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# a little math

Member Posts: 1,520
if you use a shop vac to winterize or empty out zones for repair  one of my favorite methods, then you need to isolate the water heater or an indirect tank with a shutoff and a hose cock tee after the shutoff away from the tank where you can apply vacuum, so that you can isolate the hot water tank, from what, the atmospheric 14.7psi against a vacuum might do to it,

surface of a cylinder = 2 ð r2 + 2 ð r h
plugging in the values for a typical 40gal tank 2x(3.14x10)+ ((2x3.14)x10)x58 = 314 + 3956.4 = 4270.4 square inches time 14.7 per = 62774.88psi or 31tons  IF THAT DONT CRACK THE GLASS LINING INSIDE YOUR WATER HEATER, NOTHING WILL!!!!

• Member Posts: 1,520
winterizing and a little math

if you use a shop vac to winterize or empty out zones for repair  one of my favorite methods, then you need to isolate the water heater or an indirect tank with a shutoff and a hose cock tee after the shutoff away from the tank where you can apply vacuum, so that you can isolate the hot water tank, from what, the atmospheric 14.7psi against a vacuum might do to it,

surface of a cylinder = 2 pi r(sqd) + 2 pi r h

plugging in the values for a typical 40gal tank 2x(3.14x10)+ ((2x3.14)x10)x58 = 314 + 3956.4 = 4270.4 square inches time 14.7 per = 62774.88psi or 31tons  IF THAT DONT CRACK THE GLASS LINING INSIDE YOUR WATER HEATER, NOTHING WILL!!!!

• Member Posts: 439

but the math? That is some crazy number, Kal!
• Member Posts: 1,640
Crazy indeed,

Because, the the vacuum suggested has been improperly trasferred from one square inch - whcih is all that really matters, to every square inch within the vessel - COMBINED!; which is AN INVALID EXTRAPOLATION!

• Member Posts: 3,796
Ummmm...

... I may be a mere homeowner, but the math seems faulty.

For one, even were you to pull a vaccuum inside the tank (which you won't with a shop-vac), the pressure on the outside of the tank will never exceed atmospheric, i.e. 14.7 psi.

The tanks are usually rated for 150 psi and tested for 300 psi at time of manufacture. As such, I'd like the see the tank do anything more than shrug at a shop-vac.
• Member Posts: 1,520
law of hydraulics

the 150 psi is inside to out  where the tensile strength of the steel holds it, but from the outside in, it will compress like a soda can

aircraft use aluminum monococue construction  ie a tube of aluminum stretched on circular I-beam type girders  that holds it and even then its only holding 5psi at 30000ft as they are only giving you 8000ft pressure in cruise

its 14.7 on every sq inch and the total load is still there
that is why, those big screen direct picture tube tvs have such thick and heavy glass  the vacuum tube has to hold back 20 tons of atmospheric pressure

when you make a solder joint, how do you test  I hold one end with rubber gloves and suck on the other, since can only apply 20inches by blowing, but by sucking and seeing if it holds vacuum  I am letting the atmosphere do the work for me
• Member Posts: 240
We're missing a key fact here

I suspect the glass on the inside of the tank is applied to the steel shell. Unless there is a vent to atmosphere between the glass and the steel the force will be taken up by the steel, not the glass.
• Member Posts: 223
Kal

4270.4 square inches time -(I think that this is where the mistake is) 14.7 per (right here - PER SQUARE INCH) = 62774.88psi < your formula does not translate into pressure per square inch, it is total pressure on ALL the surface of the tank.

but each individual square inch still has 14.7 psi. which is not to say that that is not enough to crush the tank if a vacuum was pulled inside!

Leo G
• Member Posts: 3,796
Perhaps I am missing here something...

Kal,

Steel is an isotropic material. As such it wouldn't care wether you apply the pressure from the outside in or from the inside out. What does matter is the shape of the material (i.e. the round cylinder), the fashion in which it was built, and the materials that were chosen.

Were your assertion true, then there would be no reason to shape submarine pressure vessels exactly alike to high-pressure vessels above ground. Ever seen the pressure vessel below the Trieste? Ever wonder why it approximates the shape of a common high-pressure LP tank? Both are balls and both resist the pressure on the inside or the outside for the same reason.

Consider that the shear strength of steel is usually measured in the thousands of PSI. As such, very little steel goes a long way.

The soda can comparison is not valid because a soda can is not uniform in shape, materials, etc. and you don't apply pressure uniformely when you attempt to crush it. That's also the reason why the lid sections are not only thicker but made of a higher grade Al as well.
• Member Posts: 1,520
if the steel only move to it's inherant spring-e-ness...

thats enough to crack the glass - which is applied to the surface of the steel

in fact the anode's job is to protect/fill in the steel and the tiny cracks caused by uneven expansion/contraction
• Member Posts: 1,520
yup - tensile strength versus modulus firmness...

ie - if you take a ring of steel and pull on it in all directions it will hold orders of magnitude more pull than the push in all directions it takes to bend it in to center - thus: it's water pressure holding capacity on the inside is orders of magnitude greater than the holding strength against outside pressure

while it's true - that a tube of metal has an i-beam like effect - that is for longitudal bending, not outside radial pressure,  a ball is quite a bit better at this, since there is, perfect pressure distribution in its shape,  so, its kind of, longitudal in all directions

the three inch thick sub walls, may have enough firmness, between pressure bulkheads, which really hold it, on longer runs they put circular bulkheads, thats how a trident sub holds 3000ft, - a few more thousand and its squish, take the same sub, with open hatches, to ten times that depth, letting it flood, then close the hatches, and hoist it up, it would not explode  the hatch hinges would blow first,  because it has a lot more pressure holding capacity than crush holding!!

Ps: subs  also stage the pressure, with double walls, between which they hold the blow air, at thousands of psi, the staging works because the inner sub is much smaller, and thus holds much less total hydraulic pressure,

area is everything in pressure, eg a cylinder hold 30000psi with a 1 micro-meter tube coming out of it, you could put your pinky on that tube ad hold the pressure, but if you increase the tube size to 1milimeter you could not hold it with a hand, and that my friends is the secret of the O-RING, the less contact surface an o-ring has, the more hold back capacity, it has, IE a couple of atoms of O-ring have enough strength to hold back that same few atoms at 30000psi, since at 30000 per square inch  at a few atoms of that square, only has microscopic pressure, its 30000 per square inch, not per square atom!!!!, - people dont seem to get that!!!

the o-ring secret, is to take a small cross-section one, in a metal groove, that backs up most of its cross-section, ie most of its pressure area, and only leaving a small part of its thickness exposed to hold back a tiny chuck out of the square inch of the pressure applied,

an o-ring is not a gasket, if you wrench it down  you have increased by a square factor the square inches it has to hold, think of the 1.5million pounds of thrust the o-rings on the shuttles solid rocket booster segments, the pressure is equal along the whole length, (and yes, while 1.5meg thrust at the nozzle spread out over the whole interior of the rocket is much less, its still tens of thousands PSI, on each inch, nevertheless, the o-rings hold it, until the Challenger accident, because water got between the metal casings, and froze due to proximity of the cryogenic hydrogen/oxygen shuttle tank, and once the metal backing u[ the o-ring was deformed, the o-ring was effectively not there  so its case failed not the o-ring,  you would be very hard pressed to come up with an example of an o-ring failure,  not due to mechanical, or chemical causes

this is whats so nice about PRO-PRESS, the crimp keeps the pipes from sliding apart  so the o-ring in its preformed groove barely making contact has to do very very very little work  as long a the pipes were de-burred and clean going in, they should last forever  cant wait for it to be code legal in nyc
• Member Posts: 1,520
but each square inch has to hold on it's neighbor...

just the springiness inherent in steel would let it bend enough to do the damage - next people will be telling me that the foam insulation will cushion that effect

trust me, the "dead men" understood all this stuff, the did not have TV, and thinking was not as painful for them as it is for us, i just love the great nuclear physicist fineman - instead of doing a complicated physics math problem on paper  he would just clench his eyes shut and give you the answer in short order, there aint no one around today that can do that
• Member Posts: 2,162
Cylindrical objects stress in vacuum

Combine a steam cleaning with a lunch break and a closed hatch & what do you get?

• Member Posts: 1,520
one picture..why didnt i think of that...

and thats only a couple of psi differential, and much thicker steel than a water heater
• Member Posts: 6,928
Isn't That Why

You design piping systems to drain back to a single point? Most particularly if the system must be "winterized".

While others are busy having their irrigation systems blow out, I just open 4 valves and let it drain. Had to plan carefully and use quite a bit of extra pipe, but it sure makes things easy.
• Member Posts: 2,162

did the same thing with an empty can, a wee bit of water and a bunsen burner by boiling the water till steam began escaping from the can and then removing it ftom the heat and screwing on the cap. Can't say as I remember anything else he did that year - with the exception of excusing himself from class one day to go to the teacher's lounge and die from a heart attack - , but the crumpling can thing never left my brain's photo album.

I've seen water heaters suffer a similar fate too - both from thermal expansion (including a 30 gallon electric model that looked like one of those blow-up punching bags with its ends rounded off - if it hadn't been in a closet, it would've fallen over)and collapsed by vacuum. Same reason why we always install a check valve at the inlet to pump tanks.

It would be interesting to take a water heater tank, install a compound pressure/vacuum gage and repeat that 7th grade science experiment. I've toured three separate water heater manufacturing plants in past years and I'll never forget how surprised I was by how thin the steel is for manufacturing the tanks. They are constructed to withstand pressure, not vacuum.