# Question re: Calculating Velocity

Discussion in 'Irrigation' started by keperkey, Apr 29, 2006.

1. ### keperkeyLawnSite Memberfrom Somewhere, TNPosts: 33

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?

2. ### PurpHazeLawnSite Fanaticfrom Visalia, CAPosts: 5,496

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.

3. ### keperkeyLawnSite Memberfrom Somewhere, TNPosts: 33

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.

4. ### Dirty WaterLawnSite Fanaticfrom Redmond, WAPosts: 6,794

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.

5. ### PurpHazeLawnSite Fanaticfrom Visalia, CAPosts: 5,496

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.

6. ### keperkeyLawnSite Memberfrom Somewhere, TNPosts: 33

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.

7. ### Wet_BootsLawnSite Fanaticfrom metro NYCPosts: 48,330

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.

8. ### keperkeyLawnSite Memberfrom Somewhere, TNPosts: 33

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?

9. ### Wet_BootsLawnSite Fanaticfrom metro NYCPosts: 48,330

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.

10. ### keperkeyLawnSite Memberfrom Somewhere, TNPosts: 33

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.