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The bladder tank conspiracy,
simplified.
When you buy a bladder tank, some
folks (like me) will tell you that it is equal in size to
a galvanized tank. Why do we do that, you ask?
The reason is simple. When you
install a galvanized tank, let's use a 42-gallon. It will
be completely empty when you first start the pump. The entire
inside of the tank will be full of air. When water enters
the tank, the air in that tank is compressed into the top
25 percent or so of that tank. That air now acts like a spring.
When you open a faucet, the spring (air) pushes water out
of the tank at the same pressure that reads on your gauge.
Now, since you started filling
the tank at zero pressure when the pump first started and
you now have a pressure switch that is going to turn the pump
on at a pre set pressure of say 30 pounds, the tank is not
going to empty. The tank will only be about ½ empty. What
happens to the rest of the water? It stays in there. Now you
shut off the faucet, the pump runs the pressure back to say
50 pounds and shuts off at ¾ full. The actual amount of water
that was taken out and put back during this cycle was 6.2
gallons.
Lets do the same thing with the 42-gallon equivalent bladder
tank. You install it on the system. The bladder is at the
bottom. The entire tank is pre charged to 28 lbs. The pump
is turned on. The tank does not take in any water until the
pump exceeds the 28lbs.
At 28lbs the tanks bladder takes in water to the tune of
6.2 gallons and the pump shuts off at 50lbs. Open a faucet,
the compressed air (spring) now pushes water out of the bladder
at the pressure reading on your gauge until the pump gets
to 30lbs. At this point the pump starts up and starts refilling
the bladder just before it is completely empty, giving you
6.2 gallons of water. If for some reason your pump did not
start at 30lbs, the tank would keep emptying until it gets
down to 28lbs, at which time there would be no more water
or system pressure. Just air pressure, trapped above the bladder.
That’s the reason we use the equivelency sizing instead of
physical size!
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