Reverse Flow 

I think this is the second video you made to answer one of my questions lol (I'm the farticle guy), I can't thank you enough! I plan on watching all your videos, I'll try not to ask too many weird hypothetical questions that are out of my league. Sometimes my thoughts get ahead of my knowledge and I start asking questions when what I really ought to do is read a book.

It is equally important what the upstream pressure does to the pressure at the junction. You have simply assumed those pressures fixed and then varied the flow rates as required to make those assumed pressures true. If you assume flow rates, rather than upstream pressures then, while holding downstream pressure constant, the upstream pressures must be adjusted to make the flow rates true. Or equally as valid, you can assume both upstream pressures and a downstream pressures as fixed, then calculate flow rates in all pipes to make the overall pressure drop true. It is NOT ONLY what the downstream pressure is that controls junction pressure. It is everything. Its only the downstream pressure that determines flows and junction pressure, because you assumed that and calculated the flows. If you assumed the flows, then you would have to calculate the downstream pressure, and as such, it would be the flows that controlled that result.

Great insights which we can't get in university teaching 👌👌

Great explanation Pat. Thanks for sharing your knowledge.

If the downstream delt P = 10 bar then how can we achieve that the 10 bar stream will move downstream ?! I very very much appreciate and like your videos. I would suggest in the future adding a full example with a DEFINITE parameter (with magnetite) and working on it for better understanding.

Interesting video Pat! Good job 👍

Great, this was bothering me on the first video, now I understand ! thank you :)

Hi Pat, I don't think the first part of the video is correct. If both streams are identical in terms of pipe size and configuration, at the mixing point they won't have the same pressure and the higher pressure stream will block off the lower pressure stream. You have to design the system so that they reach the mixing line in an equal pressure

Hello Pat, Thanks for posting this. I am considering changes to my water system, and this video (plus your video "Pressure, head, and pumping into tanks") seems to confirm my plan would work. However some people tell me my plan won't work, and I would really like the opinion of a person who understands hydrodynamics. My water source is a spring. The output from the spring is piped some distance downhill to a transfer tank and then pumped to storage tanks located at the highest point on my property. From the storage tanks the water flows downhill to a gully, uphill to the main building pad, and downhill again to a lower pad through the distribution line. Before I installed the transfer tank and solar pump the system was gravity-powered using a Highlifter pump located at the lowest point on my property, so the tank and pump were located near the existing input line. This makes the system difficult to maintain as I age and can no longer climb up and down the hill like a mountain goat, so I am considering moving them to a point near the spring and pumping the water into the storage tanks via the distribution line. In this scenario the storage tanks would occupy the 15 bar location on your diagram, the pump from the transfer tank would occupy the 10 bar location, and the tee to the house would be the mixing point. I have been told that pumping water back into the storage tanks via the distribution line will cause 'back flush" issues when I turn on a faucet or flush the toilet. I don't see why this should happen. If the pump is not running, the water will flow into the house from the storage tanks as it does now. If the pump is running, the water flowing into the house will come from the transfer tank, as the pump is operating at a pressure sufficient to lift the water all the way to the storage tanks. I can't see how it would make any difference whether that water going into the house flows in from the storage tanks via gravity or is pumped in from the transfer tank. Your comments would be greatly appreciated; thank you.

Thank you very much for the amazing explanation

Is the NRV sufficient for reducing backflow if system dP is 12 bar and source a and b are 12 bar and 10 bar respectively? If the system dP is 10bar and you want to maintain the flow at converging point to be the sum of source a and b, can you just put an NRV and pressure reducing valve on source A so they have equal pressure at the tee? Asking as designing a water treatment system for data centre cooling where I need to mix mains water with ro water to dilute it and mains water is 8.5 bar (from booster set) and ro water is at 1 bar. Thanks

With a positive displacement (gear) pump, never; with a centrifugal pump, always; this assume a liquid circulating system. If a gas system where pressure is controlled by a regulator, never, the 10 PSIG system's gas regulator would simply never open, so no reverse flow.

Waao💥💥

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