Page 199 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 163

widely used piping material for trenchless methods such as horizon-
tal directional drilling (HDD) and pipe bursting (PB) for new instal-
lation and renewal of municipal pressure and nonpressure piping
systems. PE used for pipes in North America are generally classified,
on basis of density (crystallinity) and suitability for pressure service.
Higher density provides greater hardness, stiffness, and tensile
strength.
ASTM classifies PE piping materials having specified density
ranges as low density (LDPE), MDPE, or HDPE. HDPE displays the
highest stiffness whereas LDPE is the most flexible. LDPE is not
used for pressure piping, but due to its flexibility is used for pneu-
matic instrument controls. MDPE pressure piping is predominantly
used for gas distribution. HDPE nonpressure piping materials are
used for electrical and communications conduit and nonpressure
corrugated and profile wall pipes. HDPE pressure piping materials
are used for solid-wall pressure pipes for gas, water, force mains,
nuclear and industrial process piping, and for solid-wall nonpres-
sure applications such as sanitary sewers and culverts. Gas pipes
are generally made from MDPE. Municipal water and sewer pipes
and industrial pipes are generally made from HDPE. At 73°F the
pressure rating of HDPE is about 25 percent higher than MDPE
pipes. In 2005 ASTM standards were modified to allow the intro-
duction of an improved HDPE with significantly improved perfor-
mance capabilities. HDPE materials commonly used for municipal
piping have a code of either PE3608 or PE4710. The PE4710 mate-
rial has a pressure rating 25 percent higher than PE3608 material
(i.e., 25% higher pressure than PE3608 material). With few excep-
tions, both materials have a design service life of 100 years in typi-
cal municipal water or sewer applications.
Manufacturing
The manufacturing process for HDPE pipe is plastic extrusion where
molten PE resin extruded under pressure through specially designed
extrusion machines and dies that form the melt into pipe. The pipe is
drawn into cooling tanks to solidify and cool the pipe. A second
method for producing large-diameter PE pipe is to extrude a wall
profile onto a rotating mandrel. Wall profiles can have hollow sec-
tions to reduce the amount of material but maintain circumferential
stiffness. Once the pipe section is created, the mandrel is collapsed
and removed from the pipe ID. A third method for corrugated profile
wall PE pipe is to extrude a thin wall pipe into a corrugator that has
vacuum blocks that draw the pipe into a corrugated wall profile and
cool it. Corrugated pipes can be produced with an ID liner for a
smooth waterway, and a noncorrugated OD layer for a smooth OD.
Solid wall, PE pipes are typically designed for internal pressure, but
are used for pressure and nonpressure services. Spiral wound and
corrugated profile wall pipes are for nonpressure service. A third