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I've had a couple questions above the sizing of a pipe, but I'm still a little confused and can't find an easy answer.

I'll use this image to get a representation of what I'm looking to have answered.

Change in sizing of a pipe

Sizes shouldn't matter, but if they were; we'll assume the small pipes are both 1/2" and the big pipe is just 3/4". What effect does it have on the water flow to increase the pipe size (into the center larger pipe)? And then what effect does it have on the flow to decrease the size?

The shape doesn't have to be a U like this; I just figured it'd knock out two birds with one stone. It simply represents an increase and then a decrease. You can treat them separately in the answers or as a whole if it does matter.

My general thoughts, from what I have been able to read through online, is that the pressure might slightly increase when going into the larger pipe or decrease when going down. Simply because of friction though - smaller pipe has more friction towards the water than the larger pipe. If this were one piece of plumbing, would the pressure out at the right be relatively the same as the pressure in at the left?

The other thing I see if the speed of the water. Increasing the pipe size slows down the water flow and decreasing the pipe size speeds it up? Again though, if it were one piece, would this balance out and the speed out would be like the speed in?

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  • You should start with a straight pipe, as it will make it a bit simpler. Also, this might be a better fit for Physics.SE
    – Tester101
    Mar 4, 2016 at 17:23
  • @Tester101 I would guess that they'd know the answer more readily, yes, but I also figured it's something worth knowing for diy plumbing.
    – TFK
    Mar 4, 2016 at 17:37
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    I don't see it making a noticeable difference expect with hot water. You notice a longer time for hot water to reach faucet.
    – justin j
    Mar 5, 2016 at 2:49
  • @justinj I agree. The larger volume of the pipe in the middle is likely to have roughly the same effect as making a smaller pipe longer. Either way, you have to push more water volume to get water to the far end, so if you're pushing hot water, you're going to have to push more cold water out of the way to get the hot water to the other end.
    – Craig Tullis
    Mar 5, 2016 at 18:23
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    I am about 4 years late to this party. First of all, assuming we are talking about the pipe once it is full of water, the volumetric flow rate in is the same as the flow rate out. That means that the same amount of water is moving through any cross section of the pipe (large or small) at any given time. The velocity is greater in the smaller pipes (since water needs to move faster through the smaller cross sectional area to constantly deliver the same amount of water). The pressure is greater at the beginning of the pipe, and it decreases as you move along the pipe. This makes it flow.
    – Joseph
    Feb 29, 2020 at 2:56

4 Answers 4

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You should see the same water pressure on both sides of the bigger section of pipe.

I wouldn't expect the bigger pipe to really make any pertinent difference.

The water will flow more slowly in the bigger pipe, but the pressure will increase (Bernoulli's law, the same thing that makes an airplane wing fly, but applied to fluid dynamics). The water will fill and pressurize the bigger pipe, and the greater pressure in the section of big pipe will force the water into the small pipe on the far side at the same velocity that the water entered on the near side. Again due to the phenomenon described by Bernoulli's law, faster-moving water in the smaller pipe will exert less pressure on the pipe. That isn't to say that there is less energy in the smaller section of pipe. Part of that energy is accounted for in the greater momentum of the fluid. So the water coming out the end of that pipe will be moving with as much force as the water that went into the other end.

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  • Nice explanation. Now if the larger pipes were much much larger and had some length to them. Might get a hell of a burst.
    – DMoore
    Mar 5, 2016 at 18:32
  • The extra fittings etc. will add to the pressure drop as well
    – StayOnTarget
    Feb 1, 2019 at 11:19
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The velocity, flow rate, and pressure will be nearly identical at the inlet and outlet of that assembly. The only difference will be energy lost while moving through the pipe, e.g. pressure drop due to resistance.

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  • The pressure loss will be greater with greater flow rate... so it could depend on the application how much of a concern that is. But I'd think just the addition of two size changes & two elbows wouldn't be a concern in most residential plumbing!
    – StayOnTarget
    Feb 1, 2019 at 11:19
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A larger pipe, and lower velocity, has less pressure loss. The fittings in a larger pipe also have less pressure loss. So, all things considered, if you want to lose less pressure through a series of pipes and fittings, you increase the size. The trade-off is that bigger pipes and fittings cost more, and, as noted in a comment here, bigger pipes would take longer to deliver hot water.

In the example you drew, the larger pipe, and the elbows on that larger pipe would mean that you would get more of the original pressure to the fitting, rather than losing it to friction in the pipe, compared to if you went with small pipe the whole way. The difference would depend on the flow rate, which is why many different sizes of pipe exist.

There is actually an additional pressure loss introduced just by changing the pipe size (and will depend on what kind of fitting you use to do that), because the water has to change directions (flowing out or in rather than straight down the pipe), so that also has to be accounted for when you decide if it's worth it to increase pipe size.

At the end, for a real world system, the losses have to be calculated to determine what is the best trade-off between cost and pressure loss.

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I'd expect the pressure to Decrease as it entered the larger pipe since it can then expand into the larger volume. However, real world application would need to determine any actual effect. Since, water in motion will buffer itself when it encounters an obstacle. Like a rock in a river, water actually makes a buffer bubble behind the rock.

So, actually flowing in from 1/2" & exiting out back into 1/2" would likely show no difference than a full 1/2" bend from a 3/4" bend. But, 1" might be the limit of that buffering as the water's cavitation may actually start to fight back the flow.

Your example is commonly done daily with compression fittings being larger diameter & conversely with SharkBite fittings being smaller diameter & there's no difference felt, sure maybe there's a measureable deminimis difference but nothing noticed by a faucet user.

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  • The water is moving faster in the smaller pipe, so pressure will be lower in the smaller pipe. Bernoulli's law applied to fluid dynamics. That isn't to say there is less force overall. The water has momentum, but there is less pressure on the walls of the pipe. In the bigger section of pipe, I'd expect higher pressure, lower velocity. I expect the water will fill the bigger section of pipe, where the greater pressure will force the water to move faster into the small pipe on the other side again. Overall, no change to the system aside from losses to cavitation and friction.
    – Craig Tullis
    Mar 5, 2016 at 2:38
  • Oh, I didn't know this was suddenly the physics forum. Thus, why I gave real world examples. There is no effect upon the end result, therefore what happens between point A & B have zero value. Your higher pressure ONLY comes from the cavitation buffering due to momentum & that there's a constricted outlet, period. I suggest looking at a waterfall, the velocity & pressure die instantly upon breaching their constraints.
    – Iggy
    Mar 5, 2016 at 4:09
  • ...because physics doesn't work in the real world. On the other hand, engineers get to hang their engineering certificate on the wall specifically because they understand things like physics well enough to design the things that builders are expected to build, in such a way that they are both safe and meet performance and cost specifications, and every aspect of that deals with physics. So... physics wins. ;-)
    – Craig Tullis
    Mar 5, 2016 at 18:15
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    Bernoulli's principle is just the conservation of energy in a fluid, regardless of its compressibility. It just says that as the fluid speeds up it has more kinetic energy, to conserve energy, the static pressure must drop. This is true for all fluid. However, the change in static pressure from a small pipe to a big pipe means nothing unless you are opening that pipe and using the water there, if you just reduce it back to a small pipe, you'll never know the pressure changed in the big pipe. The losses (friction), on the other hand, matter, that's why we change pipe sizes.
    – Joel Keene
    Mar 5, 2016 at 21:11
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    Don't matter? No, that's not what I said. Of course compressibility is relevant if you're trying to compute the actual pressure in the system, since density of the fluid is part of the equation. With water, the density is just a constant, but it's the same equation. Thus compressibility is irrelevant to this discussion. You said that if you feed a large pipe from a small pipe, the pressure will drop, and that simply isn't true. You also blanket-lambasted at least three old, respectable professions which require education, certification and licensing for good reasons, and that ruffles me a bit.
    – Craig Tullis
    Mar 5, 2016 at 21:11

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