20. Creep and Fatigue
Pipe experiences vibration when a fluid flow inside the piping. Vibration of piping under flow of fluid outside the piping is more critical. When the fluid flow is outside the piping and across, the vibration is considerable. Vibration of blanks of tubes under cross flow vibration has been extensively researched. The piping at high temperature undergoes creep. The American Society of Mechanical Engineers (Power Boilers) consider 100,000 hours of creep life. This is equivalent to eleven years of continuous operation. The behavior of the piping subjected to fatigue and creep, simultaneously is not known.
The life of piping is expected to be twenty five years. Hence, the number of fatigue cycles expected:
25 years X 300 cycles/year = 7,500 fatigue cycles
An approximate treatment of the interaction (based on numerical experiments) of fatigue and creep is given in the following:
1. Design temperature = 400 Degree C
2. Design pressure = 100 bar (g)
3. Number of cold starts = 100 per cent life lost (fatigue)=12%
4. Number pf warm starts = 1,000 per cent life lost (fatigue)=6%
5. Number of hot starts = 10,000 per cent life lost (fatigue)=10%
6. Number of load variations = 100,000 per cent life lost (fatigue)=25%
7. Duration at 430 Degree C = 100 hour per cent life lost (creep)=2%
8. Duration at 420 Degree C = 1,000 hour per cent life lost (creep)=8%
9. Duration at 410 Degree C = 10,000hour per cent life lost (creep)=13%
10. Duration at 400 Degree C = 100,000 hour per cent life lost (creep)=10%
Total life lost (fatigue+creep) =86%
Expected life of piping = 25 years / 0.86 = 29 years.