Page 241 - Buried Pipe Design

Design of Pressure Pipes 215

Figure 4.13 Prequalification flattening test (after flattening).

pipes. Also note that to reach ten million cycles takes about 1 year
of continuous operation. The temperature was held at 22 2 C
(71.6 3.6 F) for all tests.
Test One was terminated after cycling 10,209,535 cycles with no
fatigue failures in the PVC pipes. However, during the process of sub-
jecting these 10 PVC pipe samples to over 10 million cycles, failure of
six pumps, five relief valves, and some other miscellaneous hardware
occurred. Test Three and Four were completed prior to Test One
reaching 10 million cycles. After examining the data obtained from
Tests Three and Four, it was determined that 10 million cycles is a con-
servative estimate of cycles to failure, and failure is more likely to
occur at around 20 million cycles. Vinson’s equation [Eq. (4.17)] pre-
dicted this test to fail at 322,000 cycles. This dramatically illustrates
how Vinson’s method can give very conservative results.
The number of cycles to failure for tests Two, Three, and Four are
summarized in Table 4.8. Only two of the five pipes tested in Test Two
failed. The remaining three pipes survived over 11 million cycles with-
out failure. Where the number of cycles to failure is high, there
appears to be a larger scatter in the results. Tests Three and Four have
very consistent results. In developing fatigue failure criteria, the low-
est number of cycles to failure for each test condition in Table 4.8 was