fluid design. You also don't consider EMC effects with fluids, so caution where caution is due. ;-)

Thanks for the really nice comment!

Also important that the piping after the junction has the combined flow of the two. So in a proper mixing situation, more pressure drop per meter in that piping than upstream.

@@mikefochtman7164 true, but you can't guarantee that blockages won't happen. You need to have back flow preventers any time that you bring two different chemical lines together. You also forgot to mention the angle of entry into the larger pipe being important as well. Ideally you want both lines to meet at a Y instead of a T to insure that the flow is toward the mixing line and not toward each other.

I really appreciate it!

No worries, fully deserved!

You bet!

Vastly depends on the process. Most solid-liquid injection processes for scrubbers can be done in piping to optimize against pluggage and make sure the slurry is homogenized ie soda ash for scrubbing. Lots of applications for static pipeline mixers as well

That's really kind. And you're welcome to come with questions. Will be happy to give it a crack it it's something I am capable of doing.

@@ProcesswithPat thanks! I will come soon with my questions. Btw, very nice topics would be overpressure scenarios and PSVs. I am pretty sure you will give an amazing insight to these topics as well. ☺️

Not Pat but: How about using a one-way valve on the booster pump path, so it delivers when the pressure goes low? You can do quite a bit of routing with one-way valves.

It hypothetically possible. The only time the 15 bar stream would flow back into the 10 bar stream is if it was easier for it to do that rather than for both streams to simply flow out of the combined pipe. This would mean that either: 1. the "outlet" (combined, T-piece) actually discharges into a pressure between 10 and 15 bar, say a pressurised tank (but that's weird because why then did we form our analysis from the point of view of the 10 and 15); or 2. The capacity from the 15 bar stream is so high that it is able to put enough flow through the combined pipe such that the pressure drop in that pipe becomes greater than 10 bar, making it possible to send some of the 15 bar stream into the 10 bar stream.

​@@ProcesswithPatif it behaves anything like electric current then there will always be a flow from higher level to lower level in a parallel system. More will flow in the route of less resistance but there will be a tiny flow in the higher resistance route proportional to the resistance. Current cannot choose to not take the route of more resistance completely as long as the voltage is high enough to overcome the resistance. Speaking in fluid dynamics I'd say the 15 bar stream will split up in a certain manner, one part flowing "out" and one flowing in the direction of the 10 bar source.

Your question touches on a really interesting theme that seems to pop up a bit which is are you calculating something because you are DESIGNING a system, or are you calculating because you are EVALUATING an existing system. I made this video assuming that I was designing, which means that after I have picked these different pressure drops (i.e. I have a pressure drop budget) then I can go and size my lines and valves to obtain the flow rates that I desire. I've actually said nothing about flows in this process. Your question assumes that you are evaluating an existing system with fixed geometry. If your mixing pressure is changing and your SUPPLY PRESSURES ARE FIXED (important assumption so that we don't comlicate this too much) then either you are clogging up/have more resistance downstream (causing mixing pressure to increase and both flows to decrease), or you have less resistance downstream (causing both flows to increase). In either scenario that decrease or increase in flows, and also the ratio of the increase/decrease in flows, will be dependent on the steepness of the system curves from the source to the mixing point. I'm basically trying to say that it is difficult to answer without fully-defining the problem and knowing the geometry. It may help to do a though experiment. Imagine one of thjose lines is a large water supply line of say DN200/8" and the other line is a small chemical dosing line of 12 mm, or ½". The mixing pressure is the pressure at the dosing point. It is possible to imagine that etiher of the streams in my video is either of these lines - you could switch them around mentally. You can then re-imagine that both lines are identical sizes instead carrying similar flows. Regardless of which of these scenarios it is, the information in my video holds true. Until you know which one of these scenarios it is, it is difficult to say more about the ratio of flows. Not sure if this though experiment helps or if it is counterproductive... The final thing I'd say is that if your mixing pressure is changing because of downstream effects it is likely that both pressures A and B will change depending on the characteristic curve of the machine supplying the pressures. Then again, you maybe have pressure regulators right upstream of points A and B that give this constant pressure.

I did a video in reverse flow using the exact same setup. Check it out!