Pressure tanks are an integral part of residential well systems. They come in all shapes, sizes and colors, and in previous articles in this publication, I’ve described them in detail, the different types, their purpose and how to size them.
The earliest pressure tanks are known as hydro-pneumatic pressure tanks. They were simply a large pressure vessel hooked to the discharge of a well pump. This type of “conventional” pressure tank is still in use today. As water is pumped into the tank, the air is compressed so that when the pump shuts off you have a few gallons under pressure ready for the next call for water. The issue with these tanks is that, over time, the compressed air is absorbed into the water until eventually
|In-Well tanks eliminate issues associated above-ground tanks, such as freezing and condensation. Source: Flexcon Industries|
there is not enough air to push out much water. Over the years devices have been invented to maintain the proper amount of air, most notably, the introduction of the captive air tank where a separate rubber bladder or diaphragm inside the tank encapsulates the air, separating it from the water. These tanks are generally smaller than their conventional counterpart in terms of “draw down” (the amount of water delivered between on-off pressure switch cycles), which has led to their present day popularity.
Not much has changed in the basic design of pressure tanks over the years, but at a recent distributor open house I saw a new concept on display that looked interesting. Flexcon Industries is now manufacturing a tank that fits down inside the well specifically designed for constant-pressure systems. Before getting into the details, let’s review the purpose for having a pressure tank in a water well system and go over some of the basic sizing criteria.
The most important reason we install pressure tanks is to store a small amount of water under pressure to keep the pump from having to come on every time someone brushes their teeth or flushes a toilet. The reason we don’t want the pump coming on for a small usage demand is to protect the pump’s motor from overheating. When a pump starts up, its electric motor draws up to six times the normal operating current for a fraction of a second just to get things moving and up to speed. This extra current creates a lot of heat in the motor windings. Once the pump is running at its normal operating speed, it begins to dissipate the start-up heat and, after a minute or so, the extra heat is gone and it is safe to cycle the pump off and on again.
Motor manufacturers publish specifications regarding the number of starts and stops their motors can tolerate over a certain amount of time. As a general rule, groundwater contractors are encouraged to provide at least one minute of run time between pump cycles. That means if you have a 10 gallon per minute pump, you should provide a pressure tank with 10 gallons of “draw down.”
The one minute of run time rule applies to systems that are controlled by a pressure switch that turns the pump on when the pressure in the tank drops to the switch’s turn-on pressure—say 20 PSI—and off when it gets up to the switch’s turn-off pressure—40 PSI, for instance. What about a constant-pressure system where the pump is controlled by a VFD or control valve, both of which vary the flow rate to keep the pressure constant regardless of the flow rate? These systems do not need a large pressure tank because the pump is not stopping and starting frequently, which is one of their advantages.
So if we’re now talking pressure tanks with just a few gallons of draw down, you can see the possibility of installing one down inside a well. The In-Well by Flexcon is designed to fit into 4-, 5- and 6-inch wells. It comes in either stainless steel or schedule 40 PVC with a total capacity of 3.5 to 4.25 gpm, depending on the model. The advantage of an in-well tank is that it eliminates the negative issues of having to deal with an above ground tank—freezing, condensation and space considerations.
In VFD and control valve systems, the tank’s job is to dampen pressure spikes, not to store water, so In-Well tanks are suitable for most residential constant-pressure applications. If you get much above 20 gpm, consult your VFD or valve manufacturer to see if their capacity is sufficient.
In-Well pressure tanks are not for everyone but where they fit, they’re a slick idea. For more information, contact your local water well products distributor or Flexcon Industries at www.flexconind.com or 800-527-0030.
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