In most situations we relate the stress range to thermal expansion stress range (min to max temperature), but in ASME B31.3 302.3.5 (d) in discussion about allowable stress range, stress range factor f and computed stress range it is said: “When the computed stress range varies, whether from thermal expansion OR OTHER CONDITIONS, Se is defined as the greatest computed displacement stress range.

I need an assurance from the ASME gurus! What are these “other conditions”? Are they only thermal and displacements driven loads?

What about change in weight and pressure pulsing? Weight and pressure are causing primary stresses, but can their “pulsation” be considered as part of stress range? The pressure pulsation has already been discussed on this forum and consensus was that it should be included in fatigue calcs…but not as a part of the stress range. So, the weight changes should be treated the same way.

If my understanding of ASME requirements about delta P and delta W are correct, then cases for different operating regimes on my job should be as follows:

L1. W1+T1+P1+D1 (T1=5degC, SG1.5, P1 =2000kPa - OPE1) – Caesar file no1
L2. W1+P1 (SUS) – Caesar file no1
L3. W2+T2+P2+D2 (T2=200degC, SG0.5, P2=1000kPa - OPE2 ) – Caesar file no2
L4. W2+P2 (SUS) – Caesar file no2
L5. WNC (line is drained on regular bases SUS) same for both Caesar files

Combination cases are:

L6. L1-L2 (accounts for delta T1+D1) EXP – Caesar file no1
L7. L3-L4 (accounts for delta T2+D2) EXP – Caesar file no2
L8. L6+L7 (ABS) should be the greatest displacement stress range – EXP combination of results from two Caesar files
L9. L2-L5 (accounts for max delta W and max delta P in FAT analysis, 20,000cycles) – Caesar file no1

All comments will be appreciated

Any change in stress can cause fatigue damage. But by using a simplified approach, (most) Codes would include those displacement-based components be they thermal strain of the pipe or any sort of support movement.

I would argue with your L8. Instead, I would use L8=L1-L3. There may be other ranges to check if the maximum range is not obvious. If you get more than one stress range that is "large", then you should be accummulating damage ala Eqn (1d).

See also Appendix S in B31.3.

Remember - simplified appproach...

Hello RS,

"I need an assurance from the ASME gurus! What are these “other conditions”? Are they only thermal and displacements driven loads?"

I think this is covered well in 319.2.1.

Regards, John.

Thanks to both of you for responses.

ASME B31.3 par. 319.2.1 in combination with 302.3.5 tells me that the max stress range is L8 = L6+L7. And the first part of Dave’s response confirms the same:
“…Any change in stress can cause fatigue damage. But by using a simplified approach, (most) Codes would include those displacement-based components be they thermal strain of the pipe or any sort of support movement….”

But then Dave said:
”… I would argue with your L8. Instead, I would use L8=L1-L3. There may be other ranges to check if the maximum range is not obvious. If you get more than one stress range that is "large", then you should be accumulating damage ala Eqn (1d)…”

L1-L3 apart from delta T (max) and Ds would include delta P and delta W! These delta P and delta W are not the full range so additional P2 and W2-WNC would have to be added…, but is this the Code requirement???

Operation of my piping system is cyclic and 2-3 times daily goes from empty to W1, P1, ambientT1, D1 condition to W2, P2, D2, T2 condition and back to empty. So it does the full cycle 2.5 times a day.

To summarize my understanding of the Code requirements in this case and in general:

1. Sustained Stress Check, as per L2, L4, L5 and any additional “hot sustained” case when non-linear supports / boundaries present

2. Full Expansion Stress Range, as per L8 = L6+L7 (319.2.1), but including stress range factor as per (1c) and (1d) considering all stress changes (due to P, T, W, D,…) to protect against fatigue. Also, in general, as Dave said:”… other ranges to check if the maximum range is not obvious…”

L9 and any other combined and / or individual FAT analysis are optional (nice to have in Caesar)

Ranka