jkal321
Mechanical
- Jul 10, 2007
- 18
Hi,
I read multiple threads about Thermal Stress Range SA ASME B31.3 Eq. 1a and 1b.
I also understand that: For ASME B31.3 the failure mechanism for secondary load (temperature) is fatigue. For primary (W, P) is
yielding.
My question is specific: I am having a piping analysis utilizing CAESAR II: I have three Temperatures T1=0, T2=70 and T3=150
My Max Thermal expansion Stress Range (T3-T1) has exceeded the Max Thermal Range allowable Sa eq. 1b
Is it justifiable to ignore the Thermal Range load case (T3-T1) "Extreme Case" and choose instead the Normal operating condition (T2-T1) and state the following:
"The stress intensification factor of 2.4 (In Plane) and 2.8 (Out Plane) calculated
internally by Caesar II. Including stress intensification for fatigue failure is highly conservative when the
system exhibits only low cyclic behavior"
My understanding I can't. Based on what I read in Peng below.
Is this correct?
As explained and written in "Pipe Stress Engineering " by Peng Section 3.5 [Allowable Thermal Exapansion Stress Range]:
This may not be very significant as we are comparing the calculated stress with a theoretical stress. However, the important thing is whether this allowable stress provides enough safety factor for systems operating at limited number of cycles. Because the longitudinal stress due to pressure and weight is generally allowed to reach hot allowable stress, that is, Spw = Sh, the allowable stress range for thermal expansion only, SA, becomes B31.3 eq. 1a:
SA = f(1.25Sc + 0.25Sh)
where/is a stress-range reduction factor varying from f= 1.0 for N < 7000 cycles, to f= 0.5 for N> 250,000 cycles. However, the newer fatigue data has shown that the f value may be considerably smaller than 0.5 when N > 250,000 cycles. The stress-range reduction factor,/, has been recently renamed stress range factor because it reduces the allowable stress, and not the stress range.
Based on the experimental data, the allowable expansion stress range given in Eq.1a represents an average safety factor of 2 in terms of stress, and a safety factor of 30 in terms of cyclic life. However, because of the spread in individual test data, the potential minimum safety factor can be as low as 1.25 in terms of stress and 3 in terms of life. This emphasizes the need for making a conservative estimate of the stress and the number of operating cycles. It should be noted, however, that these safety factors were based on the original (1955) allowable stress, which was taken as 5/s of the yield strength instead of the current 2 h of the yield strength.
The stress-range reduction factor cuts off at 7000 cycles with no increase in allowable stress-range permitted when the operating cycles are less than 7000. With f=l.0 at 7000 cycles, the allowable stress has already reached the benchmark stress limit. Any stress beyond that might produce gross yielding in the system, thus invalidating the elastic analysis. Therefore, the actual safety factor increases as the number of cycles reduces. The 7000 operating cycles represents 1 cycle/ day for 20 years. This number is more than most piping systems experience nowadays when batch-operating processes requiring daily turnaround are rare.
I read multiple threads about Thermal Stress Range SA ASME B31.3 Eq. 1a and 1b.
I also understand that: For ASME B31.3 the failure mechanism for secondary load (temperature) is fatigue. For primary (W, P) is
yielding.
My question is specific: I am having a piping analysis utilizing CAESAR II: I have three Temperatures T1=0, T2=70 and T3=150
My Max Thermal expansion Stress Range (T3-T1) has exceeded the Max Thermal Range allowable Sa eq. 1b
Is it justifiable to ignore the Thermal Range load case (T3-T1) "Extreme Case" and choose instead the Normal operating condition (T2-T1) and state the following:
"The stress intensification factor of 2.4 (In Plane) and 2.8 (Out Plane) calculated
internally by Caesar II. Including stress intensification for fatigue failure is highly conservative when the
system exhibits only low cyclic behavior"
My understanding I can't. Based on what I read in Peng below.
Is this correct?
As explained and written in "Pipe Stress Engineering " by Peng Section 3.5 [Allowable Thermal Exapansion Stress Range]:
This may not be very significant as we are comparing the calculated stress with a theoretical stress. However, the important thing is whether this allowable stress provides enough safety factor for systems operating at limited number of cycles. Because the longitudinal stress due to pressure and weight is generally allowed to reach hot allowable stress, that is, Spw = Sh, the allowable stress range for thermal expansion only, SA, becomes B31.3 eq. 1a:
SA = f(1.25Sc + 0.25Sh)
where/is a stress-range reduction factor varying from f= 1.0 for N < 7000 cycles, to f= 0.5 for N> 250,000 cycles. However, the newer fatigue data has shown that the f value may be considerably smaller than 0.5 when N > 250,000 cycles. The stress-range reduction factor,/, has been recently renamed stress range factor because it reduces the allowable stress, and not the stress range.
Based on the experimental data, the allowable expansion stress range given in Eq.1a represents an average safety factor of 2 in terms of stress, and a safety factor of 30 in terms of cyclic life. However, because of the spread in individual test data, the potential minimum safety factor can be as low as 1.25 in terms of stress and 3 in terms of life. This emphasizes the need for making a conservative estimate of the stress and the number of operating cycles. It should be noted, however, that these safety factors were based on the original (1955) allowable stress, which was taken as 5/s of the yield strength instead of the current 2 h of the yield strength.
The stress-range reduction factor cuts off at 7000 cycles with no increase in allowable stress-range permitted when the operating cycles are less than 7000. With f=l.0 at 7000 cycles, the allowable stress has already reached the benchmark stress limit. Any stress beyond that might produce gross yielding in the system, thus invalidating the elastic analysis. Therefore, the actual safety factor increases as the number of cycles reduces. The 7000 operating cycles represents 1 cycle/ day for 20 years. This number is more than most piping systems experience nowadays when batch-operating processes requiring daily turnaround are rare.
