Page 235 - Buried Pipe Design

Design of Pressure Pipes 209

Diameters ID base, OD base, tubing
Pressure classes 125, 160, and 200 lb/in 2
DR 17.0, 15.0, 13.5, 11.5, 11.0, and 9.0
Again, a wide selection is available. The safety factor is also 2, and no
routine test is required for each piece of pipe.

Cyclic life of plastics
Cyclic fatigue of plastic pipe. Cyclic surging is a regularly occurring
pressure fluctuation produced by action of such equipment as recipro-
cating pumps, undamped pressure control valves or interacting pres-
sure regulating valves, oscillating demand, or other cyclic effects.
Cyclic surges may cause fatigue damage and should be designed out of
the system.
A transient surge is often referred to as water hammer. Any action
in a piping system that results in a change in velocity of the system is
a potential cause of a water hammer surge.

Cyclical pressures. Water hammer surges in a water system normally
occur on a rather infrequent basis, are transient in nature, and are not
cyclic in character although they may be repetitive. Transient surges
are discussed in a previous section (“Surge Pressure”). However, if a
system is operating out of control, cyclical pressures can occur, and
may be somewhat continuous. It is this type of condition that may
require additional design considerations for plastic pipe. Research
work has shown the following for plastic pipe:

1. Plastic pressure pipe has three independent modes of failure or
three independent strengths (life funds).
a. Failure occurs because the internal pressure has exceeded the
pipe short-time strength. Strength due to this mode of failure is
called the quick-burst strength. A failure due to a water hammer
pressure wave is this type of failure.
b. Failure due to a long-term sustained high internal pressure can
only take place if the internal operating pressure is much higher
than the design pressure. This type of failure is time-based. The
strength (stress/life) for this mode of failure is also life (time)
based and is determined from the pipe’s stress regression curve.
c. Fatigue failure can take place if the water system experiences
continuous cyclic pressures. The strength of plastics is indepen-
dent of the number of cycles experienced by the plastic. This is
no different from fatigue in metals. That is, if a specimen is
cycled to, say, 80 percent of it fatigue life, taken off test, and
then tested for strength, the strength will not be diminished.