I have seen plenty of them. As to "useless", I can't personally say.
Second, collapse of a tank is virtually
Don't take "vacuum" too literally. They would more accurately be called
"negative pressure preventers". They let air in as soon as the water
pressure is less than atmospheric. So there is still absolute positive
pressure in the pipe and on the water.
Water heaters are designed to hold pressure. It is very easy for a thin
wall tank to hold pressure (e.g., soda bottle). A thin wall does not
hold negative pressure ("vacuum") at all. I don't have figures, but I
would guess that a couple of psi would crush a tank. For example, a
tank 50" high and 30" in circumference has an area of 1500 sq in. 1 psi
would exert 1500 pounds force on the tank.
Awaiting an experienced reply,
Done it (intentionally).
My son and I cleared out an old shack of a house preparatory to salvaging
the ancient pine floors and joists.
Just for fun, (on the excuse it would make more room in the trailer on the
way to the scrap yard) we collapsed both a water heater and a water pressure
tank with a cheap 115v vane pump. The water heater was disappointing. We
_heard_ it collapse, and saw some distortions in the outer casing, but
didn't see the tank squinch down, because the inner tank wasn't hard-coupled
to the outer housing. The pressure tank almost didn't go with the limited
vacuum we could pull; but finally it caved -- rather suddenly. The water
tank was origianlly almost "square"; the heater tank had a very narrow
I've used water heater tanks for building furnaces and barbeques. The metal
is quite thin.
What do you mean don't take it literally. I said
put a vacuum on one end, that's what I meant.
What negative pressure preventer? I didn't say
anything about a negative pressure preventer? I
set up a scenario and said what would happen. You
change the scenario? What's with that?
At a perfect vacuum you are correct, with a
correction for the two ends the total pressure
would be 1500 pounds spread evenly over the
surface. And that impresses you? Do you know
that women in high heals can exert 200-300 psi on
the walking surface with each step. That
1 psi of pressure would exert 1 pound of force on the tank. Besides, water
tanks are not glorified soda cans, they have liners, insulation, and some
limited cross bracing.
I bet if you calculated the total force needed to crush a water tank under
vacuum it would be around 1000 psi
Unless we are using hard suction hose we cannot draw the pressure down
below about five pounds before the hose will collapse and stop the flow.
Even with "Hard Sleeve" the maximum pressure reduction a modern fire
pumper in good condition can achieve is roughly two thirds of
atmospheric. Would negative ten pounds actually collapse a water heater
Well we aren\'t no thin blue heroes and yet we aren\'t no blackguards to.
I didn't assume electric, I specifically mentioned
what would happen with electric and what would
happen with gas.
Of course electric elements in water heaters burn
out. Why do you think the manufacturer's
instructions always caution you to fill the tank
with water before turning on the power?
Certainly some do, what is your point? No
backflow preventers in my lines, In fact, I don't
believe I have ever lived in a house that had a
backflow preventer at the city, county, or
whatever water line.
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