Hmmm... I see some flaws... some seem fatal...
Boxer indicated that the tank is being pressurized by the pumps. His exact words were...
"counting on tank pressure" and "using pressure caused pressurizing the tank from the pumps".
Now, in as much as water is not compressible... the compression has to occur on something else... that something else would be "air".
Is this tank supposed to be Air-Tight??? It better be!
Boxer did not indicate the shape of the tank... so I am free to assume that the 300-Gal (40-cuft) tank is 40-sqft by 1-ft high... not much column pressure there. I'll ignore the column pressure for this discussion.
Has any consideration been given to monitoring the water level and knowing what the water level is when the tank is "at pressure"? In order to KNOW that the system is re-charging properly, you have to KNOW what the water level is when the system pressure is at set point.
There is a special relationship that exists between pressure and volume... this relationship is known as Boyle's Law. This law indicates the relationship between pressure and volume in a gas (This law assumes constant temperature. Charles' Law takes temperature into account).
P1 V1 = P2 V2
P1 and V1 are the initial pressure and volume.
P2 and V2 are the pressure and volume after a change.
The equation can be manipulated into a ratio...
P1 P2
---- = ----
V2 V1
In a nut-shell, this relationship indicates that a particular initial volume of air, at some pressure, is required to maintain a particular range of pressures over a range of changes in the volume.
Consider the following...The tank is 300 gallons. The pressure set point is 30-psi.
Let's say that when the tank pressure is 30-psi, the tank has 275 gallons of water.This means there are 25 gallons of air pressurized at 30-psi.
P1 30-psi P2
---- = ----
V2 V1 25-gal(air)
Now, if only one gallon of water is removed from the tank... that means there is now 26-gal of air... but, at what pressure?
P1 30-psi P2 28.8-psi
---- = ----
V2 26-gal(air) V1 25-gal(air)
By drawing off one gallon of water from 275 gallons (.36% of the available volume) the pressure drops by 4%.
What if 20 gallons of water is removed from the tank... that means there is now 45-gal of air... but, at what pressure?
P1 30-psi P2 16.6-psi
---- = ----
V2 45-gal(air) V1 25-gal(air)
By drawing off 20 gallons of water from 275 gallons (7.2% of the available volume) the pressure drops by 55.3%.
Now, let's say that when the tank pressure is 30-psi, the tank has 100 gallons of water. This means there are 200 gallons of air pressurized at 30-psi.
P1 30-psi P2
---- = ----
V2 V1 200-gal(air)
Now, if only one gallon of water is removed from the tank... that means there is now 201-gal of air... but, at what pressure?
P1 30-psi P2 29.8-psi
---- = ----
V2 201-gal(air) V1 200-gal(air)
By drawing off one gallon of water from 100 gallons (1.0% of the available volume) the pressure drops by 0.7%.
What if 20 gallons of water is removed from the tank... that means there is now 220-gal of air... but, at what pressure?
P1 30-psi P2 27.2-psi
---- = ----
V2 220-gal(air) V1 200-gal(air)
By drawing off 20 gallons of water from 100 gallons (20% of the available volume) the pressure drops by 9.4%.
Recapping...
*By drawing off one gallon of water from 275 gallons (.36% of the available volume) the pressure drops by 4%.
*By drawing off 20 gallons of water from 275 gallons (7.2% of the available volume) the pressure drops by 55.3%.
*By drawing off one gallon of water from 100 gallons (1.0% of the available volume) the pressure drops by 0.7%.
*By drawing off 20 gallons of water from 100 gallons (20% of the available volume) the pressure drops by 9.4%.
It can be easily seen that having less air in the system results in greater pressure changes as water is drawn from the system.
Here is an extreme example to drive the point home...
Let's say that when the tank pressure is 30-psi, the tank has 299 gallons of water. This means there is one gallon of air pressurized at 30-psi.
P1 30-psi P2
---- = ----
V2 V1 1-gal(air)
Now, let's draw one gallon of water from the tank... that means there is now 2-gal of air... but, at what pressure?
P1 30-psi P2 15-psi
---- = ----
V2 2-gal(air) V1 1-gal(air)
By drawing off one gallon of water from 299 gallons (.33% of the available volume) the pressure drops by 50%!
Let's try drawing 20 gallons of water from the tank... that means there is now 21-gal of air... but, at what pressure?
P1 30-psi P2 1.4-psi
---- = ----
V2 21-gal(air) V1 1-gal(air)
By drawing off 20 gallons of water from 299 gallons (6.6% of the available volume) the pressure drops by 95.4%... you are approaching a vacuum state!
The whole point of this exercise is to show that, if you are using a pressurized system, the ratio of air to water at the particular pressure really, really matters! Your system needs to monitor the level of the water at set point and it needs to be able to add air if the air is being lost.
Next issue... in the next post... The VFD
Hey Phil... the editors SUCK!!! WYSIWYG??? Not Hardly!