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DOORZ

Advanced Member

1

Tuesday, September 13th 2005, 10:03am

Backflow Suggestions

I had a question about what backflow preventer to use. My sitution is that my property is about 270 long, and slopes about 5 or 6 feet from one end of the property to the other.

My problem is that the water meter is installed at the low point on the property, so I am not sure what's best to use in my situation. If I use a pressure vacuum breaker I have to run a main line about 270 to get to the high point on the property, before then going to the valves. Then all of my zones will run downhill from there.

I assume I could use a double check assembly right at the tap, but I have read that they can be unreliable if they get garbage in them. I am getting my supply from the county, so supply should be clean.


In this sitation, no matter which backflow preventer I use, should the valves be installed uphill from the zones or does that even matter?


Tom

Supreme Member

2

Tuesday, September 13th 2005, 10:09am

use a reduced pressure backflow

DOORZ

Advanced Member

3

Tuesday, September 13th 2005, 10:35am

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Tom</i>
<br />use a reduced pressure backflow
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

But doesn't this still need to be installed above the highest head? Which means I would still have to run 270' uphill to turn back around and run in the system in the opposite direction downhill.

Tom

Supreme Member

4

Tuesday, September 13th 2005, 1:18pm

no

Wet_Boots

Supreme Member

Posts: 4,082

Location: Metro NYC

5

Tuesday, September 13th 2005, 1:38pm

The RPZ (reduced pressure zone) device is the one choice that will meet any code requirement, no matter where you're located. As for elevation, it need only be a foot or more above ground, even if the ground slopes uphill from that location.

HooKooDooKu

Supreme Member

6

Wednesday, September 14th 2005, 6:47am

Just to add to what Wet Boots is saying...

An RPZ is very much like a double check assembly. Actually, it is a double check valve assembly, but in contains "extra" stuff such that a failure in the check valves results in water spewing out of the RPZ rather than having any chance of flowing back into the supply line. (The Reduced Pressure part comes into play because a part of what this "extra" stuff does is attemt to maintain a pressure loss within the device. If something goes wrong with the check valves such as a piece of trash get caught and make the valve stick open, the RPZ will not be able to maintain the pressure loss and this will cause the device to start venting water rather than allowing a backflow).

So just like a double check, an RPZ can be installed BELOW control valves and sprikler heads. However, if the RPZ were to become submerged or something block this "spew" valve, the backflow protection of the RPZ could become compromized, therefore the RPZ itself must be install ABOVE GROUND so that it's free to release water.

A double check assembly, on the other hand, doesn't have any way to release water (during NORMAL operation) and therefore a double check can be installed below ground (in a meter box so that it can still be easily be accessed for testing).

DOORZ

Advanced Member

7

Wednesday, September 14th 2005, 7:16am

Ok, so now another part to my question

Should I use a RPZ valve instead of a pressure vacuum breaker?

Would adding 270' of piping to the system to get a vacuum pressure valve above the highest head hurt the overall system? As it is my pressure and GPM are borderline, and it has been suggested to run and 1-1/4 main line to the valves, and then use 1" after that to all of the heads.

I don't know if this extra 270' would make a differerence to the system, or does it matter?


Also what size Backflow preventor should I use? My supply from the county is only 3/4", so not sure if I could use 1" to the pressure valve and then step up to 1-1/4 after the valve, or should I come off the supply at 1-1/4.

Thanks for any input that can be provided


Wet_Boots

Supreme Member

Posts: 4,082

Location: Metro NYC

8

Wednesday, September 14th 2005, 10:17am

There isn't any rule that forces the PVB location to be at the point of the property's highest elevation. The only requirement is that the PVB device itself be at least a foot higher than any pipe that feeds from it. That means you could have it at the house, at seven-plus feet above ground. Hide it behind a chimney, maybe. Unbeautiful, yes. But a PVB will save you almost 10 psi compared an RPZ, not to mention the hundred-plus dollars difference between the two devices. A 3/4" device will be sufficient, although a one inch might be easier, since your sprinkler plumbing is usually one inch.

DOORZ

Advanced Member

9

Wednesday, September 14th 2005, 10:38am

Thats my problem; in order to get backflow 12" above the highest pipe, it has to be run 270' away from the meter, which just happens to be the highest spot on the property.

Will this hurt the system by having to add another 270' of pipe before even thinking about going to the first zone valve?

HooKooDooKu

Supreme Member

10

Wednesday, September 14th 2005, 10:53am

Using an extra 270' of pipe in the supply line will obviously cost extra money, but it will also result in a loss of pressure, because water moving through a pipe loses some pressure. The amount of pressure loss is a function of pipe size an water flow (gpm) (here's where to find a converter tool http://www.irrigationtutorials.com/formulas.htm#sec8).

As an example, if you are using Sch40 pipe and designing for a flow of 10gpm, then the 270' of pipe will lose about 6psi in a 1" pipe, 2psi in a 1.25" pipe, and 1psi in a 1.5" pipe.

How ever, RPZ is going to lose about 10-15 psi. As for deciding what size of RPZ to use, you have to look at pressure loss tables and find out which size RPZ loses the least amount of pressure for your desired flow rate.

Two alternate ideas, but you'd have to find out what may or may not be allowed by building and plumbing codes.

#1. You say that the change in elevation in the yard is only 5-6 feet. Is there any reasonable way to install a pressure vacuum breaker about 7 feet in the air at the bottom of the hill? There might be various code that limit the height of the PVB, but regardless of codes, this would only be practical if you use strong metal pipe to and from the PVB, there is something (like a building) to attach the pipe to, and there is some reasonable way to access the PVB for testing without getting on a ladder. While it could be done, it might be so iffy (i.e. must be done just right) that codes may not allow it.

#2. Assuming your local codes allow a double check, you could improve its reliability (and the reliability of your entire irrigation system) by installing a WYE filter up stream of the backflow preventer. Last time I looked at the literature from companies such as Watts, they always show the installation of their backflow preventers being proceeded by a WYE filter. Of course this could only be suggested if you are not injecting anything into the irrigation water (i.e. fertigation).

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