Initially a system known as
iron pipe size (IPS) was established to designate the
pipe size. The size represented the approximate inside diameter of the pipe in
inches. An IPS 6 pipe is one whose inside diameter is approximately 6 inches (in).
Users started to call the pipe as 2-in, 4-in, 6-in pipe and so on. To begin, each pipe
size was produced to have one thickness, which later was termed as
standard weight (STD.WT.). The outside diameter of the pipe was standardized.
As the industrial requirements demanded the handling of higher-pressure fluids,
pipes were produced having thicker walls, which came to be known as
extra heavy (XH). The higher pressure requirements increased further,
requiring thicker wall pipes. Accordingly, pipes were manufactured with
(XXS) or double extra heavy (XXH) walls while the standardized
outside diameters are unchanged.
With the development of stronger and corrosion-resistant piping materials, the
need for thinner wall pipe resulted in a new method of specifying pipe size and
wall thickness. The designation known as
nominal pipe size (NPS) replaced IPS,
and the term
schedule (SCH) was invented to specify the nominal wall thickness
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.
(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
INTRODUCTION TO PIPING
Pipe Size Designators: NPS and DNNotes:
1. For sizes larger than NPS 80, determine the DN equivalent by multiplying NPS size designation number
without a millimeter symbol. For example, DN 50 is the equivalent designation of
NPS 2. Refer to Table A1.1 for NPS and DN pipe size equivalents.
Pipe Wall Thickness
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
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.
ASMEB36.19M 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.
It is usual industry practice to classify the pipe in accordance with the pressuretemperature
rating system used for classifying flanges. However, it is not essential
Piping Class Ratings Based on ASME B16.5 and Corresponding PN
Class 150 300 400 600 900 1500 2500
PN 20 50 68 110 150 260 420
1. Pressure-temperature ratings of different classes vary with the temperature and the material of construction.
2 For pressure-temperature ratings, refer to tables in ASME B16.5, or ASME B16.34.
that piping be classified as Class 150, 300, 400, 600, 900, 1500, and 2500.
rating must be go
verned by the pressure-temperature rating of the weakest pressurecontaining
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 A1.2 lists the standard pipe class ratings based on ASME B16.5 along with
pression nominal (PN) rating designators. Pression nominal is the
French equivalent of pressure nominal.
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.
(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 A1.2 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. Refer to Chap. B2 for a more
detailed discussion of class rating of piping systems.
OTHER PIPE RATINGS
Based upon a unique or proprietary design of a pipe, fitting, or joint, the manufacturer
may assign a pressure-temperature rating that may form the design basis for
the piping system. Examples include Victaulic couplings and the Pressfit system
discussed in Chap. A9.
In no case shall the manufacturer’s rating be exceeded. In addition, the manufacturer
may impose limitations which must be adhered to.
The piping systems within the jurisdiction of the National Fire Protection Association
(NFPA) requirements are required to be designed and tested to certain required
pressures. These systems are usually rated for 175 psi (1207.5 kPa), 200 psi (
kPa), or as specified.