Choosing a Sprinkler Pump

I need a pump to run my sprinklers. Only 6 heads are on at a time. Each head has a max of 3GPM I believe. So thats max 18GPM.
Can you recommend both a brand and a horsepower of pump I should use. The water is stored at or above pump level so I don't need to pull it up any. I'm concerned about just walking into Lowes and buying a pump. I want a good one. Though my sump pump from lowes has worked well.
I'm thinking I need 30-40PSI for the Hunter PGP heads I have in my lawn.
Thanks!
CL
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The equation for horsepower of a pump is;
hp = gpm x head ____________ 3960 x efficiency
where;
gpm is flow in US gpm head in feet = pressure in psi/ 0.4333 efficiency in decimals = assume 0.75 for most pumps, ranges from 0.65 to 0.85
You don't say the size of length of the pipes so assume about 10 psi losses for pipes, bends, etc. That makes your pressure range from 40 psi to 50 psi - note pipe and fitting losses could be higher but likely not double the 10psi guess.
The hydraulic head ranges from 92ft to 116ft.
hp = (18gpm x 92ft)/(3960 X 0.75) = 0.56 hp to
hp = (18gpm X 116ft)/(3960 X 0.75) = 0.70 hp
So a 1/2 hp pump might be a bit small and a 3/4 hp pump might be a bit large.
You should look for a pump that can supply 18 gpm at a head from 92 to 116 feet. Most, but not all pumps have a table of flows and heads printed on the box - make sure the 18 gpm flow is for a head of at least 92 feet. Odds are that you won't find a pump that exactly meets your specification - it is better to have too much flow than too much pressure because the flow of a pump is naturally limited to what is shown on the table by the capacity of your pipes and sprinklers. If a pump makes too much pressure then it leads to all sorts of hassles such as too fine a spray from the sprinklers so winds move the water around to much.
The issue gets very complicated because you can make the 1/2hp pump work just fine by using larger pipes so the losses are lower than the 10psi I assumed. Or you can make a 3/4hp pump work by using smaller pipes so the losses are higher than the 10psi value - or use a throttling valve to throttle down the flow and pressure for an over-sized pump.
Designing a pipe system requires consideration of the inter-action of many variables but a simple procedure is to size the pipes according to the flow velocity - try the table at this web site;
http://www.aqua-man.com/TechTips/PVCPipe.asp
Keep velocity under 3 ft/sec for the minimum head loss case to make the 1/2 hp pump work. Use a gate valve or a globe valve throttle down the flow if required. Globe valve is cheaper and might last longer and is better for throtling purposes. If you are pushing the water uphill then you should go for the larger pump.
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ast.com:

Or run it through a pressure tank and use a pressure switch. Even a small pressure tank will do the job. Another method is to use a constant pressure switch. The main thing is to pick a pump that will produce both pressure and flow sufficient to meet the needs.
Harry K
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