The function of a pressure tank is to:
- Reduce (on/off) cycling of the pump motor
- Provide water under pressure for delivery while the pump is off
With a Pressure Tank only system the pump will continue to cycle on and off, filling and draining the pressure tank, according to the demand. Only when the water being used is equal to the maximum output of the pump, will the pump run continuously. It is almost impossible to match every zone or water outlet to the maximum output of the pump. Therefore pressure tank only systems are notorious for cycling the pump to death. The larger the pressure tank the less the pump will cycle. The larger the pressure tank, the longer the system will be at low pressure before the pump starts, and the longer the system will be at high pressure before the pump is shut off. These extremes in pressure, constantly changing from 40 to 60 PSI, can be aggravating for people taking showers, and make irregular patterns with sprinklers.
Drawdown from a pressure tank is good to have for times when the pump is off. You don't want the pump to come on every time the icemaker fills or a toothbrush is rinsed. However, for long term uses of water like showers, sprinklers, or heat pumps, a pressure tank only system will cycle the pump excessively. A Cycle Stop Valve or CSV will allow you to use all the water in the pressure tank before the pump is started. Then the CSV will maintain a constant pressure and keep the pump from cycling for longer term uses of water.
Selecting a Pressure Tank
When selecting a pressure tank, certain information must be known:
- Pressure required
- Pump capacity
A differential pressure switch is used to control the pressure produced by the pump, at or above the minimum required system pressure. The pressure switch turns on the pump at a predetermined low pressure (such as 40 PSI), and turns the pump off at a predetermined higher pressure (such as 60 PSI). A 40/60 pressure switch gives an average pressure of 50 PSI, as the pump cycles on and off between 40 and 60 PSI. The volume or draw down amount of the tank is expressed as a tap is opened, causing the pressure to drop from 60 PSI to 40 PSI. The more draw down available in the tank, the longer the pump runs when it cycles on, and the longer the pump is off when it cycles off.
Subtract the GPM being used at the time, from the GPM produced by the pump, to determine the fill rate of the tank. Divide the draw down available in the tank, by the calculated fill rate, to get the time in minutes it takes to fill the tank.
By dividing the draw down available in the tank, by the GPM being used at the time, you get the time in minutes it takes to empty the tank.
The tank should be sized to allow the pump to run a minimum of 1 minute to replace the draw down amount in the tank. I.e. 15 GPM pump, requires 15 gallons of tank draw down. The tank should also be large enough to allow water to be supplied from the taps, for a minimum of 1 minute before the pump is restarted. 1 minute on, and 1 minute off, is the minimum run and off time, to allow the motor to dissipate heat properly. A tank with twice the volume or draw down amount, will allow the pump to run a minimum of 2 minutes, and be off a minimum of 2 minutes. 2 minutes on, and 2 minutes off, requires a tank with 2 times the draw down capacity as when using 1 minute on, and 1 minute off. 2 minutes on and 2 minutes off are preferable, as the less number of times a pump cycles, the longer the pump system will last.
Some pressure tank controlled systems use a pressure switch with 10 PSI between on and off, instead of 20 PSI between on and off. This reduces the pressure fluctuations seen by the water user. Achieving an average pressure of 55 PSI when low flows are required, the pump cycles on at 50, and off at 60 PSI. Reducing this bandwidth by 50% also reduces the available draw down of the pressure tank by 50%. To maintain the same draw down available from the tank, and the correct run time for the pump, a system with only 10 PSI between on and off, requires twice the size of pressure tank as when using 20 PSI between on and off. A pressure tank large enough to deliver 2 minutes of run time with a 40/60 pressure switch, will only allow 1 minute of run time with a 50/60 pressure switch.
When the points in the pressure switch are open and the pump is off, no power is being consumed. When the pump is running, it is using maximum power, and delivering maximum flow. The pump is either running close to it's best efficiency point, or the pump is cycled off. Pumps are made for continuous duty, and care should be taken to prevent the pump from cycling on and off. High starting current from rapid cycling can increase power consumption, and decrease pump and motor life. To prevent cycling with a pressure tank system the pump must be sized to exactly match the amount of water used for long-term requirements, such as irrigation systems and heat pumps.
Types of Tanks and Total Tank Volume
Selecting the pressure tank total volume for typical systems will consider the tank draw down, as compared to the pump capacity and required pump run time. Total tank volume is not a measure of tank acceptance volume, which is typically considered to be available amount of water, or draw down from the tank. Total tank volume is a measure of the total tank size needed to provide the required amount of draw down. Most of a tanks volume is filled with compressed air, which is used as a spring to force water out of the tank. Therefore the actual size, or total tank volume is much larger than the tank draw down, or usable amount of water in the tank. The total tank volume will vary depending on the tank type. The two most common are the captive air tank, and the hydro-pneumatic or steel tank.
The captive air tank has a diaphragm or bladder that separates the air chamber from the water chamber. In a captive air tank, the air cannot mix with the water and be drawn out of the tank with the water. These tanks have very little wasted space, as even the last gallon of water in the tank can be expressed under pressure by the pre-charge in the air chamber. These type tanks do not require any additional air charging equipment, and need very little maintenance.
Hydro-pneumatic or steel tanks have no means of separating the air and water chambers. These tanks must incorporate some means of continually adding air, to make up for the air that mixes with the water and is lost with use of the water. Normally a bleeder orifice in the well is used in conjunction with an additional check valve, schrader valve, and Air Volume Control (or AVC). Each time the pump shuts off; the water is drained between the bleeder orifice, which is about 5' down in the well on the drop pipe, to the additional check valve at the tank. When the pump is restarted, this pipe full of air is pushed into the pressure tank. An air volume control or AVC is mounted about half way up the side of the tank. The AVC has a float protruding inside the tank that adjust the air level by expelling excess air, and maintains the correct air to water ratio. Air injectors and compressors can also be used to add additional air as needed. Air charging devices require periodic maintenance. This type of tank can be needed for wells that have sulfur or other contaminants, where mixing air with the water is advantageous.
When using 10 PSI between on and off, the size of the tank should be doubled. Differences in the types of pressure tanks require that you follow the manufacturers guidelines for selection and installation.