Wastewater Conveyance - Wastewater Operators - page 159

Note: Another important pipe consideration not listed above or shown
in Figure 4.3 is weight per foot , which varies according to the pipe mate-
rial and wall thickness.

In the continuing effort to standardize pipe size and wall thick-
ness of pipe, the designation nominal pipe size (NPS) replaced the iron
pipe size designation, and the term schedule (SCH) is used to specify
the nominal wall thickness of pipe. The NPS (approxi-
mate dimensionless designator of pipe size) is gener-
ally somewhat different from its actual diameter; for
example, the pipe we refer to as a “3-in.-diameter pipe”
has an actual O.D. of 3.5 in., while the actual O.D. of a
“12-in. pipe” may be .075 in. greater (i.e., 12.750 in.)
than the nominal diameter. On the other hand, a pipe
14 in. or greater in diameter has an actual O.D. equal to the nominal
size. The inside diameter will depend upon the pipe wall thickness
specified by the schedule number.

Key Point: Keep in mind that
whether the O.D. is small or
large, the dimensions must
be within certain tolerances
in order to accommodate
various fittings.

4.8.3.1 Pipe Wall Thickness

Original pipe wall thickness
designations of STD (standard),
XS (extra-strong), and XXS (double
extra-strong) are still in use today;
however, because this system allows
no variation in wall thickness and
because pipe requirements became
more numerous, greater variation was needed. As a result, today the pipe
wall thickness, or schedule , is expressed in numbers (e.g., 5, 5S, 10, 10S,
20, 20S, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160). (Note that you will
often hear piping referred to either in terms of its diameter or schedule
number.) The most common schedule numbers are 40, 80, 120, and 160.
The outside diameter of each pipe size is standardized; therefore, a par-
ticular nominal pipe size will have a different inside diameter depending
on the schedule number specified. For example, a schedule 40 pipe with
a 3-in. nominal diameter (actual O.D. of 3.500 in.) has a wall thickness of
0.216 in. The same pipe in a schedule 80 (XS) would have a wall thickness
of 0.300 in.

Key Point: 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 schedule numbers
followed by the letter S are per ASME B36.19M, and they
are primarily intended for use with stainless steel pipe.

4.8.4 Piping Classification

The usual practice is to classify pipe in
accordance with the pressure-temperature rat-
ing system used for classifying flanges; how-
ever, because of the increasing variety and
complexity of requirements for piping, a number
of engineer ing societ ies a nd st a nda rds groups have dev ised codes, st a n-
dards, and specifications that meet most applications. By consulting
such codes, (e.g., ASTM, manufacturer's specifications, NFPA, AWWA)
a designer can determine exactly what piping specification should be
used for any application.

Key Point: Because pipelines often carry
hazardous materials and fluids under
high pressures, following a code helps
ensure the safety of personnel, equip-
ment, and the piping system itself.