A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow - liquids and gases (fluids), slurries, powders, masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.
In common usage the words pipe and tube are usually interchangeable, but in industry and engineering, the terms are uniquely defined. Depending on the applicable standard to which it is manufactured, pipe is generally specified by a nominal diameter with a constant outside diameter (OD) and a schedule that defines the thickness. Tube is most often specified by the OD and wall thickness, but may be specified by any two of OD, inside diameter (ID), and wall thickness. Pipe is generally manufactured to one of several international and national industrial standards. While similar standards exist for specific industry application tubing, tube is often made to custom sizes and a broader range of diameters and tolerances. Many industrial and government standards exist for the production of pipe and tubing. The term "tube" is also commonly applied to non-cylindrical sections, i.e., square or rectangular tubing. In general, "pipe" is the more common term in most of the world, whereas "tube" is more widely used in the United States.
Pipe Size
Size choices include standard nominal English sizes, metric sizes, wall thickness, and pipe schedule if applicable. English sizes are approximations of the pipe size; smaller size industrial pipe carries an internal diameter (ID) roughly approximate to the nominal size, and larger pipe matched the outside diameter (OD) more closely. Metric sizes range from less than 10 millimeters up to greater than 1000 millimeters. Wall thickness may be specified numerically, or a schedule size may be chosen for pipe that conforms to the standard schedule dimensions. The American National Standards Institute (ANSI) assigns "schedule numbers" to classify wall thicknesses for different pressure uses. ANSI schedule numbers cover all pipe sizes from NPS 1/8 through NPS 36 identified as Standard (STD), Extra Strong (XS) and Double Extra Strong (XXS) and all wall thicknesses by ANSI schedule number. Choices for section shape include round, oval, square, and rectangular.
Nominal pipe size (NPS) is a dimensionless designator of pipe size. It indicates standard pipe size when followed by the specific size designation number without an inch symbol. For example, NPS 2 indicates a pipe whose outside diameter is 2.375 in. The NPS 12 and smaller pipe has outside diameter greater than the size designator (say, 2, 4, 6, ...). However, the outside diameter of NPS 14 and larger pipe is the same as the size designator in inches. For example, NPS 14 pipe has an outside diameter equal to 14 in. The inside diameter will depend upon the pipe wall thickness specified by the schedule number. Refer to ASME B36.10M or ASME B36.19M. Refer to App. E2 or E2M.
Diameter nominal (DN) is also a dimensionless designator of pipe size in the metric unit system, developed by the International Standards Organization (ISO). It indicates standard pipe size when followed by the specific size designation number without a millimeter symbol. For example, DN 50 is the equivalent designation of NPS 2. Refer to table below for NPS and DN pipe size equivalents.
Pipe Size Designators: NPS and DN
| NPS | DN | NPS | DN | NPS | DN | NPS | DN |
|---|---|---|---|---|---|---|---|
| 1/8 | 6 | 3-1/2 | 90 | 22 | 550 | 44 | 1100 |
| 1/4 | 8 | 4 | 100 | 24 | 600 | 48 | 1200 |
| 3/8 | 10 | 5 | 125 | 26 | 650 | 52 | 1300 |
| 1/2 | 15 | 6 | 150 | 28 | 700 | 56 | 1400 |
| 3/4 | 20 | 8 | 200 | 30 | 750 | 60 | 1500 |
| 1 | 25 | 10 | 250 | 32 | 800 | 64 | 1600 |
| 1-1/4 | 32 | 12 | 300 | 34 | 850 | 68 | 1700 |
| 1-1/2 | 40 | 14 | 350 | 36 | 900 | 72 | 1800 |
| 2 | 50 | 16 | 400 | 38 | 950 | 76 | 1900 |
| 2-1/2 | 65 | 18 | 450 | 40 | 1000 | 80 | 2000 |
| 3 | 80 | 20 | 500 | 42 | 1050 | - | - |
For sizes larger than NPS 80, determine the DN equivalent by multiplying NPS size designation number by 25.
Pipe Wall Thickness
Schedule is expressed in numbers (5, 5S, 10, 10S, 20, 20S, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160). A schedule number indicates the approximate value of the expression 1000 P/S, where P is the service pressure and S is the allowable stress, both expressed in pounds per square inch (psi). The higher the schedule number, the thicker the pipe is. The outside diameter of each pipe size is standardized. Therefore, a particular nominal pipe size will have a different inside diameter depending upon the schedule number specified.
Note that the original pipe wall thickness designations of STD, XS, and XXS have been retained; however, they correspond to a certain schedule number depending upon the nominal pipe size. The nominal wall thickness of NPS 10 and smaller schedule 40 pipe is same as that of STD.WT. pipe. Also, NPS 8 and smaller schedule 80 pipe has the same wall thickness as XS pipe.
The schedule numbers followed by the letter S are per ASME B36.19M, and they are primarily intended for use with stainless steel pipe. The pipe wall thickness specified by a schedule number followed by the letter S may or may not be the same as that specified by a schedule number without the letter S. Refer to ASME B36.19M and ASME B36.10M.
ASME B36.19M does not cover all pipe sizes. Therefore, the dimensional requirements of ASME B36.10M apply to stainless steel pipe of the sizes and schedules not covered by ASME B36.19M.
ANSI Pipe Schedule Chart (Click to enlarge)
How to read this chart?
- Blue figures indicate wall thickness in inches
- Black figures indicate weight per foot in pounds
Piping Classification
It is usual industry practice to classify the pipe in accordance with the pressure-temperature rating system used for classifying flanges. However, it is not essential that piping be classified as Class 150, 300, 400, 600, 900, 1500, and 2500. The piping rating must be governed by the pressure-temperature rating of the weakest pressure containing item in the piping. The weakest item in a piping system may be a fitting made of weaker material or rated lower due to design and other considerations. Table below lists the standard pipe class ratings based on ASME B16.5 along with corresponding pression nominal (PN) rating designators. Pression nominal is the French equivalent of pressure nominal.
Piping Class Ratings Based on ASME B16.5 and Corresponding PN Designators
| Class | 150 | 300 | 400 | 600 | 900 | 1500 | 2500 |
|---|---|---|---|---|---|---|---|
| PN | 20 | 50 | 68 | 110 | 150 | 260 | 420 |
- Pressure-temperature ratings of different classes vary with the temperature and the material of construction.
- For pressure-temperature ratings, refer to tables in ASME B16.5 or ASME B16.34.
In addition, the piping may be classified by class ratings covered by other ASME standards, such as ASME B16.1, B16.3, B16.24, and B16.42. A piping system may be rated for a unique set of pressures and temperatures not covered by any standard.
Pression nominal (PN) is the rating designator followed by a designation number, which indicates the approximate pressure rating in bars. The bar is the unit of pressure, and 1 bar is equal to 14.5 psi or 100 kilopascals (kPa). Table above provides a cross-reference of the ASME class ratings to PN rating designators. It is evident that the PN ratings do not provide a proportional relationship between different PN numbers, whereas the class numbers do. Therefore, it is recommended that class numbers be used to designate the ratings.
Pipe Forming Processes
There are basically two types of pipe and tube forming processes, namely, seamless and welded. Each process imparts unique properties to the pipe or tube. Seamless pipe or tube does not have the presence of a welded seam along the length of the pipe. This seam has traditionally been believed to be a potential weakness. The development of automated welding processes and quality control, however, has made this a virtually nonexistent concern. The control of thickness uniformity and concentricity is relatively easy with welded pipe and tube. In general, the seamless pipe is more expensive to produce. The classification of cylindrical tubular products in terms of either pipe or tube is a function of end use.
- Seamless Pipe
Seamless pipe are manufactured by first producing a hollow tube which is larger in diameter and thickness than the final tube or pipe. The billet is first pierced by either a rotary (Mannesmann) piercer or by a press piercing method. For tubes of small diameter, the mandrel mill process is used. For medium outside diameter tubes of carbon or low-alloy steel, the Mannesmann plug mill process is used. Large-diameter, heavy-wall carbon steel, alloy, and stainless pipe is manufactured by the Erhardt push bench process and vertical extrusion similar to the Ugine Sejournet type extrusion process. High-alloy and specially shaped pipe are manufactured by the Ugine Sejournet extrusion-type process. These processes are performed with the material at hot metal forming temperatures. Further cold processing may or may not be performed to obtain further dimensional accuracy, surface finish, and surface metallurgical structure. - Welded Pipe
Welded pipe is produced by forming a cylinder from flat steel sheets coming from a hot strip mill. The strip mill takes the square bloom from the blooming mill and reduces it into plates, skelp, or coils of strip steel to be fed into the particular welding process equipment. Butt-weld pipe is made by furnace heating and forge welding or by fusion welding using electric resistance, flash, sub-merged arc-welding, inert-gas tungsten-arc welding, or gas-shielded consumable metal-arc welding. The welded seam is either parallel to the tube axis or in a spiral direction about the tube center line.
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