Need help with a pump problem


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Old 07-20-10, 08:19 PM
R
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Need help with a pump problem

1.Situation: I live on a mountain riverbank, and pump water from the river, for watering the lawns and the veggie garden, during the summer months; have been doing this for thirty-two years. The river is about 17 feet below the level of the house. In the past, I have had two 1/2 h.p. pumps hooked in tandem, sort of a "push-pull" arrangement, where the first pump sits down on the river, about 40 feet from the river's edge, and is level with the water; the output from this pump then goes up the riverbank to the second pump. The second pump sits up in a pumphouse which is, as stated, about 17 feet above the first pump. The horizontal distance from the river's edge to the first pump is about 50 feet, and the horizontal distance from the first pump to the second one, in the pumphouse is about 40 feet. This arrangement has worked fine, and I have been able to set my pressure switch to shut off at 60 p.s.i., which has allowed me to run all of my sprinklers at the same time, is so desired.

Problem: However, the other day, I noticed that my sprinklers weren't running, so I went to the pumphouse, only to find that the pump was really hot and had shut off. I immediately went down to the river, and inspected the foot valve, the suction line to the first pump, the first pump, itself, and the output line from it, which is the suction line for the second pump. All of that was fine. So I shut the power off and took the whole pump in the pumphouse apart, and tried to run the motor by itself, with nothing attached. No go. I tested the thermal overload switch, and it was working fine. I applied 110 vac directly to the pump, and it still wouldn't go. When spinning it by hand, it spun just fine, so there was no mechanical binding of any sort going on. So, I'm guessing that the windings on the motor are fried, for some reason. Don't know how to test for that, though. And I don't know why it would've happened, to begin with.

Then I got to thinking whether or not I really have needed to use two pumps in tandem like that, all these years, and whether or not just one might work. They are both 1/2 h.p. jet pumps. So, I hooked everything back up again, but didn't connect the wiring to the now-dead pump in the pumphouse.

My theory was that the pump down on the river should be able to pump the water through the dead pump, as though it wasn't there. The "dead" pump now just sits in between the good, river pump and the sprinkler system, with the water passing through it, so I figured this wasn't really a problem. But, not being a plumber, I'm not so sure that this is possible. Maybe it's offering too much resistance to the water flow.

When I got it all reconnected and fired it all up, I was able to get only about 22 p.s.i. out of that river pump, which, of course, was not enough to build up enough pressure to shut of the pressure switch, and sure as heck not enough to do any watering.

So, what's the deal? Is my basic assumption that a single 1/2 h.p. jet pump is enough to do the job wrong? Or is the problem that the first pump (the one down on the river) can't push water through the dead one in the pumphouse, because it represent too much of a bottleneck to the water flow? If so, then I should be able to just remove the pump in the pumphouse and replace it with a straight section of pipe, right? I can see where that pump being in the line would be the problem, because the water is encountering lots of barriers, on its way through the now-dead pump, which would seem to reduce its efficiency, kinda like a kink in a hose.

Both of my neighbors water their lawns with just a single pump, and I'm pretty sure they're both just 1/2 h.p. pumps, so I'm pretty safe in thinking that a 1/2 h.p. jet pump should be able to do the job, alright. The only difference between their set-ups and mine seems to be that mine has a longer horizontal run than theirs, but that doesn't really enter into the whole equation, does it? Isn't the factor that counts, when talking about a pump's capability, the vertical lift required? Or am I wrong?

Added information, just in case it might help, and a question: I have the lower pump connected to run on 110 vac, and so I'm assuming that it wouldn't make any difference, if I changed the internal wiring to have it run off of 220 vac, because the power would be the same, so if I double the voltage, then the current would be cut in half...correct? I was thinking of doing that, though, because if the pump down on the river has half of the current running through it, then it wouldn't heat up as much in the hot summer sun. Is my thinking, here, right or am I way off-base? I had previously had it wired for 110 vac just for safety's sake, in case any kids came along and were messing with it. If I rewire it for 220 vac, then I'm going to have to rethink all of that.

Last question: Is it really necessary for me to hook that pump up down on the river? Or can I just hook it up in the pumphouse? That would make it a horizontal run of 90 feet, with a height of about 17 feet above the river? Can a 1/2 h.p. jet pump do that? That would be great, because then I wouldn't have anything but pipe running from the river to my pumphouse, which would make the pump safe and I wouldn't have to drag it up and down the bank, every year, to hook it up.

Any help would be greatly appreciated!!

--Ray
 
  #2  
Old 07-21-10, 09:59 AM
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It takes 1 psi to lift water 2.31 feet. So if your vertical lift is 17 feet as you say, then it will take about 8 psi to lift water that high. Horizontal runs need to be taken into account too, because of friction losses. A long suction line will drastically reduce the pump capabilities, as well. Just as piping has head losses due to friction, so does the second pump that's in the first pump's discharge line. That second pump will need to be removed.
The piping size has alot to do with whether your single pump will do the job. With a long suction line, the pipe size should be increased (at least one size, if not two). If the suction opening is 1.25", you need at least a 1.5" line, and possibly 2". If the horizontal run for the discharge is long, the pipe size should be increased as well.
It sound as though the friction losses are hurting you right now.
Ron
 
 

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