Page 247 - Buried Pipe Design

Design of Pressure Pipes 221

Minimum design pressure
P min WP P RS 160 psi 30 psi 130 psi
Design stresses: use Eq. (4.4)
Mean hoop stress (P max P min )(DR 1)/4
(190 psi 130 psi)(18 1)/4 1360 psi
Stress amplitude (P max P min )(DR 1)/4
(190 psi 130 psi)(18 1)/4 255 psi
Number of cycles:
Anticipated # of cycles (# cycles/day) (365 days/year)
(design life in years)
55 365 50 1.0E6 cycles
Factor of safety 2.0
Required number of cycles 2 1.E6 2.E6 cycles
Actual number of cycles to failure using Fig. 4.18.
Start at the left at 1360 psi mean stress
Move to the right reaching a curve for a stress amplitude of
255 psi
Drop vertically to read the number of cycles 2.E7 cycles
These values can be checked with Eq. (4.19)
Check allowable versus required cycles
Allowable 2.E7 cycles > 2.1 E6 cycles required
Result: DR 18 OK for recurring surge
Conclusion: DR 18 pipe is adequate for the design conditions

Stress amplitude – psi (MPa)
3000 (20.7) 2000 (13.8) 1500 (10.3) 1000 (6.89) 700 (4.83) 500 (3.45) 300 (2.07) 200 (1.38) 150 (1.03) 100 (0.69) 50 (0.34) 20 (0.14)

3000 255 20.7

2500 17.2
Positive/Negative Line
Mean stress (psi) 1360 13.8 Mean stress (MPa)
2000
1500
10.3
1000 Design
Space
500 3.45
2000 psi Lim it
0 0
3
10 10 4 10 5 10 6 10 7 10 8 10 9
Number of cycles 2x10 7
Figure 4.18 Number of cycles to failure for Example 4.2.