Centrifugal Pump Pressure

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Sethro

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I have a 1.5hp centrifugal pump (Flotec sprinkler pump) that is probably 25 years old, though rarely used. It came with the house when I bought it. Last year I decided to drive a sand point and put the pump to use. I live along a river, and the well is in my boathouse which sits about 6 feet above normal river level. I drove two 5' sections of 1 1/4" pipe with the sand point, and hit water with that first section with the point. Main use was for washing the boat down with a hose as I pull it out of the water. It worked great, though I got tired of having to turn it on and off as it's a continuous run pump, and I didn't want to worry about accidentally leaving it running.

This spring, I added a 13 gal pressure tank/switch as my friend gave it to me. I wasn't able to set the switch to cut in and out like I thought it should. It would cut in ok, but would never cut out till I turned the flow of water off. I can set to cut out without turning water off, but that makes cut in way too low. This was by trial and error to find just the right setting, as I liked the pump running all the time at max pressure when using water instead of cycling with such a low cut in pressure. Anyway, I could tell cut in pressure seemed pretty low, so I added a gauge to see what was going on. Cut in is about 18psi, while cut out is about 35psi. I have adjusted the cut out down to try to keep the cut in as high as possible. I'm essentially using this centrifugal pump as a jet pump I realize. Also took the switch out of operation, and pump pressure and function is the same with or without.

That leads me to my question. I'm roughly 8' above water in my pipe, and I want to install an inground sprinkler system. Running two sprinklers with hoses only yields a pressure fluctuating between 25-30psi. I'm also only pumping about 11 gpm in my test out of the two hoses running together. I'm wondering if I'm expecting too much as I thought the centrifugal pump would be able to maintain 40ish psi with several sprinklers running and a much higher gpm. I'm not sure if I don't have enough water going to the pump, or the pump is a little tired as it can't build past 35psi even with no water discharging. I don't have a problem buying a jet pump, and I'm thinking the 1hp Flotec (FP4032) but I'm just afraid I might not be able to supply the water it's capable of pumping. Should I drive another section of pipe...two more sections...I want to get it right as I'm tired of my usual trial and error ways.

Thanks for any help!!!
 

Valveman

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A jet makes a centrifugal pump build pressure. A straight centrifugal pump delivers lots of volume, but very little pressure. Most of those "sprinkler pumps" max out at 35 to 45 PSI. They are not made to be used with a pressure switch and tank.

Get a good jet pump. One that has a max pressure of 70ish PSI. Then you will have the pressure needed to wash a boat or work with a pressure tank/switch.

The jet pump will deliver higher pressure but less volume than the centrifugal, so the well should be fine. But you are right that the cycling on and off is a bad thing. It is almost impossible to set a jet pump pressure switch to prevent cycling at lower flow rates. Adding a Cycle Stop Valve as comes in the PK1A kit will give you strong constant pressure, works with a little 4.5 gallon size tank and a pressure switch to automatically supply water anywhere or anytime you open a faucet or sprinkler zone.
 

Sethro

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So, I was pumping water yesterday for an extended time. Right off the bat it was running 3 sprinklers at maintaining 34-35psi. After a while, the pressure slowly fell to 25-30psi and the pump/tank was kind of wobbling. It maintains that but doesn’t go higher even when I shut outflow off manually. If I then run water and let the pressure drop to the cut off point, and shut the outflow off, it will build back to 35psi and shut out off with my installed switch. There was also some debris on the sprinkler screens where
the hose connects.

This makes me think I have an issue on the suction side. I was kicking myself for not installing 2” to begin with. I’m thinking I need to pull the point to check the screen and for any air leaks. At that point I’d just out 2” pipe down and stop wishing I did in the first place. I’d use 15’ instead of 10’ of pipe as well.

Any thoughts given this additional information?
 

Valveman

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You could have a suction leak, or the small point could be a restriction. The chart for that pump says it should do 50 GPM from 10' of lift and 25 PSI. You say you are only getting 11 GPM? A sound like it is pumping gravel is a sign the suction is restricted. If you test that pump in a barrel or bucket and it works up to spec, then you most likely have a suction line problem.
 

LLigetfa

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They are not made to be used with a pressure switch and tank.
I agree. I have a centrifugal pump on my work truck which I use to deliver water to portable washrooms. It is connected to a hose reel with 50 feet of 1/2 inch hose. The friction loss on that much hose makes it impossible to dial in the pressure switch so that it doesn't short cycle but yet reliably still turn off the pump when I close the full port ball valve. I have to create water hammer by fast closing the valve to trip the pressure switch.
 

Sethro

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I like the idea of testing the pump, so I’ll pump directly out of the river and see what happens. I have a suction line I can make work for that.

If I had an air leak on the suction side, wouldn’t that be all the time and not show up after several minutes of pumping water? My gut feeling is the sand point screen is somewhat clogged, especially after finding debris in the sprinkler screens.

I think I’ll still get a jet pump regardless, as so much of its use will be running a hose, and the sprinklers I’m interested in like an optimum 40psi. Just need to figure out if I need to pull the well too.
 

Sethro

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So, I hooked the pump up directly to the river and the pump acted the same as hooked up to the well. Even dropping pressure to the mid 20’s on occasion before building back up. It seems pretty logical that the pump just isn’t capable of any more than it’s providing.
 

Reach4

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There are suction pumps called irrigation pumps, but instead of a jet to build pressure, they have two or more centrifugal stages in series that add together to provide more pressure.
 

Valveman

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So, I hooked the pump up directly to the river and the pump acted the same as hooked up to the well. Even dropping pressure to the mid 20’s on occasion before building back up. It seems pretty logical that the pump just isn’t capable of any more than it’s providing.
If it is building back up the pump is probably still good and you have a suction leak.
 
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