Shallow Well Jet Pumps
Shallow well jet pumps are widely used in the United States - for a variety of reasons. Most of the time they are used on a well that can vary in size from as small as 1-inch diameter to large diameter augured wells. I've also seen them used on dug wells and for small irrigation systems pulling out of a lake, canal or pond. Another use is for boosting pressure on domestic water systems.
Shallow well pumps come in two basic varieties:
Convertible jet pump - This is a pump with a bolt-on kit that contains the jet assembly consisting of a nozzle and a venturi or diffuser tube. This kit can be removed and the pump converted to a deep well jet pump.
Shallow-well-only jet pump - This is a pump with the nozzle and tube built into the housing, and can be only used as a shallow well unit.
So those are the two types. Now let's take a moment to look at what constitutes a shallow well. And what is a tube and nozzle anyway?
A shallow well has nothing to do with the depth of the well; it has everything to do with the pumping level at which the pump will be working. Remember that we have available at sea level 14.7 psi of atmospheric pressure and a pump is going to pull a vacuum. If a pump were able to pull a perfect vacuum, the atmospheric pressure that pushes down on everything, including ground water, would be able to push the water up approximately 33.957 feet to fill that vacuum. All a pump does is reduce the atmosphere in a suction pipe and let Mother Nature force the water to it. Then it's going to move this water through its impeller with a centrifugal pump.
If you divide 33.957 by 14.7 psi you get a powerful number - 2.31. 2.31 is a number that you can use to convert psi to feet of head and vice versa. Because a pump will not pull a perfect vacuum, the industry uses 25 feet as the maximum lift that you should expect out of a shallow well jet pump. Naturally, if you are operating at less than 14.7 psi for atmosheric pressure, you won't be able to lift as far.
So what are these tube and nozzle things anyway? Basically, the tube and nozzle combination will create more vacuum than just a standard impeller, and it also will increase pressure substantially. Be it a shallow-well-only or a convertible jet pump, the pump will move water off the impeller and direct some of this water back to the jet assembly (tube and nozzle). The nozzle has a very small opening that will vary with the horsepower and flow desired. The net result it that it moves the water at a very high velocity, much like putting your thumb on a garden hose to make the stream go farther and with greater velocity. The tube is just beyond the nozzle and allows the high velocity water to be channeled into pressure. The small space between the tube and nozzle is where the maximum amount of vacuum will be formed. This part is open to the suction side of the system (drop pipe), and the water is forced from the well to fill this vacuum by the 14.7 pounds of atmospheric pressure. Once there, it feeds all of the water that the jet has picked up from the well, plus the water the pump sent to the jet, and gives it all to the impeller. Remember that an impeller will take what is given and then add its own design pressure, so, in essence, we have a two-stage pump here. Hard to believe as it is, that little tube and nozzle acts just like a pump.
A couple of other notes of which to be aware:
- Most jet pumps will have a diffuser over the impeller. This allows the pump manufacturer to use one case for many different horsepower needs.
- Also, like all of the systems that require lift, the system must initially be primed and have some way to keep it primed, such as a foot valve or a check valve.
A final note about boost applications: In boost applications, most of the energy of the pump is going to develop pressure, and the units usually placed are in-line with a small tank used to turn the pump on and off. In most cases, a 3/4-inch water meter will only be supplying approximately 8 gallons per minute, so there is little chance for the pump to cut down the flow of water since the pump should be sized to have the capacity to handle the incoming capacity. In cases where the incoming capacity is larger than the capacity of the pump, you can use a bypass line with a check valve. This will allow full flow and also add the increased pressure from the pump.