Piping and tubing are essential to the operation of nearly all industrial equipment and can be found extensively throughout the home. From the pipes transporting 2,000 psi of steam in a power plant to the tubing carrying hot water from a hot water heater to the bathtub, all piping and tubing is standardized and regulated for safety.
Pipe and tube products are classified according to their application, not their manufacturing method. Although they are all similar in appearance, pipe and tube products are used in very different applications that should be understood thoroughly before use.
A pipe is defined as a tubular product sized by a nominal inner diameter, a constant outer diameter, and a schedule number. Pipes are mainly used to convey a flowing substance, but can also be used as structural support. Pipes can be manufactured in a variety of methods; however, the sizing criteria will always remain the same. Tube differs from pipe in that it typically has more stringent engineering requirements than piping. Pipe sizes up to 12 inches are designated by nominal sizes, whereas, in most cases, tube sizes are identical to the actual outside diameter of the tube.
Piping Schedule Numbers
In addition to standard, extra strong, and double extra strong pipe weights, steel pipe is also manufactured in assorted schedule numbers. These schedule numbers range from 10 through 160 and are commercially available in schedules of 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. The thickness of the pipe wall increases with the schedule number; for example, a 2” NPS schedule 160 pipe has much thicker walls than a 2” NPS schedule 40 pipe. Schedules followed by the letter “S” designate stainless steel pipe sizes.
Classification of Tubing
As mentioned previously, tube differs from pipe in that it typically has more stringent engineering requirements than piping. In most cases, tube sizes are identical to the actual outside diameter, but the inside diameter, or ID, or wall thickness of the tube can be used as well. Tubing is classified into three major types: structural tube, mechanical tube, and pressure tube. Structural tube is used in the construction of such items as building frameworks, roadway median barriers, bridge structures, and other general structural applications. Mechanical tube is used in a variety of mechanical and structural applications and, like structural tube, is not intended to carry fluids or gases under pressure. Pressure tube is the type of tubing used most often in the piping industry. It is designed to carry fluids under pressure.