Hemispherical head is made in form of half-sphere and technically is the ideal shape for a vessel head. This is because forces are spread out equally across the head. The radius of the hemisphere is constant as opposed to an elliptical head. The radius is equal the the radius of the shell that it attached to.
2. Elliptical (2:1)
Also known as an ellipsoidal head and is the most common type of head. This is also called a 2:1 Elliptical head. The shape of this head is more economical, because the height of the head is just a quarter of the diameter. Its radius varies between the major and minor axis.
3. Torispherical
These heads are sometimes referred to as flanged and dished heads after the manufacturing process. The geometry of the head is composed of the crown or dish radius, the major portion of the head. The dish radius is a fixed radius whose size id dependant on the type of head. The knuckle radius is the radius to the outer edge. This type of head has good drainage when used in the vertical position.
4. Conical
Conical heads possess more inherent strength than flat heads. Conical bottoms, depending on the angle of the cone, provide excellent bottom drainage. Placing a nozzle at the bottom of a conical head will allow for solids and precipitate to immediately get flushed out. The ASME code limits conical heads without a knuckle radius to a maximum included half angle of 30 degrees. The conical angle is specified differently depending on the manufacturer, some specify the included angle while others the angle of the cone in reference to the tangent line of the head.
5. Flat
This is a head consisting of a toroidal knuckle connecting to a flat plate. This type of head typically is not used on pressure vessels since it is inherently very weak. But sometimes they are used in form of welded cap to cap configuration for making external thick wall expansion joints.
1. Hot or cold forming with moulds
With this method, discs or segments are pressed into their definitive size and shape using one or more press runs. Selecting cold pressing or hot pressing is depended to power of forming facilities. But a simple rule states that cold forming is mainly used when head diameter is small and thickness is limited and hot pressing is used to manufacture heads made by very thick plates.
This method is used for heads of large diameter with a moderate thickness. The disc is first pressed into a spherical cap and the spherical cap is then flanged by spinning machinery to obtain a defined knuckle radius with a straight flange. Naturally, these products can undergo heat treatment if required in order to meet certain specifications.
3. Fabricated from spherical cap and segments
This method is used for larger sizes and/or thicknesses and with special materials and/or material combinations. The product is fabricated from hot or cold-formed segments. After pressing, all segments are calibrated and cut to size in accordance with the required form and dimension. The segments are then set up, test assembled and marked. The fabricated products can then be fully welded as required
