How to calculate pump size for sprinkler system

I am trying to figure out what size of water pump I will need for a yard irrigation system. I live next to a lake so I will be drafting water out of the lake. There are about 18 sprinkler heads and the furthermost head is about 225 feet from where the pump will be located. There is approximately a 12’ rise in elevation. I am using PVC for the lines. This will not be a zoned system. It will be either all on or all off.
What I would like to know is what horsepower motor and what GPM pump I would need.
How do I calculate this where I can get adequate volume and pressure? Thanks

You need to look at the packaging for your sprinkler heads. They should say something like “2 GPM @ 25 PSI”. In that very simple example, you would need a pump capable of supplying 36 gallons per minute. With PVC you can ignore friction loss in the piping.

It’s really a very small application and will require a very small pump with probably a 1/4 horse motor. If you go too large, you will blow the heads off or blow your joints apart.

As stated above, you calculate the usage requirements of pressure/flow at the sprinkler. Work upstream towards the pump, calculating head losses in piping and due to elevation. Add these to the pressure required at the sprinkler. That total pressure and flow are what the pump needs to deliver.

Piggybacking on the example above and assuming a 60% efficiency pump at the same level as the reservoir, let’s put some numbers to the exercise…

Total Flow = 36 GPM
Pressure required @ sprinkler = 25 psig = 58 ft h2o
Head(Friction Loss) = ~2 ft (a guess and depends on pipe routing and fittings)
Head(Elevation) = 12 ft
Total delivery of head for pump = 72 ft

So, your pump needs to deliver 36 GPM @ 72 ft

Brake HP = [Flow (gpm) x Head (ft) x Specific Gravity] / [(3960 x Efficiency)]
= [36 gpm * 72 ft * 1] / [3960 * 0.6]
= 1.1 BHP

So, in this example, throw a 1.25 or 1.5 hp motor on it. Something else to keep in mind about pumps is that pump operate on a curve, independent of the installed motor HP. You could put a 1.25 or 1.5 or 15 hp motor on the pump and it’ll make no difference on the pressure or flow that the pump delivers.

What!? With pvc you can ignore fiction loss? A 1/4 HP to pump 36gpm? That’s going to overcome the friction loss you’re ignoring right?

If the sprinkler main is 100ft 1"pvc the pump would have to overcome about 30 pounds of friction loss. A pump capable of pumping 36gpm at 55psi will be in the 3hp range.
The 2gpm minimum 25psi is a good rule of thumb for sprinkler heads. When designing a large system you should figure out exactly how much water you want per head. Generally you’re going to want 40-50psi for optimal performance.

You need to look at lower capacity heads or more than one zone. A pump that can provide 36gpm at 40psi or more is not practical. Larger pumps are cost prohibitive. If you ran 2 zones you could use an off the shelf 3/4 or 1hp jet pump.

Are you allowed to pull out of a pond? In many areas this is strictly forbidden.

I’d recommend HDPE piping over pvc, it’s more durable and easier to install.

Is there any reliable cite for your friction loss statement?

I agree that HDPE would be better, it sounded to me like the PVC was already in place.

Friction loss for schedule 40 PVC pipe.

I was shooting from the hip with the 30psi number. DagOtto’s chart only goes up to 30gpm, it’s 200psi loss at that point. I’ll find a better chart a later. Friction loss is a thing with all materials. 1 inch piping hits a wall around 18gpm so that’s when you typically start upsizing.

And only a 20psi loss if my phone doesn’t auto correct.

It is an 83,000 acre lake so water supply is not an issue. I have to pay a $50 per year fee and I can irrigate as much as I want. I am resurrecting a old system.
The piping for this system is already in the ground. There was a pump at one time but it is long gone, so I do not know the specs on it. The discharge of the prior pump was 1 1/2" in diameter, so I think the main lines are that size and diminishes to 3/4" for the branch lines.

Was the original pump in the water or on land? Can you post a picture of where it was?

Is there power run? What size circuit? Wire? Distance from the main panel? That will also give a clue as to what was used.

It’s impossible to size a pump without knowing demand. More research into the sprinkler heads is needed.

Submersibles are more effecient at moving water, but some care needs to be taken to assure proper cooling and getting power to them.

Jet pumps are the most ubiquitous for your applicaion. They suffer more loss because they depend on suction. The higher above the water level they are, the less efficient.

1 1/2 pipe should be plenty large enough to minimize friction loss, it’s good for up to the 50gpm range.

My guess is it had a 1.5hp jet, they handle the 20-25gpm range.

The pump was on land. The intake pipe is about 8’ long and drafts directly from the lake. The pump itself was about 2’ above water level. The power is 115V on a 20A circuit. I have a single pole switch in my dining room that operates the pump. Distance from the main panel to the pump is about 90’. I am going to look closer at the sprinkler heads when I get a chance. I know there are several different types. I am not opposed to replacing all the heads so they will all be the same.

115v puts it 1.5hp or below. A 1.5hp set to run on 115v draws about 20amps. So a 1hp is going to be your max with that circuit. If there is nothing else on the circuit you’d be better off changing it to a 230 circuit.

At only a 2ft lift you could run a straight centrifugal pump. They can’t make pressure like a jet pump but they can make more volume. Cap out at about 40psi

A 1hp centrifugal will move about 40 gpm@30psi or 30gpm@40psi

I like Goulds pumps. For your application I’d be looking at a GT10. If they are lower flow sprinkler heads and you need more pressure a jet would be more appropriate a J10S.

If you can change the circuit to 230v move to a 1.5hp.

Thanks for the info. So I guess I will need to see how much pressure the heads require to operate. I will check out Gould pumps.

This chart

Goes up to 35gpm with 1" pvc. That’s a loss of 28.74. It’s certainly not a stretch to say 36gpm would be a 30psi loss.