Page 191 - Trenchless Technology Piping Installation and Inspection

156 Cha pte r F o u r

PVC pipe is manufactured by first blending PVC resin with stabi-
lizers, pigments, lubricants, processing aids, and functional additives;
and heating this mixture to a temperature in the 400°F range. This
causes the components to properly fuse and convert into a malleable
state. In this molten form, the material is mechanically extruded into
the pipe. Following the completion of the extrusion process, the pipe
is allowed to cool, after which QA/QC testing is performed, before
the final delivery to the end user.

Types of PVC Pipe
There are three distinct types of PVC pipes manufactured in the
world, each differentiated by either the way in which it is manufac-
tured (which dictates the directional orientation of the molecules), or
by the content of modifiers in its chemical formulation (which affect
the ability of the pipe to withstand impacts by absorption and dissi-
pation of the energy). The term PVC is a generic designation, which
includes PVC-U (unplasticized PVC), PVC-O (molecularly oriented
PVC), and PVC-M (modified PVC).
1. PVC-U: Unplasticized PVC is widely used piping material
in water and sewer systems in North America. The molecular
structure of PVC pipe is a random arrangement of long chain
molecules, where molecular entanglement is prevalent
throughout the length of the pipe. In general, the PVC mole-
cules do not exhibit any definite directional orientation, and
therefore, a generally uniform strength prevails in both the
radial (circumferential) and longitudinal directions. Testing
has shown that the modulus of elasticity in 15-year-old PVC
is only slightly higher in the longitudinal direction than in the
radial directions. For simplification, the term PVC as used in
this chapter will denote PVC-U, the conventional type of PVC
most used in pipe manufacture. PVC pipes are manufactured
for both pressure and gravity applications. All PVC pressure
pipes (ASTM and AWWA standards) must meet the cell clas-
sification 12454, indicating a tensile strength of 7000 psi, and
a modulus of elasticity of 400,000 psi. PVC pressure pipes
have a hydrostatic design basis (HDB) of 4000, per ASTM
D2837, to which a design factor of 2.0 is applied. This reduces
the long-term design stress to 2000 psi. PVC gravity pipes are
typically manufactured to cell class 12454 or 12364, where the
latter has a tensile strength of 6000 psi and a modulus of elas-
ticity of at least 440,000 psi.
2. PVC-O: Molecularly oriented PVC is made in the United
States by the expansion of the conventional PVC pipe; during
the expansion process, the molecules become oriented in a
generally radial or circumferential direction. This molecular
reorientation increases the strength of the pipe in the hoop