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.
The equation for horsepower of a pump is;
hp = gpm x head
3960 x efficiency
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
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
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
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;
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.
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.
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