Submersible well pump pressure tank?
I have a well at my house that I am currently using for irrigation of my lawn and garden. The well is 50" deep with a 6" casing. The well pump is a Grundfos MQ and is a jet pump. My problem is that the water line has fallen to around 15' deep and the lower it goes with the jet pump the less water it produces. It stops lifting the water, I believe, around 20' deep or so - here in Denver, CO. I would like to use a submersible well pump to take advantage of the depth of the well.
So here is my main question: Do I need a pressure tank? Are there submersible pumps that don't require one since it will be used for irrigation? Can anyone offer recommendations for an economical pump?
Also, I am hoping that the well performs better with the pump at a deeper level. My theory is that as it pumps the water out of the casing, water will flow into the casing faster because it would increase the pressure around the casing. Is there any truth to that?
You can certainly install a submersible pump, I and most people who have a potable water well use submersibles. So far as I know, you need a pressure tank with a submersible pump in order to reduce the cycling of the pump, which is detrimental to the life of the pump. For irrigation, you might only need a small pressure tank, I have a 50 gallon tank for my house, which is pretty typical.
The maximum withdrawal rate of water from your well is called the safe yield, and is typically tested when the well is installed. It has to do with the type of soil or rock surrounding the well, and the depth of the well, and has nothing to do with the type of pump you use. The safe yield means the rate at which you can pump without drawing your well down excessively. My well, which is 350 feet deep, has a safe yield of 1.75 gallons per minute (quite low), because the well is in granite bedrock and apparently intersects only a few fissures. Your well of course is going to be unique. If you do not know the safe yield, it can be tested by a qualified well drilling company.
Installing a submersible pump at a deeper level than the jet pump you say you have will have no influence on the safe yield, but will increase the allowable short term yield, since there is typically water sitting in the pipe above the level of the submersible pump which can be utilized down to a few feet above the level of the submersible pump. Should you install a submersible, make sure you do not draw the water down to the level of the pump, as it will suck in sediment and be destroyed very quickly (I know this from painful experience).
A pressure tank is not always required with a submersible pump. What a pressure tank does is maintain pressure at set range. If you don't need pressure to be maintained within a set range a pressure tank is not required.
But as discussed above pump sizing is critical--the amount pumped must always be less than the rate the well recharges--particularly with a well that is only 50 feet deep and has relatively little storage in the well. The amount pumped will vary, for any given pump, with the effective head the pump sees. The head varies with the level of the water in the well and also varies with the amount of piping and fittings as well as nozzles or other control devices in your irrigation system. Unless you are comfortable using pump curves and have good data for the recharge rate for your well (what is referred to as safe yield in the post above) I recommend to seek the services of a professional in sizing the pump and deciding on how the installation will be done.
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