Well Pump Output

[handyman923]

I recently installed a new 1hp convertible jet pump on an existing 20' shallow well. I have a precharged pressure tank connected to a second output of the pump. The water level is about 3' below grade and draws down to about 5' below grade when the pump is running.

My problem is that I am only getting about 9.5 gpm at 40 psi. (For those that read my other question, I mis-measured the first time.) The manufacturer says I should get 15.0 gpm at 40 psi at a pumping depth of 10 ft. I know that the manufacturer probably gives ideal conditions, but I would have thought I would have come closer to 15 gpm. Do you have any ideas why I wouldn't be getting a higher flow?

I have a 1-1/4" PVC suction line down about 15 ft with a foot valve. And I have a 1" galv discharge line. I don't lose any pressure when the system is off, so I don't think there are any leaks.

Thanks for any help!

Dave

[Bob NH]

When it says you should get 15 GPM at a pumping depth of 10 ft it means when the suction pressure at the inlet of the pump is 10 ft (4.33 psi) below atmospheric pressure. Check the flow for 15 and 20 ft of lift.

Analysis:
You should find what the real lift is when all variables are considered.

First determine the height of the suction of the pump above the water in the well. The water draws down to 5 ft below grade when pumping. How far is the inlet of the pump ABOVE that grade, or above the water in the well when pumping?

In addition to that difference in elevation, you must add whatever pressure loss there is through the foot valve, and the friction loss in the pipe from the well to the pump. If there are any valves or elbows in that suction line, there will be more loss. There will be more loss if it is quite far from the well to the pump. You could measure the vacuum at the inlet of the well with a special gauge, or you could set up a manometer to measure it.

Another cause of poor performance in a jet pump is an air leak on the suction line. Polyethylene pipes with clamped joints are notorious for air leaks. PVC pipes are usually less susceptible to that problem. If there is such a joint on the inlet of your pump, you could try to seal it up with some kind of putty to see if it helps. That would not be permanent.

That exhausts my ideas for the moment.

[speedbump]

What brand pump did you buy? And did you get the shallow well jet to go with it? You said you have convertable pump, but didn't mention the jet. If you did get the jet with the pump, are you sure it's for a 3/4hp?

bob...

[handyman923]

Thanks for the suggestions. I did install the jet, but I will check to make sure I installed the correct size...

[handyman923]

Thanks for your help! Speedbump, you were right. The jet converter came with three different sizes of internal jets. Evidently, I installed the wrong one. Now I have 14.5 gpm at 40 psi and 11 gpm at 50 psi. Much better!

Thanks again.