Just need to vent...

That is, just as a higher current through a wire involves a higher voltage drop between 'upstream' and 'downstream' on the wire, the higher flow through a pipe involves a higher pressure drop between upstream and downstream of the pipe.

What the voltage or pressure is on some 'absolute' scale at either point doesn't come into it.

 

Try it the other way around. There is no flow without pressure. If you observe a change in flow, there already was a change in pressure causing it.

 

So change in pressure in which direction? Again for my current question, it greater flow rate 50gpm flowing in a pipe has greater or lesser or the same pressure than when only 5gpm flowing in the same pipe?

So, with this statement, if I measure the water pressure in the pipe at a single location, whether the flow rate is 50gpm or 5gpm at that instance makes no difference?

 

The pressure at the pipe inlet is the source PSI, the pressure at the open pipe outlet is zero, and the pressure changes linearly along the pipe length. The flow rate is then determined by the source PSI and the "resistance" of the pipe (proportional to the pipe length and inversely proportional to the pipe cross sectional area.

JeffD

 

If you measure a "single location" you're still measuring the difference between input pressure and output pressure, because to get flow you must a) put pressure on it and b) give it an outlet somewhere.

Since we are talking about a hypothetical plain pipe of constant size?

Then you must exert more pressure on the water in the pipe in order to hit 50gph than you would to hit 5gph.

 

Wound up installing the first 8,000 BTU Midea AC unit I received today. It took a good 1 hour or so. But that's because I was watching the video and going through the official installation very carefully. There were some tricky parts where I miss-positioned the bracket and I had to restart from the beginning after realizing that the bracket was too far inside of the sill. Also, I had a hard time screwing the metal parts to the side of the unit because there is not enough space between the window frame to fit a screwdriver. But all and all, it was not very difficult, though not as easy as the old unit which was basically just place it on the sill and shut the window. And for this 8,000 BTU unit. It was a perfectly fine one-man job.

Operationally, it is much quieter than the old unit. Plus at least a 50% increase in efficiency. It is a good unit. I have not set up a WiFi function yet. Plus even though I got this unit as "used" from Amazon Wearhouse, as far as I can tell, it is brand spanking new. No sigh it was even unboxed, let alone installed and used before. I got it far better price for this unit than the current Amazon Prime price, so no complaint there.

If the 12,000 BTU unit that is supposed to come tomorrow is only a few pounds heavier, I should be able to install it in no time now I know how to do the tricky part.

 

Sorry for going back and force on this topic. You guys are such good sports.

In both explanations, I can understand that a higher PSI drop is needed to produce more flow rate. My reasoning was that if a higher pressure drop is needed to produce greater flow, then the greater flow of the water exerts more pressure along the pipe than less flow of water. But so far all the hydrodynamics lessons I am getting here from great teachers led by @ChapmanF seem to suggest that it doesn't. That's the part I don't understand. I mean 50 gallons of water gushing through the pipe every minute surely seems to exert more pressure on the walls of the pipe than 5 gallons of water trickling down the same pipe.

 

Here you seem to be literally going back and forth. As Leadfoot suggested, it's easier to think about the pressure being responsible for the flow. If nobody is changing the pipe size, then higher pressure drop is the only way to make higher flow happen. So, when you see the higher flow, can you infer the pressure drop is higher? Absolutely.

In the first of this string of posts, the physical context was water supply plumbing, where pipes have the source PSI upstream and atmospheric pressure at the outlet (though usually there is some flow-restricting aerator or showerhead there).

But in later posts Sal has been wanting to divorce the question from that physical context, and just ask it completely abstractly.

In the abstract, it doesn't matter how the pressure drop along the pipe is made. You could increase the upstream pressure, reduce the downstream pressure, or both.

But it might be that before much more can be said, it would help to put the question back into some physical context, so we're able to picture the same thing.

The hydrodynamic effects get superimposed on top of that picture: in the middle of the pipe, say, you'd expect a pressure midway between the upstream and downstream ends, but if the middle of the pipe is a narrow spot, the pressure there will be lower than that.

 

I am so happy about iPhone "Silence Unknown Callers." SPAM calls never ring and their source addresses are strange places. I simply delete their call record. The same is true for voice mail and text.

If my former land line had caller ID with a record of my outbound calls being the "contact list," I might have kept it. But in the last year, it became just a SPAM line ... unusable. But I remember paying $30/month.

In contrast, my cell phone and two mobile hot spots cost $60/month and provides alternate Internet access if the cable modem provider is down or I'm on a road trip.

Bob Wilson

 

I think I got it. Not that I understood everything you have explained to me. But the root of my mistake was that I was imagining that somehow the flow of water can exert or produce pressure. It is the other way around and only in that direction. The pressure drops in the pipe are responsible for the flow of water. The flow of water does not generate pressure the way I was imagining.

Thank you for the great lessons in hydrodynamics 101. I know I did not Ace it, but did I pass? Or do I have to take it over? LOL

 

And we are not even addressing water that's pressurized to pump uphill !
LOL
Some of these discussions mirror the dynamics of CNG .... bigger pipe at the head and smaller at the source.
.

 

Hey, don't even try going there. That's for the Advanced Hydro Dynamics 302 topic. I am not going to major in physics, I don't need that course!

Seriously, I really hate physics. As much as I love and have been immersed in science, physics is one subject I never grasped well. Oh, and to some extent chemistry too, but I did finish with a minor in chemistry. P-Chem stopped my attempt to double major in both biology and chemistry. LOL

 

So just to dial it back to the pipe and pressure that started it, I just finished installing the new sump pump.

It exerts enough pressure on the output pipe to flow the water out of my sump approximately twice as fast as my old one.

 

It might be one of those chicken/egg things, just like Ohm's law, which only tells you how the voltage drop is proportional to the current flowing. It's people who have this extra itch to say which one is causing which, and it's usually easier to think about the flow happening because of the drop. The math doesn't care.

The hydrodynamic effects, like the pressure being lower than you expect where the pipe has a narrow spot, those are easier to think of as being caused by the flow.

Those are normally less interesting secondary effects—nobody plumbing a house thinks about narrowing a pipe to get venturi effects—but for certain special purposes, like a jet pump for a well or the sump backup pumps back in post #780, they become the whole point.

 

Here is another interesting type of pump that many who didn't grow up in a rural area with abundant water may not have ever heard of. It harnesses the kinetic energy from water hammer to pump water.

 

OH YES you are so right , it seems to me that some people suffer a drop of IQ when they get behind the wheel of a car.

 

Easy mistake. Pumps have gallon per minute ratings, so one can forget the pump is actually creating pressure to push the water.

Neat. For those that prefer reading, https://en.wikipedia.org/wiki/Hydraulic_ram

 

I've been struck in the crosswalk, with the WALK light in my favor, ending up sprawled on the hood and leaving angry fist dents in the hood. The driver did perform a full stop before accelerating into me to make a right turn. I thought I had made eye contact, but apparently he was looking through me and focusing only on the cop stopped at a different point on the intersection. The cop witnessed it all and cited him for reckless driving.

Much later, mom was struck, also in the marked crosswalk with the WALK light, from behind by someone turning. She was already losing mobility, and this injury stole half her remaining mobility. Then, the joint inflammation had not sufficiently healed when she was diagnosed with cancer and needed chemotherapy, which then attacked the inflamed joints too, adding to the disability and misery of her terminal year.

I've since been less tolerant off, and more 'assertive' against, drivers who violate pedestrian right-of-way.