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schmatt

New Member

1

Saturday, December 20th 2014, 5:48pm

Figuring out design GPM

Let me start by explaining what I am attempting to do. I live in Texas, so we have very hot dry summers. I already have a sprinkler system. However, it was very poorly designed. There are dry spots all over the place, and I can't even water without watering the sidewalks as well. I've tried using different nozzles and adjusting spray radius with no good results. What I want to do is modify the entire system, as best as possible, to a proper design. I am really hoping that I will not have to add any new zones, but will if I have to. I was hoping to just cap the lines where the current heads are and tap off a new line to wherever the new heads should go. I know this probably isn't the best approach, but I really don't have the time to do a full rip out and reinstall. Any advice on this approach would be appreciated as well.

My main question is figuring out the design GPM. I am on city water with a 1" meter and 1" supply line. The static pressure is almost 70psi. However, when I do a simple bucket test on a hose bib, I only get 8 gpm. That just seems completely wrong for this setup. I know the hose bib is fed from the house water which is all 1/2" lines. I would imagine that is probably affecting my measurements. Can I just assume that the flow is much higher than this? What design gpm should I assume?

Wet_Boots

Supreme Member

Posts: 5,133

Location: Metro NYC

2

Sunday, December 21st 2014, 10:17am

You could expect much more than 8 gpm from a one-inch supply line and meter. Do you know where the existing system connects to the house supply line?

schmatt

New Member

3

Sunday, December 21st 2014, 11:21am

The main valve box is located probably 5 feet away from the meter, right along the path of the supply line. I haven't dug down to look at how it is tapped off though.

Wet_Boots

Supreme Member

Posts: 5,133

Location: Metro NYC

4

Sunday, December 21st 2014, 6:53pm

Have you tried having your pressure gauge attached to a hose bib on the house, and then turning on a sprinkler zone? (while at the same time not running any water in the house itself) - if you do that, while reading the flow from the water meter, you will get accurate numbers to work with, because none of the flowing water is running through the house plumbing, which means the pressure showing on the gauge is the same as the pressure at the point where the sprinkler system connects to the supply line.

schmatt

New Member

5

Monday, December 22nd 2014, 4:33pm

Wow! That was a great idea. Here's what I got for all 5 zones.

Zone 1: 21.6sec/5gal => 13.9gpm
Zone 2: 17.1sec/5gal => 17.5gpm
Zone 3: 20.0sec/5gal => 15.0gpm
Zone 4: 38.2sec/5gal => 7.9gpm
Zone 5: 16.3sec/5gal => 18.4gpm

In all cases, the static pressure in the house never dropped. Not sure if that is correct, but the pressure gauge I had on the hose bib remained constant. Does this mean I can easily design with an 18gpm flow rate per zone? Is this the max? The only reason I ask is because I have a long narrow section, 10' x 120', that is currently on one zone and will require a lot of heads if designed properly. I was hoping to be able to keep it on one zone. If not, I'll add a zone, but that seems like it would be a much bigger job.

Wet_Boots

Supreme Member

Posts: 5,133

Location: Metro NYC

6

Tuesday, December 23rd 2014, 7:07am

Without a pressure measurement to associate with each flow, I can't comment on what you have to work with. If the pressure didn't vary at all, then there might be a check valve interfering with accurate pressure readings, and what you saw at the house wasn't what the system was seeing.

You might want to phone the water department and ask what the street pressure is. You might have a pressure reducing valve interfering with measurements.

This post has been edited 2 times, last edit by "Wet_Boots" (Dec 23rd 2014, 7:19am)


schmatt

New Member

7

Tuesday, December 23rd 2014, 7:10am

The pressure is staying constant at around 70psi while the sprinkler system is running. There is no change in the pressure between a static measurement and when each zone is running. I don't see the gauge needle move at all when running each zone.

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