04m. Model Cryogenic piping in AutoPIPE


 

Applies To
Product(s):AutoPIPE
Version(s):ALL
Area: Modeling
Original Author:Bentley Technical Support Group
Date Logged
& Current Version		 Apr. 2016
10.01.00.05

Problem:

How to model Cryogenic piping in AutoPIPE?

Solution:

See AutoPIPE help FAQ #113:(reprinted below)

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.

Furthermore:

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, "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.

Note:

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.

See Also

Modeling Approaches in AutoPIPE

Bentley AutoPIPE