There are two types of F&D heads:

• Standard – the inside dish radius of this type of head is equal to its diameter. The inside knuckle radius is three times the thickness o the heads metal. Standard D&D heads are rarely used on pressure vessels.
• ASME F&D Head – Also known as the Code F&D head, this head should have a dish radius no greater than its diameter. The knuckle radius should be no less than 6% of the diameter or three times the metal thickness, whichever is greater.

The point on the head where the knuckle radius meets the straight flange.

Refers to the point of contact (tangency) between the cylinder and the knuckle portion of the vessel head. The distance from the tangent line on one head to the tangent line on the opposite head is known as the straight side or tangent-to-tangent (T/T).

Vessel heads can be constructed in one of three shapes:

• Elliptical Head – an oblate semi-ellipsoidal surface with the inside diameter of the head equal to the long axis of the ellipse while the depth of the head is ½ of the short axis. Pressure vessels using an elliptical head will have an axis ration of 2:1. This gives an inside head depth of ¼ the inside diameter.
• Torispherical Head – this head is made up of two surface parts. The center of the head is known as the crown or dish and is a spherical segment. The portion between the dish and the cylinder of the vessel is called the knuckle; it is a part of the torus or doughnut. Torispherical heads are typically referred to as F&D (flanged & dished) heads.
• Hemispherical Head – Put simply, this type of head is ½ of a sphere.

The size of a vessel is normally calculated by its diameter and length. The diameter is expressed in inches and is given as the inside diameter (ID) or outside diameter (OD). The length of the cylinder is measured from tangent-to-tangent or seam-to-seam, in inches or feet and inches. Increasingly, pressure vessel design specifications from customers use metric or SI unites of measure (i.e. meters for dimensions, kilograms per centimeter, bar or pascals for pressure and degrees Celsius for temperature). The National Board requires that the Manufacturers Data Report show measurements in traditional “English” measurements of feet, inches and degrees Fahrenheit.

Pressure is normally calculated according to pounds per square inch (psi). Pressure as read on a gauge with zero at atmospheric pressure is psig. Vacuum refers to external atmospheric pressure and can be measured by inches of mercury, inches of water or pounds per square inch below atmospheric pressure. It can also be measured in the same units as absolute pressure. However, this is noted as absolute pressure while vacuum implies a measurement below atmospheric pressure. To eliminate confuse, use of Torr (1 Torr = 1mm HG absolute) is recommended.