| Applies To | |||
| Product: | AutoPIPE | ||
| First Affected Version: | 09.04.01 | ||
| Found in Version: | 09.04.01.05 | ||
| Fixed and Released in Version: | 09.05.01.02 | ||
| Area: | Calculations | ||
| Severity: | Critical | ||
| Backlog Issue Number: | 138808 (10782) |
Problem Description
B31.3-2010 Occasional allowable may be unconservative for T<Tcr or Tcr=0
Reason
ASME B31.3-2010 alternate occasional allowable may be calculated incorrectly in any of the following cases:
a) If the Pipe Properties dialog for an existing Pipe ID is opened and closed by pressing “OK”, with or without making any modifications, other than changing the pipe material. This can cause the program to incorrectly use the equation prescribed for calculating allowable for operating temperature greater than creep temperature of the corresponding material (T>Tcr) when the operating temperature is actually less than or equal to the creep temperature (T<=Tcr).
b) If the creep temperature (Tcr) of a pipe material is zero “0” in the material library, and yield strength value at the operating temperature used for calculating the alternate occasional allowable is non-zero.
For “a” and “b” above, program will incorrectly use 0.9*Syt (Austenitic) or 0.72*Syt (Non-Austenitic) instead of 1.33*Sh. The calculated allowable may be un-conservative.
Note 1: If the cold allowable (Sc), hot allowable (Sh), cold yield (Syc), or hot yield (Syt) is user defined for any material, then the calculated alternate occasional allowable will depend upon the specified values and may be un-conservative.
Note 2: The following material specifications will produce conservative alternate occasional allowable stresses provided that the cold allowable (Sc), hot allowable (Sh), cold yield (Syc), and hot yield (Syt) values are taken from the B31.3-2010 material library:
A53-B, A106-B, A106-C, API5L-A25, API5L-X42, API5L-X46, API5L-X52, API5L-X56, API5L-X60, API5L-X65, API5L-X70, API5L-X80, A333-6, A335-P2, A335-P11, A335-P12, A335-P21, A335-P22, A135-B, A285-B, A105, A181-60, A181-70, A335-P1, A335-P5, A335-P5B, A335-P5C, A369-FP2, A369-FP11, A369-FP12, A369-FP22, A182-F12, A182-F22, B547-5083, B241-5083, B209-5083, B210-5154, B619-C22, B366-N10276, B165-OD <= 5, B165-OD > 5, B241-5086, B247-6061-T6, B247-6061T6W, B221-5083, B-584-C92200, A516-G60-SHT, A516-G70-SHT, A387-G11-CL2, A283-GRC-SHT, A182-F316-C, A312-TP321-C, A312-TP347-C, A312-TP348-C, A376-TP347-C, A376-TP348-C
To illustrate this problem the following scenario can be considered:
Step 1: Open walkthru.dat sample model and change piping code to B31.3-2010.
Step 2: Modify the already defined Pipe ID “8SCH40” to set Pipe Material = A312-TP304H.
Step 3: Select the entire model and update Operating Temperature T1 to 100 deg F.
Step 4: Under result model options, enable “Use Alt. Occ. Allowable”.
Step 5: Analyze the model and generate code compliance report.
Notice that Occasional Allowable is correctly calculated as 1.33*Sh = 1.33*20000 = 26600 psi
Step 6: Open Pipe Properties dialog for Pipe ID “8SCH40” and press “OK” without making any changes.
Step 7: Repeat Step 5.
Notice that now the Occasional Allowable is un-conservatively calculated as 0.9*Syt = 0.9*30000 = 27000 psi
The following considerations are provided to assist in your evaluation of the potential impact of this defect on the results of a model.
1. Evaluate whether this defect has affected any existing models by considering the criteria below:
a. Was the model created in program version 9.4.0 or 9.4.1?
b. Was the selected piping code B31.3-2010?
c. Was “Use Alt. Occ. Allowable” option checked in Result Model Options?
d. Was any one of the following conditions true?
i. The Pipe Properties dialog for an existing Pipe ID was opened and closed by pressing “OK”, with or without making any modifications, other than changing the pipe material? And the temperature of the operating case used for alternate occasional allowable calculation was less than or equal to the creep temperature (Tcr) of the corresponding pipe material?
ii. The creep temperature of a pipe material was zero “0” in the material library? And yield strength value at the operating temperature used for calculating the alternate occasional allowable was non-zero.
A 'No' to one or more of the above criteria (a, b, c, d) means that this defect does not affect the model’s results. No further consideration for that model is necessary.
2. Evaluate possible mitigating conditions on the impact of this defect on the results.
The reported allowable stress will be un-conservative only when 0.9*Syt (for Austenitic materials) or 0.72*Syt (for Non-Austenitic materials) is greater than 1.33*Sh at the operating temperature used for occasional allowable calculation, otherwise the effect of this defect will be mitigated and conservative allowable values will be reported.
Note: The occasional allowables are corrected if the model is opened in v9.5.0 or later after the consistency check is performed.
Solution:
When calculating alternate occasional allowable stress at an operating temperature, program should use the following equations:
For T<=Tcr, Alternate occasional allowable = 1.33*Sh
For T>Tcr, Alternate occasional allowable = 0.9*Syt
For Tcr = 0 in the library, Alternate Occasional Allowable = 1.33*Sh
Additionally, to assist in the screening of the models created in previous program versions for any potential impact by this defect, the program shall automatically examine the model database after opening the model and generate a report (model name + "_D10782.TXT") listing the pipe IDs with material specifications which may have resulted in un-conservative allowable values.
The program will display the following confirmation message:
One or more pipe IDs in this model may be affected by a previous program defect in which the calculated allowable occasional stress may be incorrect. Would you like to review the evaluation report?
Avoidance:
Define a user allowable stress for sustained combinations using “Code Comb.” tab of Load Combination dialog.
OR
When there is no Tcr value specified for a material in the library, update the library with a valid Tcr value. Otherwise, open the Pipe Properties dialog for the affected Pipe ID, change the pipe material to another material and then change it back to the original material by clicking “OK” on the dialog. Note that user allowable values, if previously entered, will be reset.
Also See:
Critical and High issues released in AutoPIPE V8i v.09.05..