# pipe pressure

So I got scolded about pressure in my drip pipes. My understanding of hydrostatics is that if there is a pressure (e.g. 25psi) in a system, it is that pressure everywhere regardless of pipe diameter (assuming no gravitational gradient). Now, of course, the water is moving when the system is in operation, but it has a regulator that tries to maintain 25psi.

I was told it was bad going from 3/4" (PVC) to 1/2" poly pipe. I should be using 1/2" PVC, so I was told.

I’m slowly replacing all my 1/2" poly, so that won’t matter in another few years. But I’m wondering if there is any truth the that claim.

My understanding of hydrostatics is that if there is a pressure (e.g. 25psi) in a system, it is that pressure everywhere regardless of pipe diameter (assuming no gravitational gradient).

That is true for static water - water that is not moving. And I suspect we can pretty much ignore the gravitational gradient unless you’re dealing with a sizeable hill on your property or are trying to deliver water to the 3rd or 4th floor.

My guess is that the issue has to do with the times when the water is moving through the system. Bernoulli enters the picture here. And I’m not qualified to discuss Bernoulli. In the paraphrased words of Bones McCoy, “Darn it poorguy! I’m an accountant, not a hydrodynamic engineer.”

My accountant’s understanding is that the flow of water changes speed as it goes from the larger pipe to the smaller one (or the other way round) and that change in speed changes the pressure. Which way and by how much is something I’d have to try to learn again. I’m not going to publicly embarrass myself by attempting that in real time.

And I’m almost certain the problem gets more complex if you have a larger feeder pipe that splits off into multiple smaller lines. The briefest of google searches turned up differential equations. While I took three semesters of calculus in college, the bulk of that learning has faded into oblivion. But if you ever need to figure out ladders sliding down walls and across floors, I’m your guy!

At any rate, I suspect there can be issues here. What they are and how bad they can be is beyond me. With a bit of luck, a real hydrodynamicist will magically appear and explain things better.

–Peter

I was told it was bad going from 3/4" (PVC) to 1/2" poly pipe. I should be using 1/2" PVC, so I was told.

I’m pretty sure that your water main is a larger diameter than the pipe into your house.

Mike

PG there is no issue. I mean there is a pipe much above your sizes in the street outside my house. If you get larger expensive houses it is not uncommon for the house to have a one inch supply pipe and distribution pipes of 3/4 to high use functions like irrigation and ½ inch to baths.

You mention 25psi. That is too low in my view. The normal pressures are something like 50psi.

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I was told it was bad going from 3/4" (PVC) to 1/2" poly pipe. I should be using 1/2" PVC, so I was told.

I’ve been doing drip irrigation for 40 years and this is the first time I’ve ever heard this. More to the point I have had systems at several houses that do exactly this and have never had an issue.

Here’s a converter made by Orbit that does it as well. (There are others, this is just the first I found.) https://www.amazon.com/Orbit-DripMaster-67447-4-Inch-Adapter…

You mention 25psi. That is too low in my view. The normal pressures are something like 50psi.

The pressure reducers made for drip irrigation bring the pressure down to 25-30psi.
https://www.amazon.com/Pressure-Regulator-Reducer-Irrigation…

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I was told it was bad going from 3/4" (PVC) to 1/2" poly pipe. I should be using 1/2" PVC, so I was told.

I’m slowly replacing all my 1/2" poly, so that won’t matter in another few years. But I’m wondering if there is any truth the that claim. - 1pg

I did all the plumbing where I presently live. I have 1.5 inch PVC exiting the pressure tank on the well. That 1.5" backbone extends about 2,500 feet around the property to feed an irrigation system. I tap into that backbone at 30 or so places using 1 inch PVC to run laterals to the sprinkler heads, where I have to step down to 1/2 inch to accommodate the sprinkler head.

I also tap that 1.5" backbone with a one inch lateral PVC to feed the living quarters. Once inside, I drop down to 3/4 inch PVC which runs throughout the living quarters to feed every sink, toilet, shower head, and the HW heater. I use 3/4" CPVC coming out of the HW Heater to every fixture that needs HW. I drop down to 1/2" as close to each fixture as I can get.

All PVC and CPVC is schedule 40. I have had no problems and fantastic water pressure for six years.

PS - I did the same sizing and step down setup at a smaller home I built twenty years ago and had no leaks, no problems, and great pressure for the the twenty years I had the place before selling it a couple of years ago.

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The only time I’ve used a pressure regulator is on our RV trailer input hose, only because of the various campground pressures one might come across… But here at home, never bothered with any on the all poly drip system, 7 RainBird or equivalent valves, times, etc… nd that has been going since the mid '70s, never a blown pipe or nozzle, sprayhead…

Relax… Have a beer, or two!

I was told it was bad going from 3/4" (PVC) to 1/2" poly pipe. I should be using 1/2" PVC, so I was told.

My irrigation has 3/4" PVC going to 1/2" poly for some of it.

But it doesn’t have a pressure regulator on it - it’s operating at the regular line pressure (probably about 50psi)

If your drip irrigation is delivering the water where you want it and in the quantity you want, I think you’re good.

But if it’s “bad” to go from 3/4" PVC to 1/2" poly, I’d like to know why it’s bad, since I have exactly that.

The other poster was correct that the narrower tube will have faster-moving water. But I don’t think it will have greater pressure. Also, the point about service from the city coming in larger pipes is a good one.

I really don’t see how that’s a problem, but thought I would ask in case I was missing something obvious.

Yes, all my valves that control drips have a 25psi regulator on them. If the pressure is too high the emitters can shoot off the 1/4" tubing. Had that happen a few times before I installed regulators.

1poorguy

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