Page 67 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.17

elevated temperatures of the fluid in the pipe. The origin of these changes within the pipe
are in the molecular or crystallographic structure of the pipe. Plastics and elastomers expe-
rience chemical changes due to the influence of light, heat, oxygen, humidity, and radiation,
all of which cause breaks in their molecular chains. One criterion for determining the onset
of aging is the measurement of thermal stability (oxygen induction time) using differential
scanning calorimetry.

Ultraviolet Radiation (UVR)

UVR is a known source of degradation to plastic pipe. The effects can be reduced by adding
pigments or covering the installed pipe with a jacket.

Flammability
During fire conditions, the degradation of plastics is greatly accelerated. In the early stages
of a fire, most plastics melt and lose their structural shape and strength. As heat is added at
a rising rate, plastics undergo a series of typical changes, which include chemical decom-
position often releasing toxic chemicals. This decomposition occurs at a lower temperature
than ignition. By the time ignition occurs or is possible, a relatively long period of chemical
emission has elapsed.
When thermoplastic pipe burns, it releases smoke and toxic gases, provides heat that
increases the intensity of a fire, and may provide a path for flame to spread along its length.
In addition, open holes may develop at wall or ceiling penetrations which could provide a
route for the passage of gases between rooms.
All organic materials are flammable, but this is particularly true of polyolefins. It is well
proven that many polymers are, as a result of their chemical composition, difficult to ignite.
Polymers can also be made much more difficult to ignite by the addition of flame retardants.

Acoustic Transmission

Because of its light weight, thermoplastic piping does little to reduce airborne sound. An
appropriate thickness of insulation must be used to reduce noise.

Thermal Expansion
The amount of movement resulting from thermal effects is relatively high, thus requiring
special attention to installation. As a general rule, runs in excess of 20 ft should be checked
for the necessity of expansion offsets.

Corrosion Resistance

Corrosion occurs in two ways, as chemical and stress corrosion. There are two general
types of chemical attack on plastic piping. The first is called solvation, which is the solubil-
ity, or absorption, of chemicals into the piping material from the fluids inside the pipe. This
causes swelling and softening. The second type of attack occurs where the polymer or base
resin molecules are somehow changed by a chemical agent, and the original properties of
the plastic pipe cannot be restored upon removal of that chemical. Stress (or strain) corro-
sion weakens the pipe due to constant and repetitive movement and/or pressure surges.

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