Basic Parts – Pressure vessels are normally constructed out of two ends, the head and the cylindrical shell.
Blank – A flat diameter that is formed into a tank head.
Bump & Spun – Process of forming a head by bumping the dish radius in a pressing machine and forming the knuckle and straight flange in a flanging machine.
Center Hole – A hole that is drilled in the center of the head prior to flanging to ensure that the true circle dimension is held during forming.
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.
Coupon – A sample of material cut from either the blank or head for obtaining test results.
Cylindrical Vessel – The most common type of vessel configurations used, although other designs exist. The name refers to the shape of the vessel.
Dimple Jacket – A type of heat transfer surface utilizing sheet that has a dimpled pattern.
Dish Radius – Also know as crown radius. It is the major radius of the formed head, usually measured from the inside.
Dishing Press – A machine that forms the dish radius in the heads.
F & D Heads – There are two types of F&D heads:
1.) Standard – the inside dish radius o f 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.
2.) 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.
Flanging Machine – A machine that forms the knuckle radius and straight flange.
Half-Pipe – A type of heat transfer design.
Inside Depth of Dish – Measurement from the inside center of the head to the tangent line.
Jacket Heads – Heads that are on the outer shell of a double wall vessel.
Jacketed Head – Heads that are covered with some type of jackets material such as half-pipe or dimple jacket.
Knuckle Radius – Also known as corner radius. It is the formed radius of the head which transitions the dish portion to the straight flange. This process is done on a flanging machine.
Manways – Usually made up of a flanged nozzle that has an inside diameter of 16 to 20” and a cover plate, a manway allows a person access to the inside of a vessel.
Spherical Vessel – Due to the inherent strength of a sphere, these vessels are mainly used for high pressures.
Straight Flange – Is the straight portion of the head measured from the tangent line to the edge of the head. The industry standard is 1 ½”.
Tangent Line – The point on the head where the knuckle radius meets the straight flange.
Vessel Heads – Vessel heads can be constructed in one of three shapes:
1.) 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.
2.) 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.
3.) Hemispherical Head – Put simply, this type of head is ½ 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 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.
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.
Pressure Vessel Construction Materials
Alloy – A mixture or solution of metals. Low alloy steel usually has less than 10% alloying elements while high alloy steel contains more than 10% alloying elements.
Flat Bar – Material that is not wide enough to be called plate.
Forgings – Hammering a metal into a desired shape after heating it to an extremely high temperature. Nozzle flanges are normally forged.
Formed Plate – Plate that has been pressed into shape to form its shape. Most vessels use this type of plate.
Plate – material that is flat and 3/16” or thicker. Normally plate is more than 10” wide although some mills may classify anything more than 6” wide as plate.
Sheet – Materials that are flat and less than 3/16” thick.
Wrought – Plate that is made by hot rolling (a process that involves flattening the metal between rollers).