Page 229 - Buried Pipe Design

Design of Pressure Pipes 203

1. Classify PVC pipe compound per ASTM D 1784 cell classification,
that is, 12454B.
2. Conduct long-term static pressure tests on pipe.
3. Submit data to the hydrostatic-design committee of PPI for
analysis.
4. Determine the long-term hydrostatic strength (LTHS). [LTHS is the
extrapolated hoop stress that will produce failure in 100,000 h (11.4
years).]
5. Determine the hydrostatic design basis (HDB) by categorizing the
LTHS per ASTM D 2837, which also involves projections to 50
years.

The pressure test data, when presented on a log-log plot, form a
straight line. It is called a stress regression curve (see Fig. 4.6 for
stress regression curve for PVC). This “declining” curve does not
represent a loss of strength with time. It does show that the higher
the stress, the shorter the life; conversely the lower the stress, the
longer the life. The line relates the life of the pipe to the level of
stress in the pipe wall due to internal pressure. It is a series of test
data points. For example, for a given stress or pressure, failure will

Time (years)
0.0114 0.114 1.14 11.4 114 1140
10000
9000
8000
7000
6000
5000
Stress (lb/in 2 ) 3000
4000

2000

1000
10 1 10 2 10 3 10 4 10 5 10 6 10 7
Time (h)
Figure 4.6 Stress regression line for PVC pressure pipe.