Quote:
Originally Posted by Montana Man
OK guys think about it. Do you really believe that a tank will burst when it fills then pushes water up 10 feet a 1 1/2" pipe? That's an extra 7.7 lbs. of water or about one gallon, on top of approximately 350 lbs. of water already in the tank. The previous post convincingly detailed a scenario in which the tank, within seconds, reached a pressure of 5000 lbs. That scenario may be applicable in a closed pressurized hydraulic system. However, in a vented system as is our tanks, it is static pressure.
When rinsing a black tank with the factory installed flusher, there is not enough volume of water entering the tank to overcome the volumetric air flow of a 1 1/2" vent pipe. All the tanks I've seen rupture did so previous to reaching full capacity.
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Quote:
Originally Posted by Montana Man
OK guys think about it. Do you really believe that a tank will burst when it fills then pushes water up 10 feet a 1 1/2" pipe? That's an extra 7.7 lbs. of water or about one gallon, on top of approximately 350 lbs. of water already in the tank. The previous post convincingly detailed a scenario in which the tank, within seconds, reached a pressure of 5000 lbs. That scenario may be applicable in a closed pressurized hydraulic system. However, in a vented system as is our tanks, it is static pressure.
When rinsing a black tank with the factory installed flusher, there is not enough volume of water entering the tank to overcome the volumetric air flow of a 1 1/2" vent pipe. All the tanks I've seen rupture did so previous to reaching full capacity.
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I see one of my previous posts was quoted, to which Montana man questioned it’s validity. The science of hydraulics is not disproved by “let’s think about it guys”....
The proof of that is Montana man’s thinking about it came to the wrong conclusion. The OP’s tank did fail while flushed with the drain valve closed. Not one drop of sewage exited the vent on his roof. The tank failed BEFORE the liquid got to the top! It didn’t fail while traveling, it didn’t fail because the vent pipe was blocked, it failed because the OP didn’t follow instructions to NOT flush the tank with the drain valve closed. Personally it think the warning label should offer the additional verbiage “ to do so you so, you assume all responsibility should the tank fail”. To return to my explanation, the black tank IS effectively a closed system up until the point the liquid has risen to the top of the vent pipe. A column of water exerts a back pressure equal to .434 psi for each foot of height. I used the example of a 10’ vent to make the math simple and easy to understand. The disconnect of understanding here is thinking a gallon on top of 350 lbs creates 5,000 psi. It doesn’t. What I am saying is the 5 psi of back pressure is multiplied times the total square inches of each of the tank’s 6 sides. The psi in the tank is not 5,000, the lifting force of 5 psi acting upon the total square inches of the top is pushing against the top and the bottom and is creating 5,000 lbs of force perpendicular to their plane. The formula is LxWxpsi= total force acting against a specific plane. Technically the semi supported tank bottom has the greatest force against it because it typically 6-8 inches lower than the top, Montana Is correct that the tank bottom is bearing the weight of water 8.33 X total gallons) but if the tank is filled to the point of water beginning to rise up inside the vent, every thing changes, and it does it in a matter of seconds! The 2 long sides are subjected the same force, but much less than the T & B. The two ends of the tank have the least perpendicular force. Hence the sides and ends of the tank would have a fraction of force acting against their respective planes compared to the top and bottom of the tank. Do tanks also fail due to not being installed improperly??? Yes they do! But not in the way the OP’s pictures show. IMHO flushing for more than 2 minutes either the dump valve closed is not only risky, but when the water covers the spray nozzle inside the tank, rinsing action ceases. Set an alarm on your smart phone for 2 minutes. Need more rinsing? Dump and start again.