What's a T&P valve for?

[George E. Cawthon]

I have never seen a vacuum break on a water supply line to a hot water tank, and it sounds completely useless.

First if your fire truck scenario even worked, it would be the responsibility of the water system to install backflow preventers at the turnouts to the houses. Second, collapse of a tank is virtually impossible because I don't know how you would be able to suck the water out through the supply line. Water has a relatively low adhesion rate, so a strong enough pull (vacuum) would break the water column.

Second, if hot water tank lines did have vacuum breaks and there were no back flow preventers and the firetruck made the water flow "backwards," then the truck would soon be pumping water and air.

What would happen to the water if one end of a tangled system of pipes filled with water is closed and a strong vacuum is put on the other end? Nothing significant. Yes if you got a low enough pressure the water would boil and vapor would be slowly drawn off.

You are not going to crush a water tank (or any part of your water system) by putting a vacuum on the system.

[Thomas D. Horne, FF EMT]

Bob 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 tank?

[Bob Engelhardt]

I have seen plenty of them. As to "useless", I can't personally say.

Yeah, right.

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, Bob

[Lloyd E. Sponenburgh]

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 width-to-height profile.

I've used water heater tanks for building furnaces and barbeques. The metal is quite thin.

LLoyd

[George E. Cawthon]

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 impresses me.

[Robert Swinney]

[Eigenvector]

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

[Bob Engelhardt]

!!! Good one! I needed a good laugh! But please don't tell me that you teach high school physics, that would take all the humor out of it entirely. Hahahahaha ...

Bob