Page 227 - Buried Pipe Design

Design of Pressure Pipes 201

Flexible pipes
Thermoplastic. All plastics are, at some stage, soft and pliable and
can be shaped into desired forms, usually by the application of heat,
pressure, or both. Some can be cast. Thermoplastics soften repeatedly
when heated and harden when cooled. At high enough temperatures,
they may melt; and at low enough temperatures, they may become
brittle. A few familiar examples of thermoplastics used for pipe are
polyvinyl chloride (PVC), polyethylene (PE), acrylonitrile butadiene
styrene (ABS), polybutylene (PB), and styrene rubber (SR).
No matter what type of thermoplastic pressure pipe, there is com-
mon terminology. A detailed review of some of the terms will be made.
The design engineer should become familiar with these terms as they
are somewhat unique to the plastic pressure pipe industry.
Plastic pressure pipe terminology

Stress regression
Cell classification
Quick-burst strength
Hydrostatic design basis
Hydrostatic design stress
Service factor
Safety factor
Pressure rating
Pressure class
SDR
DR
PVC compounds. The original method for classifying PVC compounds
was by types and grades, for example, for PVC:
1. Type I, grade 1: Normal impact, very high chemical resistance, and
highest requirements for mechanical material strength. Type I,
grade 1 compounds are by far the predominant material used today
for pipe. Other types and grades of compounds are as follows:
2. Type I, grade 2: Essentially the same properties as grade 1, but pos-
sesses lower requirements for chemical resistance. Grade 1 has
about 5 percent higher hoop stress based on 50-year strength.
3. Type II, grade 1: High impact strength, but sacrifices chemical
resistance and tensile strength.
4. Type III, grade 1: Medium impact strength, low chemical resistance.