What is “Static Head” in ASME Section VIII Vessels?
In designing the Pressure Vessels according to the ASME Section VIII Division 1, aside from not only considering the design pressure that will occur within the Pressure Vessel, but we also need to consider the weight of the Liquid within the Vessel acting on it.
The pressure caused by the weight of Liquid in Vessel is the “Static Head” which is based on the density of the liquid (Liquid Density called the p) and will have greater value as the height of the liquid from the bottom of the tank or Bottom of a Vessel, so the Pressure used for calculating the Thickness according to ASME VIII will have to use pressure as below:
Internal Pressure (at bottom area) = Design Pressure + Static Head
Example of “Static Head” in ASME Section VIII Vessel Calculation;
The pressure used to calculate the thickness of the Pressure Vessels according to the ASME Section VIII Division 1 must be equal to the pressure that occurs within the Vessel (Internal Design Pressure) plus the “Static Head” or pressure caused by the weight of Liquid in Vessel at the lowest points of the Vessel Part such example is the pressure the most. Do with the part of the Left Ellipsoidal Head, which will be used for calculations (SG = Specific Gravity, HS = Liquid Height).
From the calculated Thickness, the Designer can Choose wisely the actual purchasing/ constructing thickness (Ex: T-calculated = 4mm, using ex-stock plate T = 8mm).
Following it, the designer can calculate again the actual pressure that the vessel can withstand and minus the Static Head, then we get MAWP.
That is the reason why MAWP is often bigger than Design Pressure.
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