Interesting, xyzer, I know what you are saying, but I thought ( I read it somewhere) that suction in a submersible well pump line was bad for the pump...so I've been trying really hard to find a pump just under the flow rate I get at the well head.
Are you sure sucking water above the submersible (at the well head) wouldn't cause cavitation in the well pump?
bitsnpieces1 the charts list "x" gpm at "x" head but no mention of at what pressure...
Head = 0 for my purposes. Needed pressure is at least 40psi.
rpg52, pressure loss iin the lines will be zero, the pipe all runs downhill at a rate that exactly cancels loss in my 1 1/4" lines.
The number of sprinkler heads will depend on the flow rate at high enough pressure. Right now if I depend on my well pump, I can get 24 gpm at 20 psi, but only about 6 gpm at 40 psi. I did find some sprinkler heads that work at 20 psi, but would prefer more water, higher pressure.(=more acres irrigated)
The "x" feet of head is equivalent to psi, same as gms is equivalent to lbs. Where you see 'ft of head' multiply the number by the 0.43 mentioned by hotater to get 'psi'. Head is used in pumping liquid situations because you can have a negative head and it's easier to make calculations from a topographic map (measured in feet). Take shallow well pumps (non-jet type) for example: Say you have a 100' head pump. If the pump sits 10' above the water, you subtract 10 from 100 to give 90. Which is the positive pressure you can get on the discharge of the pump. 100ft x 0.43psi/ft = 43 psi discharge. This is related to the amount of vacuum you can get at sea level. A positive displacement (piston) pump can lift water about 27 ft before popping a vacuum and losing prime. Centrigugal impeller pumps are at less, say about 20 ft at best (assuming you start with a fully primed pump).
In a submersible pump, the pump sits below the level of the water and
!VERY IMPORTANT! requires the water to keep cool. Therefore you do not EVER want to have a negative suction (pump is above the water level) on a submersible pump. However, if your submersible sits, say 20 ft below the water surface, that is 20 ft of positive suction pressure which can be added to the actual output of the pump itself. Say you have a submersible pump that is rated at 100 ft head and it sits 50 ft below the water level. You start with 50 ft of head and the pump adds 100 ft to it for a total discharge head of 150 ft. NOTE: the pump still only adds 100 ft. NOW, you need to know the level of non-water above the submersible pump which has to be subtracted from the head. Say you have a submersible rated at 100 ft sitting 20 ft below the water level, BUT the water level is 10 ft below the discharge. You add 20 ft, then add 100 ft, then subtract 10 ft to get 110 ft total discharge head. +20 +100 -10 = 110
PLUS, when the submerssible starts running it will pull the water level down in the well. So, you need to know the distance from the discharge point to the suction of the submersible, and the distance of the water level AFTER RUNNING for a few minutes (known as 'drawdown') from the discharge point. In the above example the total distance from discharge point to water level is 10 ft, the distance from there to the suction is 20 ft (plus a few feet for cooling) [or 30 fet from discharge point]. So the pump just sitting there has the water level up to 20 ft above its discharge, = 20 ft positive head(in relation to the pump itself), then it has to move it 10 ft, which is a negative number for head, then the pump itself adds the 100 ft for a total of 110 ft.
The upshot is that you can plumb the suction of the second pump directly to the discharge of the first pump. AS LONG AS, at no point will the water level fall enough to allow the submersible to run hot and possibly burn out. Somewhere along the way whoever drilled the well should have supplied you with some numbers for the static level of the water, the MAX pulldown volume ( max flow, or some such) and the pulldown depth.
Now you have 42 gpm at 0 head (o psi) [actually think of it as 0.01 psi]. So the submersible pump sits down far enough and has power enough that the positive head at suction, minus the negative head( lifting from the top water level to discharge) point is sufficient to produce 42 gpm at 0.01 ft(psi) head. If you can find the pump curve (or rating chart) for the pump you have it should give you the head it will produce at 42 gpm(or maybe 40). That is the total discharge head that your pump is presently putting out. Now you want to add 40-50 psi (92.27 - 115.33 ft, head). So, you're looking for a pump that will produce 40-50 psi (92.27 - 115.33 ft, head) at 42 gpm. OR LESS The 'or less' will make sure that you don't pull the water level down far enough to expose the submersible pump and overheat it.
NOW, if you can arrange it so that you can run the submersible pump into an open tank (only has to be a small opening to atmosphere) and then use the second pump to pull from there you completely eliminate the problem of running the water level low and burning up the submersible.
Look for a second pump that will give you a flow of 40gpm at 92-115 ft, head (40-50 psi): SAY 40 gpm at 100 ft, head.
i know it's long, but that's what it takes. And I hope it helps. That's also why I suggested you find a municipal water plant, the operators there deal in these kinds of calculations a lot.
Note: Your situation is comparable to one of those plants because they have a submersible(most likely) well pump that runs to the treatment process (holding tank), which then feeds into a booster pump to supply the water pressure to the pipes.
