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keperkey
04-29-2006, 08:19 AM
This is my first irrigation project.

Static water pressure per the water company is 120 psi. After the PRV, pressure is 55 so I plan to connect after the PRV.

Supply line coming from the meter to the PRV is 3/4 copper.

I have been reading Jess Stryker's Irrigation Tutorials and am confused.

Am I limited to 6 GPM due to connecting after the copper pipe coming into the house? Or do I plan based on pipe sizes after the Irrigation system point of connection?

PurpHaze
04-29-2006, 10:31 AM
This is my first irrigation project.

Static water pressure per the water company is 120 psi. After the PRV, pressure is 55 so I plan to connect after the PRV.

Supply line coming from the meter to the PRV is 3/4 copper.

I have been reading Jess Stryker's Irrigation Tutorials and am confused.

Am I limited to 6 GPM due to connecting after the copper pipe coming into the house? Or do I plan based on pipe sizes after the Irrigation system point of connection?

Feet Per Second velocity of water has been calculated out by pipe size so that undue friction does not occur thus reducing final pressure running through a particular pipe to particular sprinkler(s). Some will increase the main line size under these circumstances which helps reduce friction but will NOT increase the amount of available water.

keperkey
04-29-2006, 10:46 AM
Since 6 GPM is necessary to stay below 5 ft/s, does that mean I am only going to be able to run 4 1.5 GPM heads. This means alot of zones. :hammerhead:

Dirty Water
04-29-2006, 10:58 AM
How much 3/4" Copper is between where your measured 120 psi, and where you measured 55 psi?

I would tap at the 120 psi point, install my own pressure reducer immediatly after the backflow, and run a 1" mainline.

PurpHaze
04-29-2006, 10:59 AM
Have you actually measured your GPM flow at the POC or are you just going to use the standard GPM that the POC pipe usually carries?

Think of available GPM as a bank account where you can only write so many checks (sprinklers) before the thing bounces due to a lack of funds (water). A particular zone cannot be more than the original amount of water you started with. If you have a small bank account then you will have to break the system into smaller (and more) zones. You'll have to find a sprinkler (maybe something like MP-Rotator) that will give you the distance you want to cover (based on available pressure) but with low GPM so you can put more of them on a single zone, thus reducing the total number of zones on the system.

keperkey
04-29-2006, 11:10 AM
I have not done any digging (yet).

Water Co tells me I have 3/4 pipe running into a 3/4 meter. Then I have about 18 feet of 3/4 pipe going to the PRV in the crawl space.

I suppose I could T off just after the meter with larger pipe allowing for more GPM. What I do not understand is why the water company is only using 3/4 pipe coming from their main to the meter. I think that I will confer with the water co. again.

This is very frustrating since I compute about 12-13 GPM coming out of the hose bib coming off 1/2 pipe immediately after the PRV. But just because I have that much GPM does not mean I should use it (at least according to Stryker.

Wet_Boots
04-29-2006, 11:49 AM
Only rarely do you ever get the supply line changed. Money. Sometimes, you can get another (larger) tap into the main, and run the sprinklers from a curbside meter pit. But, the property has to be quite large for this to make any sense. Five gpm is way too small a limit for your flow. Because there is a PRV in the supply, you can go beyond the usual limits, and the PRV will prevent water hammer. The only hassle is that you can't easily figure performance by looking at any tables. You really have to make your connection and then do some flow tests, whenever there is a PRV in the circuit.

I would agree with the idea of teeing off before the existing PRV. As for having a PRV with a 120 psi supply pressure, it's not a bad idea. I might be more tempted to use a pressure regulating master valve in the system, and get more flow that way.

keperkey
04-29-2006, 07:05 PM
Studied some more. The pipe coming out of the meter is PVC and not copper. I was assuming that it was copper since that is what I have coming into the house.

What about increasing the pipe size at the meter to the T going to the irrigation system?

Wet_Boots
04-29-2006, 08:05 PM
Increasing the pipe size in the sprinkler system is an easy call. You'd do that just to cut friction losses, and to get as much useful flow as you can manage. But for just 18 feet of 3/4 PVC supply line, from a 3/4 meter, I doubt that you gain eough to justify the effort of changing it to larger pipe.

keperkey
04-29-2006, 08:24 PM
Agreed.

What I am thinking is that I would increase pipe size out of the Irrigation T and run a PRV at that point if pressure dictates once I measure it.

Wet_Boots
04-30-2006, 01:52 PM
This comes down to your particular situation (property size and soil conditions) and how much flow you need. 120 psi is more than standard sprinkler heads are rated for, and more than standard poly pipe (100 psi) is rated for. A PRV has a heavy penalty in pressure loss. Since a control valve can withstand a 120 psi supply pressure, there is some logic to using a pressure-regulating version of a zone control valve. A number of standard valves can become pressure-regulating valves by adding a pressure-regulating module to the valve. These give you the pressure regulation you want without the losses you get from a standard brass PRV ~ those would be a choice for when your supply pressure is really high (over 150 psi) and more than a zone valve is rated for. Some heavy-duty zone valves have a 200 psi rating. Haven't used any of them, so I can't advise to their effectiveness.

HooKooDooKu
05-01-2006, 12:19 PM
I have been reading Jess Stryker's Irrigation Tutorials and am confused.

Am I limited to 6 GPM due to connecting after the copper pipe coming into the house? Or do I plan based on pipe sizes after the Irrigation system point of connection?

Your 6 GPM limit is coming from the idea that you don't want the velocity of water to exceed about 5 feet per second ANYWHERE in the system.

Moving water has kenetic energy, and as speed doubles, the amount of energy quaturples. When a valve closes, it forces the water to stop almost instantly and all that energy has to go somewhere (think water pressure spikes stressing the system and banging pipes in extream situations).

Based on modern plumbing standards and materials, 5 feet/second has become the plumbing universal "speed limit". Your system very well might be able to withstand exceeding this "speed limit", but the more you exceed it, the more stress you're putting on your system and the more likely you are of having a crash (busted water line or joint).

And as others have elluded to, the higher the speed of the water, the greater the friction on the pipes, and the greater the friction losses. At 6 GPM in a 3/4" copper pipe, you will lose about 5.5 psi for every 100 foot of pipe. But if you cut that speed in half (3 GPM = about 2.5 ft/sec), the friction losses drop to about 1.5 psi for every 100 foot of pipe.

Let me know if that doesn't clear up the confusion and perhaps we can find a simpler way to explain it.

keperkey
05-01-2006, 12:48 PM
That makes sense completely. Thanks. I guess the primary issue is making sure you don't stress the system past its breaking point and making sure you set the system up in such a manner that you are comfortable that you are not past a breaking point.

Wet_Boots
05-01-2006, 08:18 PM
Do understand that there is a long tradition of ignoring water velocity, in favor of getting increased flow, to get your lawn coverage in a limited time period. The zone pipes aren't really a sealed system, so they don't get water hammer. It's the pipes up to the zone valves you want to be aware of. Backflow performance charts mark 7.5 fps as a mark to design to. (But the devices are rated for flows that are way beyond that 7.5 fps)