Selecting the most Economical Vessel Head
Selecting the most Economical Vessel Head:- The cost of any pressure vessel head is based on a combination of metal costs and manufacturing costs. Manufacturing cost is based on combination of shape, thickness and method of forming. As the metal becomes more expensive, it becomes necessary to use thinner and small blank diameter for forming the head. Thinner materials require more severe configurations that increase the cost of fabrication. The designer should select a head thickness that can be formed as economically as possible.
- Hemispherical, torispherical and elliptical heads are commonly used for process equipment.
- Hemispherical head is used only for special purposes because of its very high fabrication cost.
- Elliptical head is used for pressures above 15 kg/cm2 and requires higher wall thickness and is very difficult and costly to fabricate.
- Torispherical and shallow flanged dished head has been very common for low pressure operations. Heads can be formed by hot forming or cold forming. Cold forming technique is preferred.
- Torispherical head with 100-6 design is common. It means crown radius equal to diameter of vessel and knuckle radius 6% of diameter. For economic reasons 80-10 head i.e. a dish radius equal to 80% of the head diameter and a knuckle radius equal to 10% of the head diameter can be used. It requires less thickness as compared to standard torispherical head.
- Thickness of torispherical head depends upon the ratio of crown radius to knuckle radius also. Torispherical heads are classified as 100 -10; 100 6; 80-10 and so on.
- 100-10 torispherical head means crown radius of the head is equal to the diameter of the vessel and knuckle radius is 10% of the diameter of the vessel. Similarly other types are designated.
- Thickness required for 80 10 torispherical head is less than of 100-6 torispherical head used for the same diameter vessel and the same operating conditions.
- Thickness required for the elliptical head for the same diameter vessel and the same operating conditions is still less than that of any type of torispherical head, but the cost of fabrication of elliptical head is more than that of torispherical head.