Page 230 - Buried Pipe Design

204 Chapter Four

occur in a given time. To establish the regression line, tests must be
conducted such that individual failures occur from 10 to 10,000 h
(1.14 years). The line is for static pressure only and temperature
controlled at 73.4 F.
For PVC pipe, long-term static pressure tests have been carried out
over more than 200,000 h (22.8 years) that confirm the validity of
establishing long-term hydrostatic strength on the basis of log-log
straight-line extrapolations.
Hydrostatic design stress. The hydrostatic design stress (HDS) is
defined in ASTM D 2241 as follows: “The estimated maximum ten-
sile stress in the wall of the pipe in the circumferential orientation
due to the internal hydrostatic water pressure that can be applied
continuously with a high degree of certainty that failure will not
occur.”
The ASTM specifications for PVC, PE, and ABS pipe indicate the
hydrostatic design basis and hydrostatic design stress for these mate-
rials. A comparison of one type-and-grade designation of each material
reveals the following:

HDB HDS
PVC 1120* 4000 2000
PE 3406 1260 630
ABS 1316 3200 1600

*Equivalent to PVC cell classification
12454B per ASTM D 1784.
The higher HDB and HDS for PVC 1120 partially explains its wide
acceptance for plastic water pressure pipe. A complete listing for these
values for PVC, PE, and ABS is as follows in Table 4.5.

Pressure-rated pipe. For the purpose of reviewing the plastic pressure
pipe design procedure, PVC pipe and ASTM D 2241, Standard
Specification for Poly (Vinyl Chloride)(PVC) Plastic Pipe (SDR-PR),
will be considered. A similar procedure exists for other thermoplastic
materials.
Throughout existing PVC standards and specifications for PVC pipe,
one still finds the older “type-and-grade” designation. For example, the
most common designation for pressure pipe is PVC 1120. It can be
defined as follows:
PVC: polyvinyl chloride.
First number (1) represents type of compound, in this case, type I.
Second number (1) represents the compound grade, in this instance,
grade 1.