Tank Question
- Thread starter BrianP
- Start date
Users who are viewing this thread
Bob NH
In the Trades
There is more than a rule of thumb. You can calculate it.
The amount of water per cycle is equal to the volume of air in the tank at the start of the pump cycle times the difference in pressure between pump on and off, divided by the absolute pressure in the tank at shutoff.
Water per cycle = (Gallons of air at start) x (off - on pressure) /(absolute pressure at shutoff)
Here is how to operate your system for maximum water per cycle. First, you should put some air in the tank just as though it were a bladder tank. You should have a compressor because you will have to repeat it from time to time.
To keep it simple, I am going to assume that atmosheric pressure is 15 psi, rather than 14.7 as it actuall is at sea level.
Start by deciding you are going to precharge the tank to 30 psi when there are 5 gallons of water in the tank; almost empty. You have 75 gallons of air in the tank. The absolute pressure at that point is 30+15=45 psia. PSIA means psi absolute.
Now you want to compress that air to 50 psig which is 65 psia.
For this application, pressure x volume = a constant value.
45 psia x 75 gallons = 3375.
Now divide 3375 by the new pressure, 65 psia, to get the new volume of air:
3375/65 = 51.9 gallons of air.
The water increases by the same amount as the air decreases, so you have added 75-51.9 = 23.1 gallons of water to the tank. That is the amount of water available per pump cycle.
Now you aren't going to want to drain your tank every time you need to put air in it. Here is how you manage that if you don't have an automatic system.
Recall from above that when the pressure is 30 psig you have only 5 gallons in the tank, and when it is 50 psig you want 80-51.9 = 28.1 gallons in the tank.
To get the correct amount of air in the tank you can skip the calculations and do the following:
Make a mark at about the 35% level on the tank. That is where it should be when the pressure is 50 psig. If the water level is higher than that (you can usually tell by condensation or by tapping the tank) you need more air. Shut off the pump, drain the water to that level, and put in air until the pressure is 50 psig. Turn on the power to the pump.
So now we come to the Rule of Thumb you were looking for. The volume of water between pump cycles for a 30/50 psi pressure cycle is about 30% of the volume of air in the tank when the pump starts at 30 psig. 23.1 gallons / 75 gallons = 0.308 = 30.8%
If you get too much air in the tank it will correct itself by coming out in the faucet.
The amount of water per cycle is equal to the volume of air in the tank at the start of the pump cycle times the difference in pressure between pump on and off, divided by the absolute pressure in the tank at shutoff.
Water per cycle = (Gallons of air at start) x (off - on pressure) /(absolute pressure at shutoff)
Here is how to operate your system for maximum water per cycle. First, you should put some air in the tank just as though it were a bladder tank. You should have a compressor because you will have to repeat it from time to time.
To keep it simple, I am going to assume that atmosheric pressure is 15 psi, rather than 14.7 as it actuall is at sea level.
Start by deciding you are going to precharge the tank to 30 psi when there are 5 gallons of water in the tank; almost empty. You have 75 gallons of air in the tank. The absolute pressure at that point is 30+15=45 psia. PSIA means psi absolute.
Now you want to compress that air to 50 psig which is 65 psia.
For this application, pressure x volume = a constant value.
45 psia x 75 gallons = 3375.
Now divide 3375 by the new pressure, 65 psia, to get the new volume of air:
3375/65 = 51.9 gallons of air.
The water increases by the same amount as the air decreases, so you have added 75-51.9 = 23.1 gallons of water to the tank. That is the amount of water available per pump cycle.
Now you aren't going to want to drain your tank every time you need to put air in it. Here is how you manage that if you don't have an automatic system.
Recall from above that when the pressure is 30 psig you have only 5 gallons in the tank, and when it is 50 psig you want 80-51.9 = 28.1 gallons in the tank.
To get the correct amount of air in the tank you can skip the calculations and do the following:
Make a mark at about the 35% level on the tank. That is where it should be when the pressure is 50 psig. If the water level is higher than that (you can usually tell by condensation or by tapping the tank) you need more air. Shut off the pump, drain the water to that level, and put in air until the pressure is 50 psig. Turn on the power to the pump.
So now we come to the Rule of Thumb you were looking for. The volume of water between pump cycles for a 30/50 psi pressure cycle is about 30% of the volume of air in the tank when the pump starts at 30 psig. 23.1 gallons / 75 gallons = 0.308 = 30.8%
If you get too much air in the tank it will correct itself by coming out in the faucet.
Similar threads
