- Low expansion foam is having a more weight to volume ratio as compared to high expansion foam.
- If we use high expansion on deck, then wind effect will blow away the foam.
- The low expansion foam having sufficient weight to counter the wind force effect. So, it is easier for low expansion foam to form a blanket over fire properly.
- In the engine room, wind effect is not present only we need to suppress the flammable gases and to give a cooling effect in the large volume of the engine room. So high expansion foam is good to cover more space in a short time
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.