ASME PTB-3 Validation – Elastic Stress Analysis

I carried out ASME PTB-3 Validation for “Example E5.2.1 – Elastic Stress Analysis” to check for protection against plastic collapse per ASME Sec VIII Div 2 using ANSYS. Here’s a summary.

Problem Statement:

Evaluate the vessel top head and shell region for compliance with respect to the elastic stress
analysis criteria for plastic collapse provided in paragraph 5.2.2. Do not include the standard flanges
or NPS 6 piping in the assessment for compliance to allowable stresses. Internal pressure is the only
load that is to be considered. Relevant design data and geometry are provided below and in Figures
E5.2.1-1 and E5.2.1-2.
Vessel Data
 Material – Shell and Heads = SA-516, Grade 70, Normalized
 Material – Forgings = SA-105
 Design Conditions = 420 psig at 125oF
 Corrosion Allowance = 0.125 inches
 PWHT = Yes

ANALYSIS STEPS:

An axi-symmetric geometry model was constructed with shell (length > \(5\sqrt{Rt}\)), nozzle and attached weld neck flange using corroded dimensions as shown in sketch below.

MATERIAL PROPERTIES

A small deformation, linear elastic, static structural analysis was carried out using the following material properties.

Young’s Modulus was taken from table TM-1, ASME II D at design temperature while poisson’s ratio was taken from table PRD, ASME II D.

MESH

A fine 2nd order quad dominant mesh was used for analysis purpose with global element sizing set to 0.15 in to get adequate number of elements across thicknesses required for stress linearization. From my experience 3 elements across thickness generally gives acceptable converged linearized results.

MESH @ Nozzle Shell Junction

MESH @ Head Shell Junction

LOADS & BOUNDARY CONDITIONS

  • Internal Pressure of 420 psi was applied on the inner surfaces of the model

  • Pressure Thrust of 982.3 psi was applied on the flange face as negative pressure

  • Axial displacement was arrested at the shell base

ANALYSIS RESULTS

Total Deformation Plot

Note that PTB-3 does not provide a deformation plot for comparision.

Von-Mises Stress

Von-Mises Stress from PTB-3 example

The result plot compares pretty well with those provided in PTB-3 (ASME Sec VIII Div 2 – Example Problem Manual)

SCL Locations

Stress Classification lines were taken at deemed critical locations similar to those given by PTB-3.

STRESS ANALYSIS SUMMARY

Stresses were evaluated at the SCL Locations and were compared against allowables. The below table is reproduced from PTB-3 in which comparitive results from this analysis have been put in orange

The analysis results are generally within 5% of PTB-3 values. The cases with slightly more error are those of relatively smaller stress, therefore actual difference between the reported stresses for these cases are generally of the order of few MPa. Also it shall be noted that the stress linearization carried out by ansys is not compliant with ASME Sec VIII Div 2 procedure. For more on this read here.

Find PTB-3 Validation Limit Load Analysis here

Find PTB-3 Validation Elastic Plastic Analysis here

 

 

 

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