What happens when you mix different pressures?

A process engineer answers the question - what happens when you mix different pressures?

00:00 Introduction
00:52 Illustrating The Problem
02:25 A Thought Experiment
04:20 You Gotta Look Downstream
07:28 Outro

Process with Pat is the place to come for perspective and to ask stupid questions. I want you to leave more knowledgeable, confident, motivated, and most importantly, curious. I also want to invigorate a field that seems tired and uninspiring, at least if you get your perspective from internet forums. These are not lectures. This is a place for you to leave thinking “Oh! That’s why...”

This channel is not only for chemical engineers - anyone who works with processes should be able to find something of value here.

#ProcessEngineering
#ChemicalEngineering
#FluidDynamics

[alelaera]

What a nice video. It explains perfectly what happens in those systems and makes lots of examples. I think this is one of those problems that just are tricky but in reality they're really simple, as you made it look at the end

[Chinguenguenza]

If you take the original problem as stated (similar lines, no valves), then the outlet pressure is 15 bar (minus pressure losses) and there is no flow from source B. If fact, some or all the flow from source A would go into B. The key here is to distinguish between Rating an existing system versus Designing a new one. If I can add valves wherever I want to, I am designing. If I need to calculate pressures for a fixed system, I am Rating.

[nicolaskoutsouvelis]

To think how many students (and engineers alike) have never truly understood this concept. Extremely clear and entertaining explanation. If only more lecturers had your gift of unwrapping a problem and coaxing out the answer in such a logical and comprehensible form.

[robertbradfield]

I’ve seen similar problems in electrical systems. It’s very easy to get trolled if you don’t consider the full context or spot what’s yet to be defined. Great video, thanks Pat!

[jackmclane]

It is the same in electrical engineering. The impedance of a line determines the voltage at a given current flow...
I love it when seemingly independent fields of engineering go hand in hand.
Just as how Navier Stokes and Maxwell equations have a lot of symmetries.

[rasmussverdrup]

I always find it amazing that intuitive understanding of pressures is such a bad guide to reality. Great video!

[waynejohnstone]

Interesting - I do underground process water systems in mining and deal with this constantly. These are generally closed systems (no atmospheric dumps unless we’re filling reservoirs etc and even then we’d use orifice plates with PRV or sustaining)so we deal with frictional losses as back pressure in miles of pipe, a lot of it horizontal. Joining two streams isn’t really ideal underground, the higher pressure from one would just push back against the other until the pressure drop was big enough to cause mixing. We do force this sometimes using sustaining valves though. Anyway, great video and great explanation on how the entire system needs to be understood.

[JoJoDo]

I actually think there's a bigger point that I notice some people oversee. You nicely demonstrate that anything that happens downstream has a big effect on what's upstream, which is also the case with external flow. And that is what I think people sometimes forget/don't know.

[tirthrajchavan]

Hey pat the way you simplify and explain things is just so helpful ...why did you stop making these videos man, we want our passionate process Engineer back man

[clytle]

Good explanation. As an electronics/maintenance person it really describes a conversation I had with a chemical engineer. I was not successful in my attempts to explain it

[ianbelletti]

The problem is that the more restricting the mixing pipe is and the following equipment the more likely source A is going to flow into source B. That is why back flow preventers often get installed.

[djv]

I’m happy to learn that I was correct to become confused when you drew a T connection but were saying “combined streams.” My gut reaction was “how they are they being combined?”

[aspen_school]

Thank you for addressing very basic but highly confusing problem. 👏

[Sgrunterundt]

As a physicist my immeadiate reaction was: "Well you're not giving me enough information".

[Southghost]

Great explanation. One thing to add is that geomtery plays a big role in the way fluids mix and pressure change occurs. For example ejectors and venturis are practical devices for mixing fluids and allowing a low pressure stream to increase in pressure past the mixing point. Whereas two pipes coming to a T can cause backflow unless pressure is reduced in the high pressure stream.

[KallePihlajasaari]

Lovely video. Another way to design the process and perhaps even a preferred way for safety and management in most cases would be to have the two pipes combine in the reactor vessel then the feed lines have a constant back pressure (as you choose) and are not affected by pressure or feed rate changes in the other reactant.

I can think of no compelling reason to combine the two feeds inside the plumbing, a mechanically coupled two gang feed pump would be preferred even if the excuse of saving on a pressurised feed pump was the excuse. Saving on a reactor fitting would be a rare reason to justify the early mixing.

Someone on Facebook recommended your video, said you were worth watching. I will keep an eye out for more.

[Hilde_von_Derp]

Another analogy that helped me is to transpose it to circuit theory and use the node laws. Voltages into a node have to be the same, and sum of currents into and out of a node always sum to zero.

I tackled this years ago with a mixing issue on fluids. Worked back from the outlet pressure and wrote an iterative spreadsheet to solve for equal remaindered pressures on both lines. Centrifugal pumps involved so I baked in the pump curves on each side and baked each line down to an equivalent Cv to make things quick to calculate.

[crashandrise]

It's a delight to see you speak again. Guess I'm gonna be a chemical engineer now

[TomasSabD]

Guess... You get to control the flow, by selecting the output pressure... and have to attach the 10bar at a point in the pipe - where the pressure has already dropped below 10... they can't be connected in "parallel".

[jamesortolano]

Holy moly, this makes me ask so many questions as I’m trying to devise my pool pump location, supply nd return piping . Parallel nd singular pipe options.
Nd you gave me all the answers I need.
Bravo, very happy you nd your channel popped up in my feed, while looking into flow, diameter nd pumps.
Thank you again