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  #51  
Old 09-15-2014, 04:56 PM
Ron Wolfarth's Avatar
Ron Wolfarth Ron Wolfarth is offline
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Pipe Size / Valve Size Ratio

Okay. I'm back from vacation! Visited the libraries/historic sites of 5 U.S. Presidents in the northeast. Very interesting. FDR library/home is definitely worth visiting. (Allow all day.) I learned some things about him, Eleanor and their 'marriage' that would be scandalous today. The press just didn't report on people's private lives back then. They also respected his wishes to not be photograped in his wheelchair or being lifted into vehicles, etc. There are about 5 photos of him in these situations.

Back to the irrigation world.
Here is a topic that might get some opinions going.

How should valve size be determined? I have seen and heard several approaches to this. Here is my opinion.

The smallest valve that gets the job done should be selected given it is still within the performance parameters recommended by the manufacturer and the pressure loss is acceptable for the supply pressure available. However, the plastic pipe or tubing should be sized so that velocity does not exceed 7 1/2 feet per second. (I used 5 feet per second when I was designing. I learned back then that 7 1/2 fps was for metal pipe and 5 fps was for plastic.) This should keep water hammer in check. The result of this is that the mainline feeding the valve and the lateral line attached to the valve are often one size or more larger than the valve size.

I know someone who testifies as an expert witness in liability lawsuits that says that the standard of the industry is that the valve should never be larger than the pipe connected to it and the velocity of the water in the pipe should never be more than 5 feet per second. I am okay with the speed limit in the pipe, but I don't see the reason for the 'over-sized' valve. Rain Bird tests its valves at all the flows and pressures (and more) that are shown in the performance charts in the catalog and website. We recommend they be used within those limits, but we know they will perform well at the extremes of those performance ranges. So, assuming you can afford the pressure loss, why not save the money and do that?

Anybody else have an opinion on this?
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  #52  
Old 09-15-2014, 05:34 PM
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Wet_Boots Wet_Boots is online now
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I remember an old Buckner catalog with a design tip to undersize a zone valve for reliable performance, based on matching flow rate with a 5 psi pressure drop in the chosen valve. I wouldn't want to vouch for any actual water velocities inside the valves, if we throttle down the flow controls for better closing.
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  #53  
Old 09-15-2014, 05:52 PM
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Ron Wolfarth Ron Wolfarth is offline
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Quote:
Originally Posted by Wet_Boots View Post
I remember an old Buckner catalog with a design tip to undersize a zone valve for reliable performance, based on matching flow rate with a 5 psi pressure drop in the chosen valve. I wouldn't want to vouch for any actual water velocities inside the valves, if we throttle down the flow controls for better closing.
I also recall that recommendation, but I was told that it was a general rule of thumb. Maybe that was before we moved to relying on the difference in the size of the top surface area of the diaphragm and that on the bottom side to close the valve and depended more on the pressure differential. (Pressure times surface area = force; larger surface area = more force on the diaphragm to close it)

Today, Rain Bird valves are not very dependant on a pressure drop across the valve seat to close reliably.
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  #54  
Old 09-15-2014, 06:42 PM
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I know one time I upsized a master valve in comparison to the zone valves (1-1/2 inch compared to 1 inch) so that I could have the master throttled down, with it closing faster than the zone valves, in order to keep water hammer away from a poly-pipe mainline.
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  #55  
Old 09-15-2014, 06:46 PM
gradioc gradioc is offline
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Ron, I'm with you. I have no problem running a 1" valve off a 1.5" main for a 30 gpm zone. It's well within the performance curve of the valve. We're very strict about 5fps on runs, but a ft of 1" to pipe a valve is insignificant. I'm the service guy and in 25 years I've yet to see one break because of velocity.
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  #56  
Old 09-15-2014, 10:30 PM
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RhettMan RhettMan is offline
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Quote:
Originally Posted by Wet_Boots View Post
I know one time I upsized a master valve in comparison to the zone valves (1-1/2 inch compared to 1 inch) so that I could have the master throttled down, with it closing faster than the zone valves, in order to keep water hammer away from a poly-pipe mainline.
i upsized an entire mainline and MV from 1" to 1.5" for this reason a little under two years ago.

Actually i think it may have been on your suggestion come to think of it boots.

Ended up working wonderfully. I think i went from 100DVF to a PGA.
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