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# Flow rate

Member Posts: 20
I was wondering if any one knows and can explain to me how pressure and flow rate relate. I'm not thinking about heating. I'm thinking about on open pipe it would seem that if you had more pressure the water would flow out of the pipe faster; is there a formula for this? I was also wondering if you have a 3/4 pipe and you increase it to 1-1/4 does the flow rate increase or do you just have more stored volume, which means more psi in the sence that it has more room (more square inches). I don't know maybe I'm crazy but I always think of thes types of things.

• Member Posts: 6,928
Think old and gravity

Water seeks it own level. Some ancient cities had running water piped into each home--just as we have now. One was even on a mountain. The ancient engineers just found a water source on a higher mountain and even though the water had to flow down through a deep valley, it emerged at their city with pressure because it was lower than the other mountain.

In fact they sometimes had so much pressure that the ancient piping materials gave way and burst!

Fluid pressure can always be translated into the height of the source above the height of the outlet. Ancient Romans knew this well and their aqueducts were so successful because they engineered artificial "rivers" with a constant, VERY SLIGHT drop. Huge volumes of water are still moved this way.

In an open system like a river or aqueduct there is no pressure buildup against the walls of a pipe--pressure translates directly to velocity which translates directly to the degree of drop.

The flow through a pipe is determined mainly by the pressure (height of the water) and the size of the pipe. (The shape and roughness of the pipe also matter, but you don't have to concern yourself with that to understand the principles.) Since this is a closed system, it builds pressure against the walls of the pipe so pressure no longer equates perfectly to velocity or drop. The thing to remember is that the pipes strive to stay completely full of water.

Consider a pipe open at its outlet connected to a water main. As long as this pipe is smaller than the main, as much water as possible will flow. Increase the pressure and the quantity flowing will increase because the speed (velocity) of the water in the pipe increases as well. There is a definite limit to this as 1) you can break the pipe with too much pressure 2) as velocity increases so does frictional resistance--so some of that pressure becomes heat and does not translate to increased flow volume. Pressure in the main drops in relation to the amount of water flowing from it.

As you increase the size of the pipe, velocity remains the same but flow increases and so does the pressure drop in the main. Once the size of the pipe equals the size of the main, the system is at maximum possible flow--it can now only be increased by increasing the pressure in the main.

If you make the pipe larger than the main, you still get maximum flow--BUT at reduced velocity and pressure. If you make the pipe much larger than the main, it no longer runs full and it begins to behave like an aqueduct.

Now put this in the context of a home connected to a large water supply like a municipal system. Once you tap into the main the ultimate quantity you can get through your house "main" is determined by its size and the pressure of the street main.

As long as no water is being used in the house, the pressure of the system will be totally dependent upon the height in relation to the street main. The pressure at a given elevation will be exactly the same throughout the entire system. This is the static pressure of the system. It's important to remember that static pressure varies with elevation!

When you run water in the house, it behaves like the example above with an open pipe connected to a larger main. The relative size of the branches to the house main is much closer than the relative size of the house main to the street main. Thus is becomes quite easy to try to draw more water from the branches than can be supplied by the house main--velocity slows to a crawl and pressure drops considerably.

In this situation, increasing the size of the branch lines won't really help--you'll only get a significantly greater flow by increasing the size of the house main.

"I was also wondering if you have a 3/4 pipe and you increase it to 1-1/4 does the flow rate increase or do you just have more stored volume, which means more psi in the sence that it has more room (more square inches)."

Increasing the pipe size WILL NOT increase the PSI--you can only increase the PSI by increasing the elevation of the water supply. Flow will only increase appreciably if the pipe is connected to a main larger than this pipe.

• Member Posts: 240

Grab a physics book and look up Bernouli's equation. It is the relationship between flow, pressure, change in elevation, etc.
• Member Posts: 6,106
The limiting factor

in residential systems, if there's where your headed, is the size of the water meter and meter setter. That where the flow restriction will occur. Generally home meter setters are 5/8 or 3/4" limiting the gpm based on the main line pressure.

Pipe resistence, length, and number and type of fittings also play into the formula. The FPS velocity ultimatly becomes the determining factor on water pipe sizing. The CDA has tables and graphs showing this. www.copper.org

hot rod

• Member Posts: 23,906
some basic relationships...

the velocity in a pipe is more or less proportional to the SQUARE ROOT of the pressure DROP -- that is, if you double the pressure drop, the flow will increase by a factor of 1.4... that being said, the amount of flow (gallons per minute) is directly proportional to the area of the pipe -- thus, to the SQUARE of the diameter of the pipe. As someone else said, the actual pressure drop for a given velocity or flow rate affected by the length of the pipe... and even more so by the collection of fittings, valves and doo dads it encounters on the way...

That help? That's what you get for having an engineer lurking on the Wall!
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 140
Remembering the teachings of...

The Bernoulli Equation just gives me a headache! I worked in the field and was driven crazy by that stuff while going to college.

Skip the book, and read these posts...that's enough knowledge for 99% of people. The other 1% either are required to know it, or they enjoy reading long. tedious and confusing things :-)

As explained by Mike, (first reply) very well, just worry mostly about static head...the dynamic head in these cases probably accounts for, at most, 5% of the friction loss. Once the water enters the home, the meter is the boss (or the first fitting off the bladder tank from my well) as far as maximum flow...well said by HR (as usual).

One other item involving wells...the well pump works off a pressure switch, so in these cases you will see a noticable difference in flow and pressure as the pump cycles. This is also, of course, slightly changed by the effects of pressure drop/pipe size/type and elevation/number of open of use points.

Hope this helps, PJO
• Member Posts: 549
The Only Thing...

...that will give you a flow, is pressure drop. The greater the pressure drop in a given pipe, the higher the velocity, and the higher the flow. A pretty good rule of thumb for quick estimates for flow is that for the same conditions, is that increasing the pipe one size will give you about double the capacity. A 3/4" line will supply a pair of 1/2", etc.

I once had to explain to a guy that his problems in feeding three 2-1/2" process water lines (all running at once...) from a 2" supply through a factory could not be solved until he increased the supply main size. The guy with the problem was the production supervisor, and also professional engineer
• Member Posts: 6,928
That was a St. Louis hookup...

...where water is unmetered.

You're utterly right that the meter is the limiting factor in most. I believe that the entrance line (after the meter) is generally one pipe size larger than the meter to limit velocity and head loss (thus maximizing pressure) in the house main and its branches.
• Member Posts: 344