How to model Cryogenic piping in AutoPIPE?
Please see the following AutoPIPE help section:Help > Contents> Contents Tab> Bentley AutoPIPE> Frequently Asked Questions> #113.
ASME B31.5 ( REFRIGERATION PIPING AND HEAT TRANSFER COMPONENTS) covers refrigerant or cryogenic piping. Companies will use Bentley AutoPIPE with the piping code set to ASME B31.3 for all of their refrigerant or cryogenic piping modeling analysis. They do this because:
a. Appears that much of the Design and SIF sections in ASME B31.5 matches with ASME B31.3.
b. ASME B31.3 has many materials that go down to -325 deg F and some even go down to -425 deg F. In addition, modeling and analyzing Jacketed piping design is something that is handle nicely in AutoPIPE.
B31.3 code compliance case to figure 323.2.2B to avoid Impact Testing for carbon steels
Combined longitudinal stress due to pressure, dead weight, and displacement strain (stress intensification factors are not included in this calculation)
divided by S at the design minimum temperature. In calculating longitudinal stress, the forces and moments in the piping system shall be calculated using nominal dimensions and the stresses calculated using section properties based on the nominal dimensions less corrosion, erosion and mechanical allowances.
S= nominal design stress at low temperature
1. Create a model with one(1) T1, P1, case with low temp and pressure so the sustained case (SUS) and (LONG) based on correct pressure.
2. Analyze with Gr, T1, "Calculate pressure extension cases, e.g. P1, P2, etc…" enabled
3. Create a user defined code combination as follows:
a. Combination Method = abs sum
b. Category = occasional
c. Combination = (LONG) + Gr + Amb to T1 + P1
i. Long = Axial stress due to pressure, Factor = 1, S = combined at the stress level.
ii. Gr = Gravity or dead weight, Factor = 1, M = combined at the moment/force level.
iii. Amb to T1 = displacement strain, Factor = 1, S = combined at the stress level
iv. P1 = Stress from imposed forces/displacements and from forces and moments under P1 load case, Factor = 1, M = combined at the moment/force level.
1. This combination does add Gr, and P1 at the moment level (abs sum) before calculating its stress which is reasonably conservative i.e. no moment cancellation in summing the load cases. Less conservative method is to use SUM combination method. Also adding thermal stress separately is conservative.
2. (Long) load case is the AutoPIPE default sustained stress calculations. You will only see this in the code combinations tab, NOT in the non-code combinations.
4. Update Allowable (k) factor and Allowable stress
a.Select Tools> Code Combinations> K – Factor> Set the K factor = 1.0
Note: uncheck "Auto Update" to enable the user to enter a custom Allowable (k) factor value.
b. Select Tools> Code Combinations> User Allowable stress> The automatic Allowable = K. SH (i.e. nominal design stress at low temperature)
Note: The K factor is only editable for occasional. Uncheck "Auto Update" to enable the user to enter a custom Allowable Stress value.
5. Select all Tees and Bends and Insert> Xtra data > Joint Type & User SIF> SIF = 1.0 (both in-plane and out-plane)
Note: See B31.3 Section 323.2.2B
6. Set "Use Nominal Thickness" = unchecked
The calculated stress ratio can be used to evaluate the temperature reduction below the minimum design metal temperature for carbon steel to clause 323.2.2d (1) and Fig 323.2.2B.
Another words, The requirements of 323.2.2(d)(1) states that the design minimum temperature for required impact testing is -48C (-55F). Should the designed stress ratio be below 1.0, users can determine the new minimum temperature for required impact testing by adding the associated temperature delta in table 323.2.2B to the design minimum temperature of -55F. For example, if the stress ratio is 0.8, the new minimum temperature for impact testing is T_min=-55 deg f+20?=-35 deg f.
Modeling Approaches in AutoPIPE