Properly Cooled Submersible Pump motor

Didn't want to hijack another thread, but read something about proper cooling for submersible pump motors and would like your thoughts:

1- Many may or may not remember, but I encountered well issues in the early summer of this past year. Loooong story short; the well is used only for an irrigation system and I monitor the water well level with 1/4" tubing and a pressure gauge and for a year or so I had been experiencing very low water levels in the well. I had dropped the pump to just a few inches off the bottom (137 feet), but the water level would still settle to just a few inches above the pump intake when running. When nothing was running, the pump had approximately 20 feet of water above the intake.

2- The pump failed in mid-June and was replaced with an F&W with the same specs as the original. This pump being new, had no problem quickly pumping the well dry. The impellers on the old pump were so worn that I do not see how it was pumping any water at all! Any way, the well was 35 years old and it was suspected the screen was becoming clogged and not allowing water to flow freely. Backwashing did not help any at all!

3- So, a new well was dug just 20 feet from the original and ended up being 151 feet deep. It is suspected this is the very same aquifer as the static water level is the exact same (117 feet) and the depths are almost identical. The original well was documented as being 145 feet, but the casing went down only 137 feet.

4- Since the original well wasn't producing very well for the past year or so AND no one knew for sure if the well was going dry OR if the screen was partially plugged, everyone decided to lower the pump to 5 feet from the bottom of the newly dug well ...... or 146 feet. This would put the pump in the new well about 10 feet lower than it had been in the "old" well. And by the way, these are both 4 inch PVC SAND wells.

5- So, in the new well, the 1HP 10GPM F&W pump is approximately 29 feet under water (this well and the original BOTH had 40 feet of 4 inch PVC strainer). This past August, on a routine water cycle day, the water level in the well would drop from 117 feet to about 123 feet ................ which leaves the pump submerged under about 23 feet of water in a 4 inch PVC casing.

My question is as follows: If this continues to be the "norm", is the pump motor getting sufficient water flow around it to keep it properly cooled. I certainly do not want to pull the pump so that it can be raised a few feet, but then .................??? note: the irrigation system has 16 zones, runs a total of approximately 6 hours per day and each zone is designed to use approximately 10GPM.


Thanks for your replies!

Ok you have a 4" PVC cased well with a total depth of 151 ft. The bottom 40 ft of casing is slotted or perforated from 111.to 151 ft. The swl is 117 ft. A 1 hp 10 gpm pump is set at 146 ft. When pumping the water level drops 6 ft to 123 ft.

The answer to your question about cooling flow past the motor is......you PROBABLY have no or very little cooling flow past the motor.normal procedure in a screened well is to NEVER place the pump lower than the TOP of the screened interval otherwise there is no way to force cooling flow past the motor.

You are now "top feeding" your pump, because it is set at the bottom of your screen. This wasn't happening in your old well because the water level was drawing down to the level of the pump intake and all the water was coming from below the moter, and cooling it.

Over the years Franklin kept shortening their motors until the new 1 HP is only 11.73” long. That means the bottom of the motor is only 11.73 inches below the water intake on the pump. Franklin says their motors are now so short they don’t need a shroud, even in a top feeding application. Of course they like to sell motors, which is the main reason they have shortened their motors to the bare minimum length possible. So their advice that a flow inducer is not needed should be taken with a grain of salt.

I would prefer to only have 3’ of water above my pump instead of 23’. As Craig said, the best practice in 4” casing is to set the end of the motor above the perforated casing. When that is not possible, I do a well test and set the pump as high in the well as I possibly can.

Do you have a controller that shuts down the pump for a while if the water gets exhausted?

Thanks for your help!

Now, just to reiterate: this is a SAND well. The top of the sand bed is approximately 110 feet and the bottom is approximately 150 feet, then the driller hit rock!!

So, 40 feet of strainer allows the "entire stream" to flow into the 4 inch casing (static water level is at 117 feet although the stream supposedly begins at 110 feet). This being a sand well, is not some water being drawn in near the bottom while the pump is running??? If so ,then I may be OK. I just don't know enough about wells to make this decsion on my own. The driller had no problem with the pump being set this low. He just wanted to make certain I did not have another dry well issue!

I have no real problem raising the pump, but what happens if the water table drops and/or we encounter a severe drought .................. which are common here in South Carolina. At this point I would be forced to lower the pump once again. I'm trying to find a happy medium, but it appears I may possibly get screwed if I do raise it or I may get screwed if I don't. What a dilemma.

OK, a few quick facts...

5 feet of screen under the pump.
23 feet of screen above pump when drawn down.
6 feet of drawdown.

Curious how quickly the well recovers after the draw stops. That will give some idea about the cone of depression versus resistance across the screen. Remember the greater the depth, the greater the pressure across the screen.

If resistance across the screen is a contributor to the drop, then the flow beneath the motor may contribute more than 25% of the flow, so more than enough cooling flow past the motor.

Reach4,

Yes, I do utilize a Pumptec. But, I would like to set things so that this piece of hardware is never needed. Who knows, it could very well fail at the time it's most needed! However, it does offer some piece of mind. But, I do also want to ensure the pump motor is adeqately cooled. I'm assuming if the motor overheats enough, it could possibly melt the PVC casing creating an even bigger issue???

tvl said:

...this is a SAND well. The top of the sand bed is approximately 110 feet and the bottom is approximately 150 feet, then the driller hit rock!!..

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Chances are there are stratified layers in the formation... perhaps finer on top and coarser on the bottom. That often is how glacial till is laid down if in fact that was how the formation came to be.

The driller should know as he likely sampled it as he went.

Thanks LLigetfa!

But, can you explain this as if I were a 7 year old. I think I like what you are saying, but I do not fully understand either????

Oh, by the way LLigetfa, I also installed a 10GPM dole valve in the system when the well was installed. This was to allow for the proper amount of water my irrigation system required, but also protect the well from over-pumping. I'm not sure if this is important or not, but I did want to mention it.

Finer sand partially blocks the screen, reducing the flow. If there is courser sand/gravel close to the bedrock, it can pass water with less resistance and it also doesn't clog the screen. Also, at the greater depth, you have greater pressure.

When you draw the water hard, the level drops. If the level only drops inside the casing but right next to the casing it doesn't, the likely cause is the resistance across the screen.

If the level drops in the aquifer surrounding the well casing, that is called "the cone of depression". The size of this cone depends on the formation.

So, if the level recovers really fast, then either the cone is not very large or the screen is causing the drop in level. So, extrapolating on that, the screen limiting the rate of flow would limit it more at the top (lower pressure) than at the bottom (more pressure)... so 25% or more of the flow could reasonably be attributed to the last 5 feet of screen.

tvl said:

The impellers on the old pump were so worn that I do not see how it was pumping any water at all!

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I get the feeling your new pump will probably suffer the same fate, namely that the pump will wear out before the motor does. My former pump wore out the wet end and the motor was still going strong.

It can go either way. A well may develop over time, opening up the aquifer, increasing the yield. It depends on the formation and whether there is bacteria or other minerals that redeposit.