The essential aspects of a solid wall PE pipe
manufacturing facility are presented in Figure 1. This section will describe
the production of solid wall pipe from raw material handling, extrusion,
sizing, cooling, printing, and cutting, through finished product handling.
Details concerning profile wall pipe are also discussed in the appropriate
sections.
Raw Materials Description
The quality of the starting resin material is closely
monitored at the resin manufacturing site. As discussed in the chapter on test
methods and codes in this handbook, a battery of tests is used to ensure that
the resin is of prime quality. A certification sheet is sent to the pipe and
fitting manufacturer documenting important physical properties such as melt
index, density, ESCR (environmental stress crack resistance), SCG (slow crack
growth), stabilizer tests, amongst others.The resin supplier and pipe manufacturer may agree upon
additional tests to be conducted.
The raw material, usually referred to as PE compound, is
typically supplied to the pipe producer as non-pigmented pellets. PE pellets are
stabilized for both heat and UV protection. Usually, color pigment is added to
the pipe at the producer’s facility. In North America, the most common colors
are black and yellow. The choice of color will depend upon the intended
application and the requirements of the pipe purchaser. Carbon black is the
most common pigment used for water, industrial, sewer and above-ground uses.
Yellow is reserved exclusively for natural gas applications, although black
with yellow stripes is also permitted for this application. Other colors are
used for telecommunications and other specialty markets. All ASTM and many
other industry standards specify that a PPI-listed compound shall be used to
produce pipe and fittings for pressure pipe applications. A compound is defined
as the blend of natural resin and color concentrate and the ingredients that
make up each of those two materials. The pipe producer may not change any of
the ingredients. In a listed compound, such as substituting a different color
concentrate that could affect the long-term strength performance of the pipe.
Any change to a listed formulation has to be pre-approved. These stringent requirements
ensure that only previously tested and approved compounds are being used.
If the resin is supplied as a natural pellet, the pipe
producer will blend a color concentrate with the resin prior to extrusion. In
order to obtain a PPI Listing, each manufacturer producing pipe in this manner
is required to submit data, according to ASTM 2837, to the PPI Hydrostatic
Stress Board. A careful review of the data is made according to PPI Policy TR-3
(5) to assess the long-term strength characteristics of the in-plant blended
compound. When those requirements are met, the compound qualifies for a
Dependent listing and is listed as such in the PPI Publication TR-4 (6), which
lists compounds that have satisfied the requirements of TR-3. Producers of
potable water pipe are usually required to have the approval of the NSF International
or an equivalent laboratory. NSF conducts un-announced visits during which time
they verify that the correct compounds are being used to produce pipe that
bears their seal.
Raw Materials Handling
After the material passes the resin manufacturer’s quality
control tests, it is shipped to the pipe manufacturer’s facility in 180,000- to
200,000-pound capacity railcars, 40,000-pound bulk trucks, or 1000- to
1400-pound boxes. Each pipe producing plant establishes quality control
procedures for testing incoming resin against specification requirements. The
parameters that are typically tested include: melt flow rate, density, moisture
content and checks for contamination. Many resin producers utilize statistical
process control (SPC) on certain key physical properties to ensure consistency
of the product. Resin is pneumatically conveyed from the bulk transporters to
silos at the plant site. The resin is then transferred from the silos to the pipe
extruder by a vacuum transfer system. Pre-colored materials can be moved
directly into the hopper above the extruder. If a natural material is used, it
must first be mixed homogeneously with a color concentrate. The resin may be
mixed with the color concentrate in a central blender remote from the extruder
or with an individual blender mounted above the extruder hopper. The blender’s
efficiency is monitored on a regular basis to ensure that the correct amount of
color concentrate is added to the raw material.
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