Pressure Vessel Design Terms
The following terms are defined as used in the pressure vessel industry. Some may have other meanings in other industries.
Basic Parts: Pressure vessels are normally constructed out of two ends, the head
and the cylindrical shell.
Connections: Vessels have two common connections: couplings and nozzles. Couplings
project through the shell or head and are welded into place. They consist of threaded female pipe.
Nozzles are made up of a pipe stub that has been welded onto the vessel and end in a bolting flange.
Cylindrical Vessel: The most common type of vessel configuration used, although other
designs exist, the name refers to the shape of the vessel.
F&D Heads: There are two types of F&D heads:
- Standard F&D Head: The inside dish radius of this type of head is equal to its diameter.
The inside knuckle radius is three times the thickness of 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.
Manways: Usually made up of a flanged nozzle that has an inside diameter of 16 to 20 inches
and a cover plate, a manway allows a person access to the inside of a vessel.
National Board: Primarily responsible for the training and certification of ASME Code pressure
vessel inspectors, the National Board of Boiler and Pressure Vessel Inspectors is an organization that
also maintains a database of all pressure vessels registered with the Manufacturers Data Report Forms.
However, not all Code vessels are required to register with the National Board. Some states may have
mandatory registration, though.
Spherical Vessel: Due to the inherent strength of a sphere, these vessels are mainly used
for high pressures.
Straight Flange: The short, cylindrical part of the head which attaches to the shell.
The majority of vessel heads have a straight flange.
The Code: Unless otherwise specified, the Code refers to the ASME Boiler and Pressure Vessel
Code, Section VIII, Division 1 or 2. The Code is an internationally recognized standard in pressure vessel
Vessel Heads: 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 one-half of the short axis.
Pressure vessels using an elliptical head will have an axis ratio of 2:1. This gives an inside head
depth of one-fourth 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 and dished) heads. While F&D does technically include
elliptical heads, elliptical heads are generally referred to separately.
- Hemispherical Head: Put simply, this type of head is one-half of a sphere.
Tangent Line: 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 Measurement: 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 units of
measure (i.e. meters for dimensions, kilograms per centimeter, bar or pascals for pressure, and degrees
Celsius for temperature). While Prentex is happy to perform the necessary conversions to work with their
customers� needs, 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 confusion, use of
Torr (1 Torr = 1mm Hg absolute) is recommended.
Vessel or Tank?: Often used interchangeably, a vessel generally implies a more sophisticated
container. While it is not incorrect to refer to a vessel as a tank, few tanks are referred to as vessels.
Vessels can be designed to be operated in a vertical or horizontal position.