Page 223 - Buried Pipe Design

Design of Pressure Pipes 197

where T longitudinal stress due to temperature
linear coefficient of expansion
T temperature change
E Young’s modulus for pipe material
An example of a situation that would cause such a stress follows:
Consider a welded steel line which is installed and welded during hot
summer days and later carries water at 35 F. The resulting T will be
substantial, as will the resulting stress. Additional information on
temperature-induced stresses in welded steel pipe can be found in
AWWA M11, Steel Pipe Manual, and in other AWWA standards on
welded steel pipe.

Pipe thrust. Longitudinal stresses due to pipe thrust will be present
when a piping system is self-restraining with welded, cemented, or
locked-joint joining systems. For example, at a valve when the valve is
closed, the thrust force is equal to pressure P times area A. The same
force is present at a 90 bend.

Thrust pressure area
PA P r 2
The stress due to this thrust is given by

PA P r 2 Pr
th
2 rt 2 rt 2t

where th longitudinal stress due to thrust
T thrust force P r 2
P internal pressure plus surge pressure
r average radius of pipe
t thickness of pipe wall

Stress risers. The pipe system designer should always be aware of
stress risers which will amplify the stresses. Stress risers occur around
imperfections such as cracks, notches, and ring grooves. They are also
present near changes in diameters such as in the bell area. Designs
that overlook stress risers can and have led to piping system failure. In
a welded bell and spigot type of joint, the longitudinal tensile stresses
are not passed across the joint without inducing high bending moments
and resulting bending stresses. These bending stresses have been
shown to be as high as 7 times the total longitudinal stress in a straight
section. For this example, the maximum longitudinal stress is given by

( L ) max ( Lb L LT th )