Fittings And Pipes Sizes.. Effect On Pressure? - Plumbing - DIY Home Improvement | DIYChatroom

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Old 05-16-2008, 09:39 AM   #1
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fittings and pipes sizes.. effect on pressure?

Are there any web resources/pages/sites out there that go in to the details of how different pipe diameters & elbow fittings (90/45 deg/T/elbows/etc.) effect water pressure? I've been searching & unable to find anything other than some things on gravity & how to angle pipes (well duh).

Or, someone here can briefly (if possible) explain?

I just recently converted all my cold-water plumming over to PEX from old galv pipes. My water pressure, probably not just doubled, but trippled (at least how I "architected" the pipes). So much so my old utility sink is having a hard time with it. But I did so based on some guessing work & it somehow all came together great.

For a few days, in one particular section, I ran about 5 feet of a 3/4 pipe from one fitting to another. Water pressure was great & such. Then I made some aesthetic modifications to better line things up & ended up cutting the pipe & setting up 2 x 90 degree angles (I made my own custom elbows out of 3 fittings instead of using the skinny PEX elbow). Anyway I have no idea.. maybe it's placebo effect or not, but I think the water pressure was even better than before. I guess a simple pressure guage would help but.. I don't have one yet.

So my questions are:
How do the different elbows effect water pressure?
(Logically it seems that the less elbows the better, but I don't think that's the case... seems like they can also be used to build pressure?)
How does a cut-over from 3/4 to 1/2 inch effect water pressure?
How does a cut-over from 3/4 to 1/2 inch (say 1 - 2 feet) and then back to 3/4 effect water pressure?

thanks again.


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Old 05-17-2008, 12:15 AM   #2
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I don't think that your pressure would have changed unless either A) your pump system was adjusted, or B ) your pressure regulator was monkeyed with.

Think about what happens when you put your thumb over the end of a hose. As you're closing off the opening, water pressure is still trying to force water out. You're relating this to an increase in pressure. What you actually have is a pipe with full volume in it. Pressure and volume are very different. I have a hard time explaining the difference between volume and pressure even though it makes sense in my own head....

Here is what I found... I hope this helps you.


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Old 05-19-2008, 09:58 AM   #3
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Pressure will drop through out the piping system due to elevation changes, friction in piping as well as frictional losses due to fittings. In engineering we refer to these losses of pressure as head losses. There is 2.3 feet of head per pound per square inch (PSI)

For example: In a 3/4" type L copper pipe with a flow rate of 5 gallons per minute (GPM) there is a head loss of roughly 8 feet per 100' of pipe. Or 3.5 PSI per 100'.

Now looking at fittings there are tables that give the equivlent length of the fittings. For example: A 3/4" regular 45 ell screwed steel has an equivilent length of pipe of 0.92 feet of pipe.

So to look at a system we would total up the length of pipe and the equivilent lengths of pipe due to each fitting. In a home plumbing system we would look at the longest run. Then we can take that total pipe length (Say 300 feet) and an assumed flow rate (5 GPM from above) and come up with the total head loss (pressure drop) of 24 feet or or 10.4 PSI. We would then add any elevation changes to that number say our longest run ended on the 2nd floor our elevaton change is roughly 16' or another 7 psi.

So to keep the supplied pressure up, limit the amount of fittings including the expansions and reductions.
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Old 05-19-2008, 10:37 AM   #4
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my head is still juggling the #s but overall thanks for helping me understand.. in the end its just common sense; less angles/less resistance is best but I never know.. maybe I was over thinking things.
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