Just need to vent...

You are still writing as if there can be a "flow rate of the water feeding the system" independently of whether the water is flowing.

I'm running out of ideas for how to respond to that. There is no water flowing except when it's flowing. Then it is flowing at the rate you allow it to flow. A maximum flow rate of your water supply is simply that: the rate where your flow will max out when you have everything open, because the capacity of the upstream system can't give you more.

 

Chap, my max flow rate is set by the 3/4 inch feed to my home from the Main under the street. I have never come near that rate as the water pressure in my home has never dropped below 60 (it would be zero if I drew the actual max flow rate - about 130 GPM assuming that the main pipe pressure would not drop). My lawn sprinkler draws water before my pressure regulator (set at 65) and drops the PSI from 85 to about 65-70 and that is at a flow rate of about 20 gallons per minute.

JeffD

 

Yeah, I think my service from the street was probably also 3/4" when it was laid. That's the size of the end I can see.

By observing the pressure drop when I draw different flow rates through it, and poking around in Engineering ToolBox, I am guessing that pipe has about a 1/4" effective internal passage by now.

I replaced all the galvanized plumbing inside the house right after buying, but still haven't dug up that service line.

If I ever do replace the service, with new pipe with a clean 3/4" bore, I will probably also want one of those leak-detecting auto-shutoff valves, given the vast amount of water such a pipe could dump into my house.

 

No, you are still not answering my question. I understand that water flow is zero when the system is closed. And it is at max if there is no resistance in the system. But that is not what I am asking.

OK, put it this way, suppose water is pumped from a submersible pump into a closed system of pipes (how long ever it is to be practical) with a single faucet at the end of the pipe. If this submersible pump has a variable power control such that it can adjust the flow rate of pumping water. For the sake of argument, there is no limit on the body of water the pump is submerged. Is there any internal water pressure difference of the pipe when the pump is set to 5 gpm vs 50 gpm? I would think, if I open the faucet at the end of the pipe, the velocity and power of the water coming out of the faucet are much larger if the pump is set at 50 gpm than at 5 gpm? Maybe not 10X more as the "Flow is proportional to the square root of delta P", but it seems a faster flow rate feeding the system will result in a faster flow rate at the faucet. So, are you saying increasing the pump flow rate from 5 gpm to 50 gpm will not change the water pressure in this system?

 

The usual pump or water supply source cannot determine flow. There won't be a "GPM delivered" knob on it for anyone to set like that. The pump only produces pressure. Whatever flow does (or doesn't) result is a dependent variable, determined by that pressure and by the downstream resistance to flow (decreasing as more faucets are opened).

Analogously, an electrical battery only produces a voltage. Whatever current does or doesn't flow is a dependent variable, determined by the voltage and the resistance of the circuit being served.

You could have two 12 volt batteries, a little dinky one and a big honkin' one, and both would measure 12 volts while you're not taking any current.

As you connect lower and lower resistances, allowing larger current to flow, the voltage from both batteries will drop, but that of the dinky one will drop off more steeply than that of the honkin' one.* Finally, at short-circuit conditions, the voltage will have dropped far below 12 and a large current will be flowing, larger from the big battery than from the little one. At that point, you'll have identified the maximum current each battery can supply. But neither battery somehow forces its maximum rated current on the circuit. The current that flows is determined by the circuit, and the voltage across it.

There is such a thing as a "positive displacement" pump: one whose every revolution delivers a fixed volume of water. If you can turn such a pump at a fixed RPM, then you know a fixed flow rate is being moved. What happens if you give such a pump nothing but closed faucets to pump through? Yes, the pressure will rise until the engine turning it stalls or something breaks or a pipe bursts. That question is analogous to "what will an electronic constant-current source do when facing an open circuit?"

There aren't any potable water pumping systems, well or municipal, that are built that way, for just that reason.


* I'm writing here only of the drop in voltage due to the battery's current capacity (modeled as an "internal resistance"), as if running this test so quickly that neither battery significantly discharges. Of course if the test continues longer, the reduction of each battery's state of charge is another reason for the voltage to drop.

 

Oh, come on. I am not asking how the pump works. I am simply asking if the different flow rate from the pump makes any difference in the water pressure in the pipe. Since you are not going along with my "hypothetical" flow rate controllable pump, then how about two different pumps, one rated for 5gpm and another rated for 50gpm. I would think a 5gpm pump is much smaller and has less HP than the 50gpm pump. If the same system of pipe and a single faucet (or it can be even a hose with a valve on the end) is connected either a 5gpm pump pumping the water to the pipe or a 50gpm pump pumping the water to the pipe, is there any water pressure difference? Doesn't the water at the end of the pipe come out much faster and more powerful with the 50gpm pump?

 

But your whole line of questioning started back here where you were responding to another poster's casual guess at the maximum flow rate that might be available from their water service.

There hasn't been anybody in this whole thread, except for you, ever suggesting there is such a thing involved as a pump trying to make some predetermined amount of water flow into your house. The post you were originally responding to didn't suggest that. Nobody else has suggested that. Maybe it makes a fun hypothetical question of the form "if water systems were built in this other way that they're not built, what would they be like?" But a bit of thinking about that question leads straightforwardly to "ah, right, that's why they're not built that way."

 

Had a peristaltic recirculating a sodium carbonate solution when a line clogged. Since finding white spots in the lab

 

Fair enough. I admit, I have totally diverted from my original comment that relates to the no-battery backup sump pump system. I am not arguing your explanation nor trying to prove or disprove any of the statements you or I made on this thread related to the backup sump pump system.

I am simply trying to understand how the flow rate relates to the water pressure. This question has diverged enough now it has nothing to do with the ordinally home plumbing system. I now understand that a home plumbing system has a series of flow rate regulators and pressure control devices such that the upstream water flow rate does not change the internal water pressure in the pipe. However, without any of those regulators and controllers, I am wondering if the increase in upstream flow rate would increase the downstream water pressure. As I said, I am not versed in hydrodynamics, so with my very naive intuition and some life experience, I thought if the upstream water flow is 10x higher, that would result in an increase in water pressure downstream. But if it doesn't, then I am totally clueless as to why that is. So, I just want to know if the flow rate upstream affects the water pressure downstream or not.

 

It is still far from clear what you even think you are talking about when you say "the upstream water flow rate". What are you thinking that can possibly even mean?

It doesn't take any fancy regulators or devices to decouple "the upstream water flow rate" (whatever in blue tunket that is) from the pipe pressure.

Consider the simplest sort of municipal water system: a tower 138 feet high (so as to produce 60 psi water pressure) with a water tank at the top. When the level starts to drop, more water gets dumped into the tank by a pump. When the level is right, the pump stops.

A pipe runs down from that tank and into your house. The pressure in that pipe is 60 psi because it can't be anything else. When you open your faucets you get water out (at close to 60 psi, but a little lower because of pressure drop in the pipe).

How much does the pressure drop when you let water flow? Depends on the length of that pipe and how big around it is. A skinnier pipe drops the pressure more than a fat pipe, for the same flow rate.

The maximum flow you can get (when you basically hack off the end of the pipe and let the water glug out into atmospheric pressure) is completely determined by the size of that pipe.

If that water tower is serving Alice's house and Bob's house, and there's a skinny pipe to Alice's and a fat pipe to Bob's, Bob has a higher available maximum flow rate than Alice. That doesn't in any way mean Bob is served at a higher pressure.

 

OK, let's back up a bit. I am and have been asking relationship between water flow rate and water pressure. My question no longer concerns house plumbing or municipal water tank or well pump. In the simplest physics explanation, if the water flow rate is 10x faster before entering into a pipe where the water pressure is measured, does it increase the psi measurement compared to the 1x water flow rate? Yes or No? If Yes, then I am satisfied. If No, I will have to ask more questions as to why that is.

 

If you measure the pressure difference between an upstream and a downstream point in that pipe, while a given flow rate is flowing through that pipe between those points, you will measure a larger pressure difference at a larger flow rate.

That may or may not seem to be the answer to your question. If it seems not to be, that might call for more carefully stating the question.

 

OK, the larger Δpressure means more drop in pressure with a larger flow rate. So, if it started with the same pressure upstream, then at the lower point measurement, it will be lower pressure... That seems the opposite of my intuition. But does the higher flow rate make the pressure upstream higher than the slower flow rate? If it does, then even if Δpressure (pressure drop) is larger, the downstream pressure measurement may be increased. In a given same pipe size, does the higher flow rate result in higher pressure?

 

i think I'm getting a pain behind my right eye .....


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What, again, do you mean by "flow rate" here?

If anything other than the actual rate of actual water flowing at this actual instant through this actual pipe, we're still not on the same page (or even in the same book).

 

I believe that Sal is confusing the speed of the water flow (meters/second) with the rate of flow (gallons per minute). when you neck down a pipe, the speed of flow can increase, but the rate of flow will decreases due to the increase in back pressure.

JeffD

 

It was seeming to me that the confusion might be more fundamental, at the difference between "flow rate" as actual flow of actual water through an actual pipe, or as some kind of specification that some kind of source may be trying to achieve.

Speaking of flow rate and velocity in a necked-down pipe, of course the flow rate has to be the same in all segments of the pipe, wide or narrow, hence the higher velocity in the narrowed segment. (The rate of flow may be decreased compared to a similar pipe lacking the narrowed segment, but within the given pipe the flow rate is necessarily the same throughout, assuming no taps along the way where water could enter or leave.)

 

Yes, I am talking about the actual rate of actual water flowing at the actual instant through the actual pipe. Well, you said, "while a given flow rate is flowing through that pipe between those points, you will measure a larger pressure difference at a larger flow rate."
At a given point in time inside of a pipe at one location inside the pipe, a certain volume of water is flowing through the pipe. So, if the GPM "flow rate" at that point is 50 GPM, would that exert more pressure at that location in the pipe than when only 5 GPM of water is flowing?

I got the difference between flow rate and speed. I might have learned that in Basic Physics 101 for non-engineer majors. I can perfectly understand (almost intuitively) that speed increases when the size of the pipe gets smaller. And I think I can see why the rate of flow may drop as the water enters the smaller pipe.

But for my question, I am asking a single-size pipe. No change in diameter. Does the greater flow rate (volume of water passing in a given time unit as in gallons per minute) exert more pressure than the lesser flow rate? Or is it the same no matter what the flow rates are? Or they are totally irrelevant and unrelated measurements? Or something else?

Again, my question is not about house plumbing. I am asking the question in the Physics 201 principal of hydrodynamics, the course obviously I have never taken. LOL

 

Sal, It does take a higher pressure drop to push a greater flow rate through a given pipe of a given length. I did take a course in fluid dynamics as an EE undergraduate student.

JeffD

 

Thanks. But that still does not answer my question. My question was does a higher flow rate result in higher pressure?

Maybe what I am not grasping is the "pressure drop" part. Does the flow rate only relate to the "pressure drop" not to the actual instant absolute value of pressure?

Yeah, this is a vent-out thread. I hate physics. I don't get it.