Page 212 - Buried Pipe Design

186 Chapter Four

Certain plastic pipe specifications refer to a dimension ratio (DR) or
a standard dimension ratio (SDR), where

D o
D o
DR or SDR
t t
Both DR and SDR are defined the same. However, SDR often refers to
/t for standard prod-
a preferred series of numbers that represents D o
ucts. By introducing D /t = SDR into Eq. (4.3), it can be rewritten as
o
follows:
P i
max (SDR 1) (4.4)
2
The above equation may be expressed as

2 max
SDR 1 (4.5)
P i
Equation (4.5) is often referred to as the ISO (International
Standards Organization) equation for stress due to internal pressure.
However, this basic equation has been known to engineers for more
than a century and was originally given by Lamé in “Leçons sur la theorie
de l’elasticité,” Paris 1852. Obviously, ISO is a relative newcomer and
should not be given credit for Lamé’s work.
To calculate these tangential stresses in the pipe wall produced by
internal pressure, either Eq. (4.2) or Eq. (4.4) are often suggested by
the manufacturer or by national standards. All forms are derived from
Lamé’s solution and will produce comparable results.

Surge pressure
Pressure surges are often divided into two categories: transient surges
and cyclic surges. Cyclic surging is a regularly occurring pressure fluc-
tuation produced by action of such equipment as reciprocating pumps,
undamped pressure control valves or interacting pressure regulating
valves, oscillating demand, or other cyclic effects. Cyclic surges may
cause fatigue damage and should be designed out of the system.
Transient surges are just that—transient in nature, occuring over a
relatively short time and between one steady state and another. A tran-
sition surge may occur, and the system then returns to the same steady
state as before the surge. Transient surges are usually not cyclic in
nature although they may be repetitive. 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 water in the system is a potential cause of a water hammer surge.