Air pipe head for tank design

[Adarsh Edakkote]

Hi,
Does anyone know why we are taking the pressure head up to the top of air pipe for the scantling calculation of tank?
For sewage tanks, air pipe height is normally more compared to other tanks.So the pressure head will be more and need to design the tanks heavily.
Please advise

 

[gonzo]

That is the maximum pressure the tank will have before the fluid overflows.