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How to model Cryogenic piping in AutoPIPE?
Below is procedure for analyzing refrigerant or cryogenic piping. Companies use B31.3 for all their refrigerant or cryogenic piping using AutoPIPE. It is not necessary to use B31.5. Also, often people will use B31.3 for this because many materials go to -325 and some to -425 degrees F. Jacketed piping design is also something we handle nicely in AutoPIPE for these applications also
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
AutoPIPE Modeling Approach
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, pressure analysis = ON
3. Create a user defined code combination as follows:
a. method = abs sum
b. category = occasional
c. combination = (LONG) + Gr + cold 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. Cold to T1 = displacement strain, Factor = 1, S = combined at the stress leve
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.
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.
4. Select Tools> Code Combinations> User Allowable stress> The automatic Allowable = K. SH (i.e. S at low temp)
5. Select Tools> Code Combinations> K – Factor> Set the K factor = 1.0
6. Select all Tees and Bends and Insert> Xtra data > Joint Type & User SIF> SIF = 1.0 (both in-plane and out-plane)
7. Select Tools> Model options> Results> 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.
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