Pressure from liquid in cylindrical tank

[ziptron]

Hi All,

So I have a cylindrical liquid storage tank that sits horizontally. Due to pitting corrosion, a small section of the tank (the pit about 1/8 inch in size) reduced the wall thickness of that tank locally. I essentially have now a tank that is of uniform thickness except the area where I had a pit form.

I started thinking about the hydrostatic pressure that the water within the tank is creating, and if its possible that the pressure of the water could eventually push out the pit and cause a leak. However, I can't wrap my mind around how to calculate the force that the hydrostatic pressure would be applying to the cylindrical wall.

I feel that if it was a square tank, I would do:

Pressure = rho*g*h ... and then it would be simple to know the pressure acting on that pit, and thus the probability that it will be pushed out.

However, in a cylinder, how do I calculate the pressure on the wall at a certain height, and THEN, how do I accommodate for any additional hoop stresses that may be created by the hydrostatic pressure within the cylinder.

Can anyone point me to a good resource for this?

 

[zdas04]

Assuming the tank is at atmospheric pressure, the hydrostatic head is [ρ]gh. The "h" is the distance from the pit to the surface of the water. Hoop stresses with just a hydrostatic gradient are negligible (do a Hook's Law calc with atmospheric pressure at the top and max hydrostatic pressure at 6:00 and average the two, you'll find it is a really small number)

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[hydtools]

The hydrostatic pressure does not depend on the shape of the container.

http://hyperphysics.phy-astr.gsu.edu/hbase/pflu.html

Approximately 1 psi per 2.3 feet of depth.

Ted

 

[ziptron]

Thanks, that definitely makes sense for hydrostatic pressure!

Do things change if I use air or vaccuum? What if I filled the tank with air pressure, and or kept it under vaccuum? What would be the best way to calculate the pressure on my little pit area? Are hoop stresses the only thing occuring there? Or, should I be treating the pit as a flat plate (as its so tiny in comparison to the radius of the cylinder) and see when the flat plat will buckle and fail under an applied pressure?

Or is there something better?

 

[bimr]

Don't understand what is your issue.

It doesn't really matter what the pressure is. With the metal thickness being unkown, there is no way to calculate the pressure that the tank will sustain.

Why don't you just install a boiler repair plug into the pit and be done with it.

http://www.ehow.com/how_6837329_repair-galvanized-water-storage-tank.html