The Semi-Elliptical (SE) Heads are designed in a 2:1 ratio, or also known as 2:1 head. This means that in this type, the diameter to the depth ratio is 2:1. Meaning that the head depth is four times the head width. This SE head appears like a half ellipse, so its head depth is usually a quarter of its diameter. This ratio is a perfect setting to hold fluids or gases at high pressure. Another fact that makes them desirable amongst buyers is that they are known to be made out of thinner materials, which essentially helps reduce manufacturing costs.
Some vessel head producers construct the semi-elliptical heads with three radii, which is close to an ellipse. The code recommends the SE head’s sphere of the radius 0.90D and other with a knuckle radius 0.17D.
The ASME code (American Society of Mechanical Engineers) design formula for 2:1 SE is similar to that of a cylinder. There the composition thickness for a cylinder is defined as 0.500″, and that for the semi-elliptical head is 0.4947″.