Our FEA BPVC consultants have successfully completed a broad range of analysis work on hundreds of pressure vessels. Within this body of work, we have applied the following codes:
- ASME BPVC Section VIII, Divison 2 (Design-by-Analysis)
- ASME Pressure Vessels for Human Occupancy (PVHO)
- ASCE 4-98 and ASCE 7-02
- AISI N690
- ABS Underwater Vehicles, Systems & Hyperbaric Facilities
These FEA pressure vessel consulting projects cover a wide variety of analyses, from differential thermal-stress analysis of heat exchangers utilizing mixed materials, to stress and fatigue analysis of large-diameter vessels, to analyses of vessels with complex internal structures subjected to sloshing, seismic and added-mass effects or lifting and transportation analyses and transient thermal-fatigue of thick-walled tanks. We have also done stringent code work under the ASME PVHO and ABS Underwater Vehicles, Systems and Hyperbaric Facilities on several types of passenger submarines. In one particular case, our FEA PVHO-ABS consulting allowed our client to certify their submarine via FEA in lieu of the standand design rules. This exception was approved by the ABS since the FEA results tightly correlated with the strain-gauge results from the dive test. More results on this investigation can be found at NASA Tech Briefs
To support our pressure vessel work, we have developed custom software for stress and fatigue evaluation of thin and thick-walled vessels. Some of this PV consulting work has helped SpaceX launch their next generation of rockets.
In the nuclear field, Predictive is certified to generate NQA-1 seismic, buckling and fatigue analysis reports on some of the most complex vessels installed at the Hanford Tank Waste Treatment and Immobilization Plant. These reports are all focused on the "design-by-analysis" rules within the ASME Section VIII, Division 2 specification. Working collaboratively with Department of Energy contractors, Predictive has pioneered many of the seismic and buckling analysis procedures based on interpretation of ASCE 4-98 and ASME Section VIII, Div. 2 Codes for pressure vessels containing large submerged internal components that are subjected to the added mass requirements within ASCE 4-98.
From seismic to buckling to fatigue analysis, Predictive can assist in validating the most challenging pressure vessel designs. Our hard-earned experience allows us to safely classify tanks and vessels as “fit-for-service” that would typically have required extensive rework by the standard ASME Section VIII, Division 1 hand-calculations.
In brief, clients come to us when they need high-quality work executed and documented to withstand the most rigorous reviews.
Some ASME BPVC Pressure Vessel Consulting Project Examples (text listing)
NEW PROJECTS FROM 2016
Large Diameter (300”) Pressure Vessel Lifting Lug Analysis
The purpose of this analysis was to verify that jacking lugs used on several large diameter pressure vessels would not cause damage to PV components (such as the skirt, shell and cooling jacket). Starting with existing FE models of the pressure vessels, the jacking lugs are modeled, meshed and attached to the vessels with a “glued connection”. This connection type allows new components to be added and joined without re-meshing existing components. A 6” diameter hydraulic ram is placed under each of the jacking lugs, centered between the gussets. There is a 1” thick doubling plate between the rams and lugs. The jacking lugs transferred load to the doubling plates and hydraulic ram through surface-to-surface contact. This analysis work showed that the jacking lug design was robust and would not cause stresses in the PV exceeding ASME Section VIII, Division 2 stress classification requirements.
Soda Ash Filter Pressure Vessel – Sliding Support Frame Analysis
The objective of this project is to calculate the stresses by the finite element analysis (FEA) method for the supports and adjacent sections of shell and head of a horizontally suspended soda ash filter pressure vessel (PV). The PV is suspended horizontally and one of the PV heads is mounted to a fixed section of the support frame. The vessel shell is mounted to a sliding portion of the support frame such that the PV can be opened, the shell can be translated along the frame and the contents can be accessed easily. The analysis of the PV includes two loading conditions. First the vessel was analyzed in the “closed” position. That is, the vessel shell was joined to the vessel head via the retaining rings and swing bolts. In the closed position, the vessel was assumed to be at maximum fluid level and maximum design pressure. The next load combination saw the vessel in the “open” position. In the open position, the vessel is not be pressurized and did not have fluid contents. This load combination needed to be considered because with the vessel shell detached, the cantilevered load at the head supports was increased. The supports and adjacent regions of the shell and head pass the stress requirements as given in the ASME Section VIII, Div. 2 specifications.
Elastic-Plastic Analysis of an ASME Section VIII, Div. 2 Regenerative Thermal Oxidizer (RTO) Pressure Vessel
Our customer came to us with an engineering drawing of a large 20 x 20 x 8ft rectangular pressure vessel with strict review requirements. This structure is part of an emissions treatment system handling noxious, pressurized gasses so it was important that they see no failures in service. The structure was modeled using surfaces in FEMAP and then meshed with plate elements in quarter symmetry. The first pass showed the structure did not meet the ASME Section VIII, Division 2 stress classification requirements. Design iterations were performed to find a solution that would work within their manufacturing limitations. With these modifications in place, linear static analysis showed that the structure would pass. As a follow-up, the customer wanted assurance that the pressure vessel would handle 2.5x the operating pressure without failure. Running the model with nonlinear material properties and inspecting the plastic strain allowed us to say with confidence that the structure was safe.
FEA of HDPE Plastic Adult Beverage Vessel
The most efficient geometric shape for an internally or externally pressurized vessel is a sphere. If that is not possible, one can fall back on a cylinder with hemispherical or domed ends. Sometimes, to fit within a refrigerator, one would like to use a shape that is cubic. The design-by-analysis rules within the ASME code provides significant flexibility to the FEA simulation engineer but the analysis of a pressurized cube does not leave one with many options. Initially our client specified aluminum and then later switched to high density polyethylene (HDPE). Interesting enough, upon switching to the low elastic modulus material (20x less stiff than aluminum) and running the analysis with nonlinear geometric stiffness turned on, we noticed a 4x drop in stress. In hindsight this seems obvious that by allowing the corners of the cube to deform and form into smooth curved structures, our cube become more like a cylinder. In combination with increased wall thickness, we were able to present to the client an acceptable pressure vessel design that would meet their pressure requirements and still provide the safe service of adult beverages.
ASME “Design-By-Analysis” Buckling Classification of an In-Line Gas Pipeline Pig
Buckling of externally pressurized structures is a fascinating phenomenon and for those aficionados of classic old-time submarine movies, a bit of cold sweat. Predictive Engineering has significant experience in the buckling of structures using ASME’s PVHO and ABS’s Underwater Vehicles code for hyperbaric chambers, submarines, pressure vessels and now for an in-line gas pipeline pig. Our client manufactures sensor’s for pipeline inspection industry. A common method for delivery of the sensor is send a “pig” or a cylinder loaded with instruments through the gas pipeline driven by a differential pressure. Operating conditions within the pipeline can reach pressures up to 1,500 psi. To ensure that their instrument package could safely operate, they contracted us to perform an ASME Section VIII, Division 2 “design-by-analysis” study on the buckling resistance of their miniature in-line inspection tool or “pig”. Their material of choice was a simple carbon steel (L360MS). The ASME allowable stress was calculated using standard knock-down formulas where the material’s original yield strength of 66.7ksi was reduced to a membrane allowable of 16.2ksi. The buckling assessment was based on 2.08 safety factor. FEA simulation started with a linear elastic stress analysis followed by an eigenvalue buckling analysis. To verify the eigenvalue results, a geometric nonlinear buckling analysis was performed on the structure. As our client had suspected, the pig had significant safety margins and the project was sealed up and our project summary was one sentence “The ….. passes the ASME Section 8, Division 2 stress and buckling classification.”
Weld Fatigue Analysis of Overhead Condenser Tubesheet Intersection
Our client had a very specific challenge in dealing with a prior design that was failing after five to seven years in service. They wanted to extend the life of the vessel but given financial and design constraints had a very limited set of options. The tubesheet was explosively clad with titanium. The material selection was a combination of stainless steel and carbon steel. A combined thermal-stress (internal pressure + temperature differential) analysis showed high stresses at the weld intersection that was an obvious driving factor toward the shortened service life of the vessel. Further analysis pinpointed the problem to the use of stainless steel for the tubesheet given its high CTE would expand push against the shell. That is to say, the internal pressure load only created about 20% of the stress load while the thermal load was 80%. Various redesign attempts were explored to move the weld zone down and way from the tubesheet intersection via a “stub” approach but every attempt only slightly lowered the stresses. After numerous meetings, it was determined that the tubesheet could just as easily be made out of carbon steel and thus with its lower CTE solve the problem of high stresses at the weld zone. Although this was not a classic ASME Section VIII, Division 2 FEA stress analysis, the skill sets we have learned over the years through the FEA simulation of hundreds of pressure vessels allows us to offer unique solutions that our clients might not immediately think of. With the change of tubesheet materials, the client was able to more than achieve their goals by lowering the cost of fabrication and extending the service life of the vessel.
PROJECTS 2015 AND EARLIER
Nuclear Waste Processing Vessels
Seismic, thermal, dead weight, nozzle and pressure analyses were performed on a series of large-scale (e.g., 300” diameter) nuclear waste processing vessels for the DOE. Engineering work was approved under ASME Section VIII, Div. 2, ASCE 4-98 for Seismic Analysis of Nuclear-Related Equipment and associated NQA-1 requirements.
100 foot-long Tube and Shell Evaporator
A 100 foot-long tube and shell evaporator was analyzed for stress and deflection based under transportation loading. Stresses were classified according to ASME Section VIII, Div. 2. The evaporator was fabricated with several different material types, each with different ASME Division 1 material allowables. The vessel was constrained with a saddle support near the top tubesheet with a Schnabel connection at the base of the skirt.
Stress and Fatigue Analysis of a Powder Transfer Vessel
A standard thin-walled pressure vessel was analyzed to ASME Section VIII, Division 2 rules given an internal pressure and dead weight loading that consists of a granular powder. Besides the analysis of the pressure hull, the vessel’s supports were analysis for a combination of seismic and wind loading per ASCE 7 specification. After this classification, a fatigue analysis was done on the complete structure per ASME Section VIII, Div. 2, Part 5.
External Pressure Loading on Sensor Pig for Gas Pipelines: Stress and Buckling Analysis
Our client had a thick-walled vessel that would carry sensor equipment to monitor the structural health of gas pipelines. The external pressure on the vessel would be in excess of 1,000 psi. By careful use of the “design-by-analysis” rules within the ASME Section VIII, Division 2 code, we were able to take their original design and make it a bit lighter and substitute a much lower cost material. This flexibility allows one to leverage the full power of a finite element analysis to maximize the design of the pressure vessel structure.
Thermal-Fatigue Analysis of High-Temperature Flue Gas Steam Generator
An ASME thermal fatigue analysis was performed on a high-temperature flue gas steam generator (i.e., boiler). Given the large thermal gradients between the flue gas inlet and outlet, the thick tube sheet presented a particular thermal-stress challenge. The vessel passed based on peak alternating stress (Sa) per Figure 220.127.116.11 of the ASME Section VIII, Division 2 code. This particular PV FEA consulting project was reviewed at several levels from the manufacture to their end-client. We sat in during the review defending our work and were able to successful get the vessel to pass muster without last minute design changes.
Thermal-Stress and Fatigue Analysis of Thick-Walled, High-Pressure Liquid Oxygen (LOX) Tank
Thick-wall, high-pressure liquid oxygen (LOX) tanks experience thermal fatigue conditions not rarely seen in normal pressure vessels. Stress classification required our complete ASME software arsenal. Membrane stresses were calculated using standard ASME stress linearization. These tanks were used in test firing of rocket nozzles where one set of tanks stored LOX and the other set RP1 (rocket fuel). All vessels were classed under ASME Section VIII, Division 2 rules.
ASME Section VIII, Div. 2 Pressure Vessel Transportation Analysis
The PV in this FEA consulting analysis was a large shell-and-tube heat exchanger analyzed under transportation conditions. The project required the design of the shipping support structures: a schnabel at the skirt and a saddle at the upper tubesheet. Once the static stress analysis was performed, the results were interrogated to ensure that the stresses within the PV did not exceed the ASME allowables (PL, PM for membrane stress and PL+PB+Q for surface stress) and the stresses within the support structures did not exceed AISC N690 stress allowables.
ASME Section VIII, Div. 2 Pressure Vessel Lifting Analysis
In this analysis the PV was analyzed for lifting loads at various angles as the vessel is installed. The main concern was the skirt, tailing lug and trunnions. The mass of the vessel was a critical parameter. To ensure proper mass was include the finite element model used beam elements to approximate the tube sheet; this greatly improved the efficiency of the model. Initially, the tailing lug was not adequate but after a couple design suggestions a final design was found.
ASME Section VIII, Div. 2 Pressure Vessel Reorientation Analysis
A large (100’”) ASME Section VIII PV needed to be reoriented within the fabrication shop to provide access to allow test fitting of external structures. This analysis helped determine the most effective combination of rollers and belly-bands that will allow the C210 Evaporator Vessel to be rotated to provide access for test fitting ladders and platforms while not exceeding ASME Section VIII, Div. 1 stress allowables.
Stress Analysis of Shell and Tube Heat Exchanger
The pressure vessel system consisted of an evaporative chamber connected to a tube-sheet heat exchanger via piping. The system is mounted on a structure steel frame. Thermal and stress analysis calculations were performed per ASME VIII, Div. 2 to determine thermal and stress margins. Some optimization work was done on the lower piping system due to wind loading effects per ASCE 7-02.
Thermal-Stress Analysis of Dual-Chamber Titanium Shell and Tube Heat Exchanger
Shell and tube heat exchanger for a boiling tube, dual-vessel evaporative system classed under ASME Section VIII, Division 2 rules. The main shell was titanium with the tubing of SA-240. The vessel analysis included a check on the tube sheet design and optimization work to reduce the wall and tube sheet thicknesses due to high material costs. Design and analysis work was performed for the Dedert Corporation.
Stress, Buckling and Seismic Analysis of T-Design Shell and Tube Heat Exchanger
Shell and tube heat exchanger per ASME Section VIII, Div. 2 analyzed for design, operating, buckling and seismic loading. The final design of the vessel required 14 design iterations due to internal baffle constraints to prevent buckling. The final buckling analysis was done by geometric, nonlinear analysis to confirm safety margins.
Stress, Thermal-Stress and Buckling Analysis of Dual-Wall 5.8m Diameter Valveless Regenerative Thermal Oxidizer (VRTO) Vessel
The VRTO was analyzed for stress, thermal-stress and buckling per ASME Section VIII, Division guidelines and with all results classed per Division 1 stress allowables. This large diameter vessel is used to efficiently burn-off process gases using a unique regenerative ceramic bed. Process gas is introduced through the bottom of the vessel into the ceramic bed which allow selective heat transfer from the ceramic to the gas stream. Without going into a lot of detail, the analysis work considered thermal expansion effects within the vessel due to the hot gas at temperatures above 500C and load combination effects due to internal pressure and external wind loading. The vessel was classed to include super-structure loading (roof mounted blowers and a walkway), dead weight loads (ceramic and steel) and then normal design loads.
The VRTO vessel passes the ASME Section VIII, Division 2 requirements for mechanical and thermally-induced stresses. It should be noted that the VRTO vessel is not an ASME stamped vessel and there is no requirement for it to be classed under the ASME code, saying that, the code provides sound guidance for verification of its “fit-for-service” design. The vessel was classed using the ASME stress allowables for 316L. An optimization study was also done to investigate the roof-to-shell weldments for possible improvements in the manufacturing process and to investigate a material switch from AL6NX to standard SS304 or at least the ability to thin the shell material from 0.125 to 0.1875”.
At the end of this project, optimization results more than paid for the cost of this FEA investigation.
Large Diameter Spray-Drying Chamber
A large diameter spray-drying chamber was analyzed for seismic, wind and lifting. Applicable codes were ASCE 7-02 with reference to the UBC and ANSI A58.1. The lifting stress requirements were borrowed from ASME, Section VIII, Division 2. The model was highly idealized and allowed sections to be optimized for seismic and lifting. The exterior of the vessel was a space frame structure idealized using beam elements. Appurtances hanging on the outside of the vessel (e.g., fans, heaters and ladders) were simulated using mass elements.
ASCE 4-98 Pressure Vessel Sloshing
For this project, the sloshing loads needed to be determined within an ASME Section VIII, Div.2 Pressure Vessel (PV) upon a large marine vessel. The fluid sloshing pressure loads were calculated with ASCE 4-98. The 3D spatially varying loads were applied to a Finite Element (FEA) model. The FEA model provided reaction loads and stress results used to design anchors and determine potential problem areas in the skirt.
Thermal-Stress Analysis of a 500 kW Generator Housing
This project involved FEA modeling, meshing and analysis of integrated power module generator housing. Model loading included thermal loads, pressure loads, static forces and interference fits. The housing was evaluated against ASME Section VIII, Div. 2 allowables.
ASME Section VIII Pressure Vessel - Iterative Vessel Design and Analysis
It is common for a PV to have non-standard geometry that requires ASME Section VIII, Division 2 analysis. If the majority of the components of the vessel are radically different from the standard components of an ASME Section VIII vessel, initial section sizing must be determined with an FEA model.
This project started with initial design proposed by the manufacturer. An efficient FEA model was constructed and multiple iterations were performed, updating the section thicknesses until a workable design was achieved. Once a workable design had been achieved, there was freedom to explore different shell shapes, stiffener geometries and flange thicknesses. The vessel evolved from something that was simply “workable” to something with optimal wall thicknesses and surface area that met the thermal requirements of the project.
Thermal-Stress and Fatigue Analysis of Dual-Material Tube Sheet Bundle
Very large ASME vessel analyzed for tube-sheet stresses arising from dual-material tube-bundle with likewise dual material shell. Thermal-stress analysis was performed to classify system under ASME stress and fatigue requirements.
Lifting Analysis of As-Built Pressure Vessel
A complex lifting analysis was performed to verify that an already built vessel system would not exceed ASME Section VIII, Div. 1 allowables. The slings and spreader bar were idealized with beam elements and pinned connections.
Stress and Buckling Analysis of Deep-Diving Submarine
Finite element analysis of an eight passenger, deep-diving (depth 1,200’) luxury submarine under ASME Section VIII, Division 2 rules. Stress results were verified by strain gauging of the submarine during its submersible proof-test. Given the close correlation of the stress results to the dive test, it become the first human occupancy submersible certified by the American Bureau of Shipping (ABS) via the finite element method. Two other submarines were likewise analyzed.
Stress Analysis of Formaldehyde Reactor
Formaldehyde reactor analyzed for thermal and mechanical performance. ASME Section VIII, Div. 2 code requirements on internal shell and tube heat exchanger.
Threaded Plastic Regulator with Pressure Load
A static stress analysis was performed on a threaded plastic regulator. This analysis required that the FE model was built with fine detail in key locations but reasonably low overall model size for quick and efficient solve times. These key locations include area of high stress (as determined by pilot models), threaded regions and areas in contact. Although not classed as an ASME pressure vessel, this small pressure vessel required the same type of analysis procedures to numerical proof the design.
ASME Section VIII Pressure Vessel - Fluid Mass Adjustment with Software Automation
Since thin walled PV FE models are most commonly built with plate and beam elements, the mass of non-structural three dimensional entities (i.e. fluid, concrete, insulation) must be distributed over the surface area or length of corresponding mesh. Fluid mass must be changed to adjust the FEA model for horizontal or vertical seismic analyses.
Using a Microsoft Excel spreadsheet and Femap's Application Programming Interface (API), a “Non-Structural Mass (NSM) Adjuster” was created to rapidly add, remove or change the FE PV. The Excel-based API uses the volumes of the components, material densities and plate mesh surface areas to calculate NSM values for the model. The program allows the user to change the NSM of the FEA model with a single button-click. When the user selects horizontal or vertical seismic conditions, the API distributes the NSM while maintaining an accurate center of gravity.
ASME Tube and Shell Heat Exchanger Vessel
Based on an initial design as developed by TEMA and ASME codes, a large pressure vessel was analyzed under ASME Section 8, Division 2 specifications with a complete seismic and buckling analysis. An interesting twist to this vessel analysis was that it was based on the client’s existing vessel and was woefully under-designed for its intended application. Although one may take analysis work for granted, it is very easy to produce wrong numbers. Predictive Engineering takes great pride that it has never delivered bad results to a client and that in over 17+ years of service, not one component analyzed by Predictive has failed due an analysis error.
Submarine Analysis Work via ABS and ASME Codes
Predictive engineering has certified two large, manned commercial submarines and a third experimental submarine destined for the Mariana Trench. We have extensive experience in ABS and ASME PVHO codes that allows us to guide the client toward the most optimized design for a manned submersible. Complete FE analysis can be done with a nonlinear buckling calculation to validate the submarine design. Our work has been strain gauged and validated under ABS surveyor requirements.
Regenerative Thermal Oxidizer (RTO)
The RTO contains a heat exchanger mass divided into sections – each part alternately serves the cooling phase, while the other serves the heating phase. The exhaust air flows through the RTO’s heat exchanger mass from bottom to top, which heats it up to oxidation temperature – approximately 800 °C. The hot purified gases then flow downwards through the other part of the heat exchanger mass, giving it their heat.
A finite element analysis (FEA) model of the RTO was built and subjected to pressure, dead weight and thermal loads. The first objective of this analysis was to ensure that the pressure boundary met ASME allowables under pressure, dead weight and platform loading. Because the vessel is designed to be subjected to a negative pressure load and a compressive load exists on its roof, a buckling analysis was performed per the ASME code. The second objective of this analysis work is to investigate the penetrations (nozzles and platform standoffs) connecting the inner and outer wall on the roof. With the inner wall at a higher temperature than the outer wall, differential expansion has the potential to create high stresses at these connections.
Horizontal Pressure Vessel Mounted on Sliding Frame
When a fabricator was hired to build pressure vessels from an existing design, a bit of reverse engineering was necessary. While the code calculations for the pressure boundary and nozzles were straightforward, the support system was a bit more complicated. The vessel is oriented horizontally and mounted to a support frame at the top head and along the shell. The vessel able to open at a set of retaining rings between the shell and top head. Once the swing bolts are removed, the vessel shell and bottom head slide away from the top head using a gantry system.
The analysis of this system required two loading conditions. First the vessel was analyzed in the “closed” position. That is, the vessel shell was attached to the top head at the retaining rings. In the closed position, the vessel is at maximum fluid level with maximum design pressure. The second load combination had the vessel in the “open” position. In the open position, the vessel is not pressurized and does not have fluid contents. This load combination needed to be considered because with the vessel shell detached, the cantilevered loads at the top head supports are increased.
ASME Pressure Vessel Analysis
Extensive experience has been earned in the analysis of dozens of simple to complex pressure vessels. Vessels have been certified under ASME Section VIII, Division 1 and 2 with every possible configuration of tube sheets, nozzle connections, partitions and operations. Work has also been executed under NQA-1 for extremely complex nuclear waste recycling vessels in seismic environments. Buckling analysis was done via ASCE and ASME requirements. In some cases, the buckling resistance of the vessel was certified by UG-2 exception using a non-linear approach to the structure. A complete rainflow counting fatigue analysis was also performed on all structural components, weld joints and piping structures.