Page 66 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.16 CHAPTER TWO

For the most part, the codes are very specific about acceptable materials for use in
plumbing systems, such as potable water and drainage, where approval of the authorities is
required. Regarding uses such as draining chemicals from laboratories or industrial work,
the codes are vague. When the chemical waste will be treated inside the project boundary,
usually the materials (and design) used for the waste system do not fall under the jurisdiction
of the plumbing code. In these cases, the engineer has the most latitude in the selection of
materials used for drainage piping.

ELEMENTS FOR SELECTION

Due to the differences in manufacture, grade, and chemical composition of the pipe, test
data must be obtained from the local supplier or manufacturer. Properties of similar materi-
als from different manufacturers are often not the same. Very often, a range of values for
properties such as tensile strength, maximum operating temperature, and hardness is given.
Resistance to chlorine shall also be a consideration.

Elevated Temperature Considerations
Service temperatures in plastic piping systems depend on the type of plastic used. A maxi-
mum service temperature is generally fixed for thermoplastics, and identifies the upper
limit to which the pipe may be heated without damage. When heated above this tempera-
ture, the pipe will soften and deform. Upon cooling, it will harden to the deformed shape
and dimensions.

Long-Term Hydrostatic Strength

The design pressure for plastic pipe is based on long-term hydrostatic strength, which is deter-
mined by finding the estimated circumferential stress that, when applied continuously, will
produce failure of the pipe after 100,000 h at a specified temperature. In addition, a service
factor is included in the design calculations. This factor takes into account certain variables
together with a degree of safety appropriate to the installation. The service factor is usually
selected by the design engineer, and referenced to a service design life of about 50 years. This
design method does not include the fittings, joints, or cyclic effects such as water hammer.
Most pressure ratings for thermoplastic pipes are calculated assuming a water environ-
ment. As the temperature rises, the pipe becomes more ductile and loses strength, and
therefore the rating of thermoplastic pipe must be decreased to allow for safe operation.
These factors are different for each pipe material.

Fatigue Behavior
When surges and water hammer are likely to be encountered, additional allowance should
be made or protective devices installed in the piping system to reduce the pressure.

Aging and Long-Term Degradation

Aging is the change in physical and chemical properties during storage or use, and is
generally dependent on temperature. These changes can occur naturally through normal
atmospheric or building temperature fluctuations, or can be developed artificially due to

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