Page 69 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.19

through plastic pipe, it is not recommended that plastic pipe be used to carry potable water
in areas of contaminated soil.

Leaching
Leaching is a process where substances, sometimes called extractables, are released from
the walls of the pipe material into the fluid, but not through the pipe walls. The most com-
mon extractables are inorganic chemicals and volatile organic compounds (VOCs).
Tests have shown that the rate of leaching from plastics in contact with high-purity water
usually decreases with time. The time it takes for any specific plastic to reach a steady state
after being subject to immersion in the fluid (elution) in dynamic systems is a function of
the water temperature and velocity. Experience has shown that leach-out in the first 5 days is
considered a burn-out period. The release of VOCs from various plastic pipes in contact with
high-purity water at 74°F is shown in Fig. 2.1. Figure 2.2 shows calculated lengths of thermo-
plastics that will increase total organic compound (TQC) level of high-purity water by 1 ppt
when used as a transfer medium (assuming 4-in pipe and a water velocity of 6 ft/s).

Creep
When a load is continuously applied on a plastic material, it creates an instantaneous initial
deformation that further increases at a decreasing rate. This further deformation is called
creep. If the load is removed at any time, there is partial immediate recovery followed by
a gradual creep recovery. If, however, the plastic is deformed (strained) to a given value
that is maintained, the initial load (stress) created by the deformation slowly decreases at
a decreasing rate. This is known as the stress relaxation response. The ratio of the actual
values of stress to strain for a specific time under continuous stressing or straining is com-
monly referred to as the effective creep modulus, or the effective stress-relaxation modulus.

FIGURE 2.1 Release of VOCs from plastic pipe.

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