Hello,

I'm reading Mechanical Engineering News Dec92 about life extension estimation of piping system (page 12 to 17). I summary as below. Hoping someone already read thi article to explain more.

In section 3 "System Modification for Life Extension" mention about piping system in Figure 3. The system has 14000 design equivalent cycles for 20 design years. After 18 years, the system undergone 12600 cycles (rate = constant = 12600 cycles/18 years = 700 cycles/year). The goal is to use the system more 20 years beyond original 20 design years.

Maximum stress range 30902 psi in first 18 years gives actual equivalent cycles 15959 cycles which permit 15959-14000 = 1959 additional cycles. It means the system can only be extended life for 1959 cycles/700 cycles per year = 2 years 9 months

Solution is reducing stress range to reduce number of cycles. After modification, reduced stress range is 19582 psi (with expect 22 years x 700 cycles per year = 15400 cycles), the new equivalent cycles is calculated base on below conditions:

a. 12600 cycles @ 30902 psi (undergone first 18 years)
b. 15400 cycles @ 19582 psi (will be undergo next 22 years)
--> N = 12600 + (19582/30902)^5*15400 = 14173 cyles

The paper concludes "this value is less than the calculated allowable number of cycles for maximum stress range, which was 15959. Therefore, with this modification, this system should be adequate for 20 years life beyond its original design life of 20 years."

My question: I don't understand why 14173 cycles smaller than 15959 cycles will help the system to extend more 20 years life?