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Why is a man hole door elliptical in shape?

Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole 
is lesser in size than the corresponding circular hole. More over it is prime concern to have a  smoothed generous radius at the corners to eliminate stress concentration. Hence other 
geometrical shapes like rectangle and square are ruled out. 
To compensate for the loss of material in the shell due to opening, a doubler ring has to be 
provided around the opening. The thickness of the ring depends on the axis length along the  dirrection in which the stresses are maximum and the thickness of the shell. It is important to  align the minor axis along the length of the vessel, as the stress in this direction is  maximum.

Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= 
thickness of the shell plating 

Circumferential stress: Pd/4t 
More over a considerable material and weight saving is achieved as minor is along the 
direction of maximum stress.

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