what is the difference in 1/2hp pump?


Old 01-07-10, 10:30 PM
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what is the difference in 1/2hp pump?

I have a sprinkler system with 2" suction. It is pumped by a 2hp Sta-Rite pump. It has two rows of sprinklers, four on each section.

The pump cost close to $400.

I've got a new one located which can be had for $150. It is only 1.5hp.

If I go to the new, lower powered one, what do I lose?

I'm not sure I'd lose much. Is it suction or pressure to heads?
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Old 01-08-10, 06:04 AM
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You have to look at the specs for the pump/motor combination. A pumps performance is determined by it's design.

Example: One could be designed for high flow volume (gallons per minute) but lower pressure, while another with the same hp could pump less water but at a higher pressure. Generally a self priming pump offers less flow and pressure than a pump that requires priming.
Old 01-13-10, 10:39 PM
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the new pump should have a performance curve somewhere in the literature. What you need to do is check that performance curve vs. the performance curve of your current pump. The difference between the two will be very simple to see and if you post the two curves (or links to the two curves) - and you know how many GPM you need to supply the irrigation system with - I will be happy to explain to you why one pump is a better fit than the other.
Old 01-14-10, 06:26 AM
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I'm a bit mystified over what you're asking.

The pump that was on the well was an old Sta Rite (approx '70 early). Where do I find the specs for it? The thing ate itself, and all I have is slag and spewed parts for it. Can't even say I have a good tag for it.

The pump I'll buy is a built up one. I'd have to ask the buyer what pump he's going to sell me. Then, find the "curves" for it. What pump he sells me depends on the time of the purchase.

Both pumps are 230vac Sta Rites. One is 2hp, the other is 1.5hp. Both pump the same well, both use the same sprinklers. What is so hard to tell me, in general terms, what I lose if I go to the smaller pump?
Old 01-14-10, 08:51 AM
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Assuming your pump was sized to match your needs, you will likely lose the ablility to maintain system pressure and flow to the heads. Like everyone has already said, the question is impossible to answer for sure, without pump model numbers. Do your actual irrigaion demand for your system? You should be able to find a pump curve for the new pump....compare that to what your actual demand is....the 2hp may have been oversized, and the 1.5 may work fine.
Old 01-14-10, 06:47 PM
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Sta-Rite has a website.
Sta-Rite Industries
They post pump curves in their literature.
There are pictures of the different pumps they sell.
Here is one of their spec sheets:

Even if you do not know what the old one did, you could pretty well guess it out from that website.

Even if you can't guess it out, if you knew what your demand was from each sprinkler (available from the manufacturer, but guesstimate 3GPM each) then look at the curve and see if it will meet the demand. Four sprinklers at 3GPM would need 12GPM, for example.

A number that you do not know, when looking at the curve is the "draw down" depth. That is the depth of the water in the well while the pumps is running. A decent well draws down no more than 4 feet. So to guesstimate that number, measure the water depth with a string tied to a weight like a nut and listen for the splash sound, then add 4. So if the well depth is 18 feet, plug 22 feet into the table.

Be aware that "built-up" pumps usually mean "rebuilt" and usually do not perform to the curve like a new one would, so degrade the output numbers by 20(?)% and that would be closer to the true number.

This is a lot of guesstimating. If the sprinkler heads use more, or the well produces less, you are screwed. I'll bet you thought this was easy?

I forgot to say that if the new pump doesn't perform, then splitting up the sprinkler heads is a cheaper fix than getting a new pump. So you just run a new pipe and hook up two & two, with a valve, either manual or electrical, for example.

Last edited by Vey; 01-14-10 at 07:16 PM.
Old 01-14-10, 07:34 PM
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Absolutely! If the sprinklers are weak, we'll just break 'em down and add on circuit. Fewer per circuit, more circuits.
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