I am trying to plan my irrigation system for this summer to pump water from my pond to vegetable gardens. I have been trying to understand the formulas and do the math but I am lost. I have a pond which is 350' from where the gardens will be located. I would like to run a gas powered or PTO powered pump from there which will feed 6 sprinklers and maybe 6 drip hoses. I know that the pond is 6 feet deep and the pump will sit on the bank 15 feet from the deepest point. The pump needs to push the water as I said 350 feet and rises 13-15 feet over this length. I don't have any idea about what I need for a pump as far as horsepower, pressure, head etc. I guess I get lost with static head, dynamic head, total head etc. Does anybody have experience with figuring out these irrigation setups or know of a calculator that can be used to enter in the variables? I have asked the farmers around me and only 3 use irrigation on their fields and it's pretty much the same setup that has been passed down to them over the years so they can't offer a whole lot. I am not sure about hiring an engineer around here as it seems a lot of these "contractors" want to get their money and run or are just as clueless as I am. I just want to do it right the first time and not waste money on the wrong gear. Thanks for any advice.
Head and pressure is synonymous. One is expressed in feet( actally feet of water) and the other is in psi. Dynnamic head is the pressue of the water if it were brought to rest from water velocity it is flowing at. It is equal to the 1/2 x water density x the velocity squared. tTe elevation head is just the difference in the elevation from a datum refernce. The total head is the static head + the dynamic head + the elevation head + the friction head
The pump needs to overcome the the elevation changes(13-15 feet), the fiction head loss due to 350 ft of pipe (the friction head loss is a function of the pipe diameter,flow rate, pipe roughness, excressance losses to to joints and valves) and deliver a certain pressure to the drip system and the sprinklers. the pump also must be capable of lifting the water from the pond to the pump.
So first decide how many inches of water you want to put on the ground during an irrigation. For example 3 inches of water over a 12 hour irrigation set on 10,000 sqft of garden. That yields 10,000 sq ft x 0.3333 ft/(12 hrs x 60 min/hr) = 3333/720 = 4.62 cu/ft per minute of water / 0.1335 cu ft/gal=
34.6 gallons per minute.
Decide on your sprnkler spacing so that you can cover the the 10,000 sq ft(or what ever the size is). Go to the sprinkler tables (Nelson, et al) that the sprinkler manuafacturers have. They will give you the throw (radius of the sprinkler spray) plus the flowrate as a function of the presssure at the sprinkler head, the sprinkler head hole diameter, at a specific sprinkler head height above the ground. So lay out the garden and figure out how many sprinklers you need to cover the 10,0000 sq ft (or whatever you have) so that you can cover the area with a flow (the sum of the individul sprinkler flows) of 35gpm. It's trail and error. the outcome will be the number of sprinklers you'll need and the pressure you'll need at the sprinklers.
Now you'll have to work the line sizing out. Bigger line size is better but you have to be practical. 3 inch sprinkler pipe is what I would use to minmize pumping requirements and four inch main line (from the pump to the sprinkler pilpe would be good. the bigger the pipe flow area the lest of your prime mover energy you use overcoming the friction head. You might be able to get by with 3 inch main line and 2 inch sprinler pipe. you'll just have to work out the head loss for several sizes of pipe and see what makes sense for you from a cost stand point. Remember that once you size the disribution system you pay the head loss penalty in power as long as you use that size of pipe.
If you have cheap power(an oxymoron?), you might want to go to smaller pipe. Nowfind the pressure loss charts since you have the flowrate , the length and callculate what pressure you have to have at the pump to deliver the required pressure at the sprinklers. Do this for several pipe sizes and then pick the one that best meets your needs. The bigger the pipe the smaller the head loss. The 13 to 15 ft of elevation change should be added to that. So you have the flowrate, the pump out put pressure and you can size the pump and motor.
To minimize the suction lift requirements of the pump, I would dig a sump so that you can pump with the shortet and biggest pipe size from your water source.
I irrigate 40 acres at a time with 2- 10 hp pumps delievering ~ 450 gpm total and 6 inch to 5 inch mainlines and any where from 4 inch to 3 in sprinkler pipe and 4 and 5 inch wheel line pipes. My pressure loss from the pump to my furthest wheel line is about 25%. I didn't design the system. It was here when we bought the ground. i would have done it differently and eventually will redesign it.
This is the basic methodology used to design irrigation systems. The real way to do this is with network analysis program like engineers use to design big systems. But this mehod will work for what you are trying to do.
I hope this helps you.
