| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | 2004, XM, V8i | ||
| Area: | Calculation | ||
| Original Author: | Bentley Technical Support Group | ||
| Date Logged & Current Version | Jan 2015 09.06.01.10 |
Problem:
Regarding B31.8 code. The program is calculating the outside diameter hoop stress instead of the mid-plane hoop stress when the 2012 code is used. The help indicates that the mid-plane hoop stress should be calculated. This is what is done when the older versions of the same code are used.
Solution:
Per the equations in AutoPIPE's online help, ASME B31.8 2012, the hoop stress is based on the Pipe's Outside Diameter and nominal wall thickness.
This matches the actual code equation, ASME B31.8 - 2012, section 841.1.1(a):
S
Note: For ASME B31.8 offshore piping code,
Please see the following AutoPIPE help section:
Help > Contents> Contents Tab> Reference Information> Code Compliance Calculations> ASME B31.8 Offshore (2012)> Hoop Stress (ASME B31.8 Chapter VIII - 2012).
Hoop Stress Calculation:
Per code eq. above.
where
S = SMYS
F = design factor from table.
E = longitudinal joint factor obtained from table
T = temperature derating factor obtained from table
P = design pressure, psig
D = nominal outside diameter of pipe, inch
t = nominal wall thickness, inch
Given the following parameters calculate Hoop stress:
Therefore,
P = 300 psi
D = 8.625 inch
t = Wall thickness = 0.322 inch
Hoop = 300 psi x 8.625 inch = 4017.86 psi
2 x 0.322 inch
Confirmed on Results grids:
If Results Model Options> "Use Nominal thickness" option was unchecked. See AutoPIPE help for details on how this will ultimately affect the results.
Note: 3. For B31.8 code, this option is checked by default and only applies to non-Hoop categories including longitudinal pressure stress which will used reduced thickness (i.e. considering corrosion + mill tolerance). For Hoop category, nominal thickness is always used.
Note 4. Hoop stress is typically not a strict code compliance stress category but is provided (inverse of minimum wall thickness calculation) as requested by users for a safety check.
Meaning that when this options is unchecked, the related code combinations will use a different wall thickness (ex. t.red) then when the option was checked ON. Note 3 tries to explain which combinations this feature is being applied too. Non-Hoop categories include: Sustain, Expansion, Occasional, Rest-Func, and Rest-Env. However only 2 categories contain Longitudinal Press stress; Rest-Func and Rest-Env. For combinations in these categories the application will calculate the hoop stress based t.red when the option is unchecked. The program does not calculate a hoop stress for those other combination categories. Likewise, combinations categories Sus, Exp, Occ have variables that depend on wall thickness. Again, these variable will use the indicated wall thickness when the feature is unchecked. Finally, note 3 mentions that for Hoop category combination, the nominal wall thickness will always be used regardless of this setting (i.e. checked or unchecked).
Hoop Stress for categories that include Longitudinal Press Stress
t.red = Wall Thickness - Corrosion Allow - Mill tolerance
t.red = 0.322 - 0.118 - 0.040
t.red = 0.164 inch
Hoop = 300 psi x 8.625 inch = 7888.72 psi
2 x 0.164 inch
Confirmed on Results grids:\