THE IMPORTANCE OF PRESSURE TANK SIZING.

Tank sizing is one of the most important elements of a water well pump system’s overall design but many of our customers don’t understand the need for pressure tanks.  Sadly, even many well contractors don’t seem to know the importance of a properly sized pressure tank.  To fully understand tank sizing, you will need to know a few things about the pumps that we typically use in wells, “submersible pumps”.

Submersible Pump:  A submersible pump (or sub) is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped.

The figure to the left shows a submersible pump and motor coupled together.  For most purposes, the complete, coupled unit is referred to as a “submersible pump”.  However, technically speaking, the pump (sometimes referred to as the “liquid end”) is the top half of the unit while the motor is the bottom half of the unit.  The pump is bolted directly to the motor and a spline shaft couples the two together.  The power is provided by a “submersible cable” (wire) that runs down the well and attaches to the motor.

When power is applied, the motor turns the motor shaft, which turns the pump shaft, which turns the pump impellers that are attached to the pump shaft.  The impellers “pull” water into the pump through the inlet screen and pushes the water up and out of the well through the “drop pipe”.

 

Similar to how a car takes more power to get up to freeway speed, than it does to stay there, each time a submersible pump starts, it requires a relatively large amount of power (current) to get things turning.  The inrush of current to start a pump can be 5x higher than the energy needed to keep it running.  This big inrush of current creates excess heat in the motor.  If too much heat is allowed to build up, the motor will fail.  But, by design, when a submersible pump is running, the relatively cold groundwater is flowing past the motor and cooling it down (neat, huh?).  The fact that the flowing water keeps the motor cool means that a submersible pump can stay running indefinitely and in fact, submersible pumps prefer to stay running for long periods of time.  Unfortunately, because we don’t need water running 24/7, we have no choice but to “cycle” the pump.  This is where pressure tanks come in.

Pressure tanks are designed to fill with water up to a certain pressure (known as “cut-off” pressure), set by the user, in which the pump shuts off.  Then, as you draw water from the tank, the pressure inside the tank drops to a certain pressure (known as “cut-in” pressure), also set by the user, in which the pump turns on.  This process is referred to as a pump “cycle”.  

The purpose of pressure tanks is to provide a buffer to allow adequate cycle times for the cooling of submersible motors.  According to manufacturers, small horsepower motors require a minimum of 1 minute of pump run time to dissipate the heat generated during starting.

Example:

You have a submersible pump in your well that produces 10 gallons per minute (GPM).  In our example, you are going to be hand washing dishes (sorry about that!).  Each dish takes 15 seconds to wash and 10 seconds to put away or 25 seconds per cycle.  The average kitchen faucet uses around 2 GPM.

Without a pressure tank, every time you turn your kitchen faucet on, your pump will turn on and every time you turn the water off, the pump will turn off.  If you wash 10 dishes, you would cycle the pump 10 times in only 250 seconds or just over 4 minutes. Each cycle (since we know it takes a minute to cool down), will cause the motor to continuously get hotter and hotter.  The continual buildup of heat would break down the motor windings and you would have a premature failure.

Now, lets put a pressure tank on your system that holds 10 gallons of water at pressure.  This means that when full, the tank has 10 gallons of “usable” water in it (we’ll get to that later). 

Now, when you turn your faucet on for the first dish, the tank pushes the water out to the faucet (instead of the pump) – 25 seconds (for the first dish) at 2 GPM or about 1 gallon of water.  So, now we have 9 gallons of water left in the tank and the pump hasn’t run.  Wash dish 2, then 3 and so on.  After 10 dishes, (which still took about 4 minutes) the tank has emptied of 10 gallons of water, hitting the cut-in pressure and the pump starts to fill the tank back up – remember, in our example, your pump produces 10 GPM, so it will take the pump 1 minute to fill the tank up and hit cut-off pressure to shut the pump off.  In this example, if you had 50 dishes to wash, you would only cycle the pump 5 times in 20 minutes!

This is a very simple example of tank sizing, in the real world, more factors may come in to play.  We size each system specifically based on your application.

To learn more about the different sytyles of pressure tank see: Pressure Tank Styles.

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  2. I like that you provided some valuable insights regarding the importance of pressure tank sizing. Actually, I don’t really know much about the whole process until I came across your post. You said that pressure tanks provide a buffer to allow adequate cycle times for the cooling of submersible motors. I will definitely be reading more about pressure tanks and how the whole cycle works for additional learning. Thanks.

  3. Regardless of whether you are installing another well or overhauling your present pressure tank, setting aside the opportunity to figure the best size of pressure tank for your well will guarantee that your pump execution is advanced and supported for whatever length of time that conceivable.

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