In the design and construction of pressure vessels, various types of heads are used to ensure the structural integrity and safety of the vessel. One commonly used type of head is the torispherical head, which includes the ASME flanged and dished head and the 10% dished head. These heads have specific geometric dimensions that need to be considered during the design and fabrication process.
ASME Flanged and Dished Head
The ASME flanged and dished head is a type of torispherical head where the dish radius is 100% of the inside diameter of the head, and the knuckle radius is 6% of the inside crown radius. Here is a step-by-step guide on how to draw the ASME flanged and dished head:
- Start by drawing a vertical line that corresponds to the inner diameter of the pressure vessel shell. This line should be drawn horizontally from the vertical centerline.
- Next, draw a horizontal line from the horizontal centerline vertically, about 0.8 times the diameter downward. This line represents the bottom of the dish.
- From the point of intersection above, draw a boundary line with a vertical centerline and a slope of 27.9 degrees in both directions. This line defines the shape of the head.
- Draw a circular arc with a radius of 1.0 D. This arc represents the crown part of the head.
- Finally, draw an arc with a knuckle radius of 6% of the diameter at the point where the horizontal centerline and the boundary line meet. This arc represents the knuckle part of the head.
The detailed dimensions of the ASME flanged and dished head are provided in the design code, and they include the surface area of the head, the blank diameter for the plate, and the diameter of the blank considering the straight flange. It is important to consider a minimum straight flange of 1.5 inches (38 mm) for welding the head to the shell.
10% Dished Head
The 10% dished head is another type of torispherical head that has a dish radius of 100% of the inside diameter and a knuckle radius of 10% of the inside crown radius. The drawing method for the 10% dished head is similar to that of the ASME flanged and dished head.
When comparing the 2:1 ellipsoidal head, ASME flanged and dished head, and 10% dished head, it is important to consider the detailed dimensions and calculated thickness. The ASME flanged and dished head is approximately 77% thicker than the ellipsoidal head, while the 10% dished head is about 13% thinner than the ASME flanged and dished head and 54% thicker than the ellipsoidal head.
Thickness Calculation and Considerations
The ASME Section VIII Division 1 provides guidelines on how to calculate the thickness of the torispherical head under internal pressure, taking into account the ratio of the crown radius to the knuckle radius. It is important to note that the plate thickness may be reduced during the forming or spinning process of the head. Therefore, when ordering the steel plate for manufacturing the head, it is recommended to consider a thickness with a margin of about 15% more than the calculated thickness to account for this reduction.
In conclusion, the ASME torispherical head, including the ASME flanged and dished head and the 10% dished head, is widely used in pressure vessel construction. These heads have specific geometric dimensions that need to be carefully considered during the design and fabrication process. By following the guidelines provided in the ASME code, engineers can ensure the structural integrity and safety of pressure vessels.
