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Pressure Vessel FEA

Finite element analysis has been widely used in pressure vessel design where code formulas are not applicable. Interpretation of the FEA results shall be performed to ASME Section VIII, Div. 2. For CRN registration projects using stress analysis, the FEA report has to be accepted by the review engineers at the jurisdictions. Follow the link FEA Requirements for some instructions when performing finite element analysis and writing report.

Model #1: pressure vessel design
This analysis is to show how central stay affects stresses in formed head and tubesheet. Use of stay effectively reduces required thickness for tubesheet. However, it also generates additional bending stresses in head. A repad is needed on head to limit peak stresses within allowables. It provides an option for tubesheet design. Material thickness and loads applied are not our concerns in this show, which are omitted.

Material for analysis is SA-516 70. Pictures below present partial results of the analysis. Interpretation to stresses at different locations is simlpy illustrated to ASME Section VIII-2 rules.

When model is relatively too big, more time will be needed for FEA runs if reasonable mesh size is used and tubes are modeled. The alternative is to create a seperate model for tubesheet only with tubes included. The last picture shows such a model and stress distribution around tube holes.

Mesh size: 40mm;                                         Model pressurized inside,
mesh at corner: 10mm.                               additional forces added on end.
                    

Stresses above 20000psi                           At corners: (PL+Pb+Q)< 3Sa, PL<1.5Sa;
(membrane limit Sa) in red.                      General locations: Pm+Pb< 1.5Sa, Pm<Sa.
                     

Stresses in tubesheet are below 1.5 x Sa (Bending stress limit), acceptable.
           

Model# 2: heat exchanger without shell, ribs added inside head
This is a further discussion on model# 1. In model #1 above, a repad is put on head to lower local stresses on openning. Can we remove the repad if head is supported inside by ribs (Ribs are designed to carry condensate out of  head)? The answer is yes. Results are detailed in complete FEA Report. 

Design Conditions:
Vessel ID: 2200mm
Shell thickness: 20mm; SA-516 70 (Left end welded to SE head, right to tubesheet)
Head: 2:1 SE, 18mm thick after forming, SA-516 70
Manhole on head: 300×400mm, 25mm thick, SA-36 rolled plate
Drain on head: reverse flange, 52mmID, 59mm wall, 25mm thick, SA-36 pad
Central pipe passing through head and tubesheet: 250mmOD, 26mm thick
Design pressure: 150psi
Corrosion allowance: 3mm
X-ray: RT-3

Calculation discussion:
This is a non-code exchanger design, calculated by code formulas and FEA.
Due to the use of support on tubesheet and the missing of shell side, UHX is not applicable. UG34, UG39 can be used to check tubesheet thickness (ASME Calculations ), but it is obviously too conservative because support is not taken into consideration. U-2(g) (FEA) is adopted, which makes required thickness calculation more reasonable.

Results:
Required thickness by UG-34, UG-39 is 156mm. FEA calculated required thickness (Support is included in model) is around 65mm. TEMA required thickness is 80mm.

                                               Download ASME Calculations 
                                               Download complete FEA Report

Model#3: platform support attachment

Support configuration not covered by WRC107 or NozzlePro can be analyzed by FEA.

Model #4: irregular pressure vessel design analyzed by FEA

Model #5: structure design
                  

This model is for an ash conveyor underneath power boilers. A quarter of the frame is modeled. 3D solid element is used in analysis. Symmetry constraints are selected for mid planes. Half circle on top for transmission shaft is the area to take loads. Guide wheel for the chain at bottom of the slope is anchored in direction perpendicular to bottom plate to ensure the model is fully defined.

Results show stresses around guide wheel are over yield point. Strength design shall be focused on this area. In this case, local reinforcement might be needed to prevent material from yielding.

Download ABSA FEA Requirements