How Does Atmospheric Pressure Affect Hydrostatic Calculations in Tank Design?

[parch]

Dear guys,

I got a common doubt regarding the pressure at the bottom of a open tank filled with water.

1) Assume a tank of 2m height, filled with water. So the hydrostatic pressure at the bottom of the tank will be 0.2 bar(rho*g*h). Wondering! it is less then atmospheric pressure(1 bar).

2) So do i need to include the atmospheric pressure for my stress calculation, which is 4 times greater than the hydrostatic pressure.

 

[Cyrus]

The pressure cannot be less than atmosphere, because this physically does not make sense. There is pressure due to atmosphere above the tank, and there is pressure due to the water in the tank. The only result is more pressure than atmosphere alone.

 

[parch]

Hi Cyrus,

Thanks for the reply.

1) So you mean that the 0.2 bar is wrong answer??

2) then we need to take atmospheric + hydrostatic pressure for stress calculations.

 

[tyroman]

Consider the case where the tank is empty. The load for stress calculations is the self weight of the tank bottom... plus atmospheric pressure ONLY IF the tank rests on an evacuated space. An empty tank which experiences atmospheric pressure on both the upper and lower surfaces of its bottom sees no net load from atmospheric pressure.

.

 

[parch]

Hi Tyroman,

Thanks for the reply.

1 )I am planned to place my tank on a flat surface where the atmosphere pressure won't comes into play on the bottom plate. So i need apply the atm pressure+hydrostatic pressure inside the tank for the stress calculations.

2) And please let me know where I can get materials for the rectangular tank design.

Thanks in advance.

 

[Su Solberg]

Hi Tyroman,

Thanks for the reply.

1 )I am planned to place my tank on a flat surface where the atmosphere pressure won't comes into play on the bottom plate. So i need apply the atm pressure+hydrostatic pressure inside the tank for the stress calculations.

2) And please let me know where I can get materials for the rectangular tank design.

Thanks in advance.

For Q1.
It's easy.
A. pressure from the water due to gravity = maximum 0.2bar.
B. pressure of atmosphere at the tank out side. = 1 bar (anywhere of the tank surface; simplified as 1 bar as 1 ATM = 1.01XXX and varies with temp + altitude).

Thinking a vacuum hot bottle, using vacuum to insulate the heat transfer. It is exactly the stuff you are talking about.

Thus the maximum pressure suffered by the tanks is the so called wetted bottom plate of the tank. 0.2+1=1.2bar

For Q2.
For Tank design, a cylinder/ sphere is better, the book "Fundamental of Hydraulics System engineering" will help you a bit. It involve stress distribution on plate and depends on your tank support.

 

[parch]

Please put some more materials for standard rectangular tank design ,,

Thanks in advance.