pump confusion

knotshawkAugust 6, 2010

Hi all... I'm trying to build a sprinkler system. I'm trying to do it VERY cheap and VERY basic because we're likely moving soon. As such I'm hoping to build a simple above ground system using PVC and a few garden hoses to be run by a sprinkler pump drawing water out of my irrigation ditch. My main confusion is this:

I've been told that it's very important to have enough impact heads coming off of my centrifugal pump so that I don't damage the pump. So the question is, how do I determine this? I guess I don't understand the relationship between PSI and GPM. How do I determine what kind of PSI/GPM I'll be producing so that I can determine how many heads I need? I'm planning on needing maybe 15 impact heads (it's about a 1/3 acre lot) and need to know if I can run that all at once off a 1 HP pump.

I know this is pretty vague, but can someone give me some good guidelines? Thanks in advance...

PS - I can provide more specifics if needed... just ask ;)

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lehua49

Hi knot,

I think I can help with the basics. The manufacturer of the irrigation heads designs the head for a certain operating pressure range(usually 45 to 60 psi). The irrigation head will have a certain flow (gpm or gph) for what ever pressure you provide in that range. You work backwards. First find the type of heads and size you want. Look in the literature or on-line to find the flow rate per pressure you will provide. The add the flow rate of all 15 heads and multiply by 1.20 that is your flow rate that your pump will need to produce. Check the flow curves of your pump which will tell you the flow rate and pressure that the pump will produce for the vertical height the pump has to push to get to your highest irrigation head. You should look into a submersible pump. The submersible will have less problems with no priming of the pump and line air leaks from the ditch to the pump. You can read some old threads describing the problems and difficulty in fixing them. You might think of placing a weir or small dam in the stream to have more available water for the pump. Also use heads that can pass a small amount of silt and don't get clogged. JMHO Aloha

    Bookmark   August 6, 2010 at 7:09PM
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knotshawk

Thanks... I was looking at some basic rain bird heads with the following specs:

Performance Chart
PSI -- Radius -- GPM -- Bars -- Meters -- M3/h
25 --- 35 ------ 2.8 ---- 2,0 -- 11,0 -- 0,68
35 --- 38 ------ 3.3 ---- 2,5 -- 11,7 -- 0,78
45 --- 40 ------ 3.7 ---- 3,0 -- 12,1 -- 0,83
55 --- 41 ------ 4.1 ---- 3,5 -- 12,5 -- 0,89

So... do I need to know my PSI ahead of time? I'm not clear on it still. Would a pump that puts out 43 PSI/63 GPM work for this?

As for the pump type, my ditch won't allow for a submersible, but I plan to dig a sort of reservoir that I can draw from. I also plan to screen over it, plus I'll be using a foot valve which has a screen built into it. The ditch isn't very deep, but it's got good flow.

    Bookmark   August 6, 2010 at 10:21PM
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lehua49

Knot,

Lets see at 45 psi flow is 3.7gpm/head x 15 heads equals 55.5 x 1.2 is 67 gpm. Design for 55 psi is 4.1gpm/head x 15 equals 61.5 x 1.2 is 73.8 gpm. Not much difference but you need a higher gpm output from your pump. In the 70 gpm range. Can your ditch sustain that kind of water use? Your ditch may be a limiting factor. What is the ditch flow. You can use a lower ditch flow rate by breaking the job into zones if you need while utilizing a smaller rated pump. JMHO Aloha

    Bookmark   August 7, 2010 at 12:49PM
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knotshawk

Two questions:
1) How do I differentiate the PSI vs. GPM on a pump? I mean, you're talking about 45 PSI and 70GPM... I don't think most pumps can do both of those, right?

2) My ditch is about 3 feet wide and 10 inches deep. I used a formula online to guesstimate its GPM and came up with over 700 GPM. So that should be plenty... yes? I used the formula at this link: extension.missouri.edu/publications/DisplayPub.aspx?P=G1681 under "float method".

    Bookmark   August 7, 2010 at 3:17PM
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lehua49

Knot,

I will try to explain but I don't know if I will make it understandable or even if I understand it all completely myself.

Centrifugal pumps are kinetic energy pumps that capture water from the suction side and impart energy or pressure to the water pushed to the outside portion of the casing from the impeller blades. Kinetic pump characteristics include high flow rate,not self priming,steady discharge stream,

1.For each situation a system curve which relates pressure psi and flow rate can be created. For example the pump needs to pull 3 ft and push 127 ft(130ft x .433 psi/ft= 56 psi. This the pressure for the irrigation system.

2. There is also the performance pump curve(provided by manufacturer literature) for the particular pump for each size impeller which gives the psi versus flow rate for that particular pump.

3. The pump operating point is the intersection of the performance and system curve. An operating range and max efficiency point will be shown and your system curve should be in that range or on that point. You select a size pump that gives you that operating point. Realize this is not an exact graph but approximate.

HP of Pump is equal to Q x Head/3956
Q(flow) is in gpm; Head is in ft (psi/0.433=ft)
example1: 63gpm x 43psi/0.433 divided by 3956 = 1.6 HP
example2: 70 gpm x 55psi/0.433 divided by 3956 = 2.2 HP

You should look at making two zones. 35gpm x 50psi/0.433 divided by 3956 = 1 HP

Pressure relates to the distance you want your heads to throw or your overall coverage. Number of total heads.

Compare costs of a 1, 1.5 and 2.5 HP Pump.

A link to explains this better than I is:

http://www.rainforrent.com/Training/PumpTraining.pdf

700 gpm is plenty. Did they use Manning's Flow Equation and where the slope is used along with the geometry of the ditch?

If you make a stilling area(section wider and deeper) in the ditch, it will settle out particles and put the in-take pipe well above the bottom.

Hope this hasn't cause more confusion. JMHO Aloha

    Bookmark   August 7, 2010 at 10:32PM
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zeuspaul(9b SoCal)

Below I have linked a pump curve for a Flint and Walling one horse power centrifugal water pump. It uses feet of head instead of psi. Multiply feet of head by .43 to convert to psi.

Note on the graph that at 100 feet of head you get 500 gallons per hour. That converts to 8 gallons per minute at 43 psi. From the same curve the same pump willl give you 625 gallons per hour at 60 feet of head which converts to 10 gallons per minute at 25 psi.

You are nowhere near getting 70 gpm at a useable pressure with a one hp pump.

As long as there is clean water flowing through the pump you will not ruin it. If you have just one sprinkler you will be at the high pressure side of the pump curve. If you have ten sprinklers you will be on the low pressure side of the curve.

Zeuspaul

Here is a link that might be useful: Pump Curve

    Bookmark   August 8, 2010 at 12:29PM
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Stewart_Yardley

I think Lehua13 has given some useful guidelines that are quite specific and comprehensive which you can refer to as a basis. More info can also be easily retrieved on the Internet but you need to be certain that those pieces of data can be trusted and are truthful. I think the main concerns here are not only the pressure and types of inlets/outlets but the water supply releasing all that water is very important to take note as well. Anyhow, it is still advisable to seek professional advises from the experts in this field to avoid any future problems.

    Bookmark   October 24, 2013 at 2:19AM
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