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Old 01-20-2013, 08:52 PM
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Weeping valve by low flow ?

I was told that the reason a system was having trouble with weeping valves was because of low flow through a large valve. This system is a waste of money on overkill. It is all Hunter. It is a two wire system, but only 12 zones, and uses 1.5" or 2" PGV valves and all MP sprays. Around 9k sq ft turf and beds combined.
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Old 01-20-2013, 09:10 PM
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Sounds like the plumbers installed it. Pull the bonnet and check the diaphragm. You prolly are going to want to run concurrent zones to keep the valves clean, though there are valves made for low flow applications, as well.
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Old 01-20-2013, 09:36 PM
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There is a mechanism in way-oversized valves that can lead to weeping and leaks. What is the system flow rate?
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Old 01-20-2013, 10:00 PM
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Quote:
Originally Posted by Wet_Boots View Post
There is a mechanism in way-oversized valves that can lead to weeping and leaks. What is the system flow rate?
I just looked at Hunters site and I think these valves are rated 20GPM-120/150GPM. I would have to calculate flow but I bet the larger areas are not much more than 20 with the MP's. I think I may need to replace them [as needed] with the 1" that is rated at .2- 30GPM.
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Old 01-20-2013, 10:06 PM
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Rookie question again.
I know you could put too much through one, but why would a low flow cause it not to work?
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Old 01-20-2013, 11:06 PM
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The closing force in a zone valve comes from the force differential on either side of the diaphragm assembly. Pressure loss through the valve is a part of that force. An oversized valve won't have the pressure loss of a standard valve, so that part of the force is absent or diminished. An oversized valve can even develop seat wear in certain low-flow situations, due to it opening incompletely (a phenomenon whose name I can't recollect at the moment)
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Old 01-20-2013, 11:37 PM
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Quote:
Originally Posted by Wet_Boots View Post
The closing force in a zone valve comes from the force differential on either side of the diaphragm assembly. Pressure loss through the valve is a part of that force. An oversized valve won't have the pressure loss of a standard valve, so that part of the force is absent or diminished. An oversized valve can even develop seat wear in certain low-flow situations, due to it opening incompletely (a phenomenon whose name I can't recollect at the moment)
It's the seldom seen Nottalottaopenup.
You described it well Shoes.
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Old 01-20-2013, 11:44 PM
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If you ever see a building water supply sent through a ladder array of three or more pressure reducing valves, that is an arrangement to fight against the seat failure that could appear in one larger PRV.
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Old 01-22-2013, 12:20 PM
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If you ever see a building water supply sent through a ladder array of three or more pressure reducing valves, that is an arrangement to fight against the seat failure that could appear in one larger PRV.
How's that work?
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Old 01-22-2013, 04:19 PM
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How's that work?
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They're set up in parallel to each other, and the smallest one alone delivers low flows, and has the highest outlet pressure. The outlet pressures of the additional PRVs are lower, by steps, so they open one at a time as flow demand increases.

What you get for the extra work, is avoiding the seat wear that happens sometimes when a small flow takes place over one portion of the seat of a larger PRV that isn't even beginning to open fully. Over a small enough seat area, the small flow can be at a velocity that can erode the seat material.
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