FEA Validation of a Multiplace Hyperbaric Treatment Chamber
How stress analysis at the rectangular door openings of a 3-patient Hyperbaric Oxygen Therapy (HBOT) chamber delivered documented proof of patient safety under ASME VIII Division 2 Part 5.
ASME VIII Div 2 Part 5 Design-by-Analysis
Medical Pressure Vessel
HBOT / Multiplace Chamber
Life-Safety Validation
Problem
A 3-patient hyperbaric therapy chamber with rectangular door openings — geometry that ASME Division 1 code-formulae cannot evaluate.
Action
Full ASME VIII Div 2 Part 5 Design-by-Analysis using a 1.78M-node ANSYS model with live-load patient masses.
Outcome
All ASME stress checks PASS across 4 load cases at every door — chamber certified structurally safe for full operating pressure with patients aboard.
Executive Summary
The Decision Problem
When a pressure vessel is designed to contain human beings as a hyperbaric therapy chamber is every stress check is a patient-safety check. But hyperbaric chambers have a structural feature that ASME Section VIII Division 1 code-formulae were never written to handle: rectangular door openings. Div. 1 reinforcement rules apply only to circular nozzles. For non-circular openings, the only defensible validation path is a full Design-by-Analysis under ASME Section VIII Division 2, Part 5 – finite element analysis backed by stress linearization.
What We Did
For a 3-patient multiplace HBOT chamber (designed to ASME Section VIII Div 1, 2017), Ideametrics performed a full ASME VIII Div 2 Part 5 Design-by-Analysis. A 1.78M-node ANSYS model captured the complete vessel, three rectangular doors, and the 65 kg patient live-load at each door position. Four load cases bracketed design pressure and hydrotest conditions, for both the main chamber and the total vessel.
Business Outcome
- Documented stress analysis across all 3 rectangular door openings, every door passed every ASME check
- Code-defensible validation package for medical device compliance and regulatory submission
- Confidence in the chamber’s safe operation for every patient, every treatment, for the full design life
- No rework, no redesign, design cleared for manufacture
Technical Outcome
- All primary local membrane (PL), primary+secondary (PL+Pb+Q), and general membrane (Pm) stresses below allowable across all 4 load cases
- Worst-case PL at Door 1 under hydrotest: 141.9 MPa vs 373.35 MPa allowable (38% of limit, 2.6× margin)
- Worst-case PL+Pb+Q at Door 1: 395.95 MPa vs 746.7 MPa allowable (1.9× margin)
- Model validated against hand-calc hoop stress (0.5 MPa difference on ~48 MPa) and global equilibrium (0.6% error)
Why This Is Repeatable for You
A hyperbaric treatment chamber is not a commodity pressure vessel. It is a medical device that directly carries human lives inside at elevated pressure. Every “PASS” in this analysis is a patient-safety certification. This is what FEA under ASME Div 2 Part 5 delivers for life-critical equipment, documented, code-defensible proof that replaces engineering judgement with numerical evidence.
1. The Engineering Challenge
A multiplace hyperbaric treatment chamber is a horizontal pressure vessel designed to accommodate three patients simultaneously at elevated pressure (up to 0.2157 MPa gauge, approximately 3.14 ATA absolute). Each patient has a dedicated access door, a rectangular opening in the chamber wall sealed by a gasketed hinged door assembly.
The rectangular geometry creates a specific, well-known structural problem:
Figure 1 — Why this analysis is necessary: stress concentration at the corners of a rectangular door opening under pressure. Classic pressure-vessel problem: Div. 1 code formulae apply only to circular openings. For rectangular geometry, FEA is the only defensible validation path.
ANSYS equivalent von Mises stress contour plot at the rectangular Door 1 opening of a multiplace hyperbaric treatment chamber under internal design pressure, stress concentrations visible at the sharp corners of the rectangular opening, illustrating why ASME Section VIII Division 1 code formulae (applicable only to circular openings) cannot evaluate this geometry and why ASME VIII Division 2 Part 5 Design-by-Analysis is required
ASME Section VIII Division 1 provides nozzle reinforcement rules for circular openings. It does not provide formulae for rectangular openings. The sharp internal corners at a rectangular door concentrate stress in ways that can only be quantified with a full-field stress solution, finite element analysis with stress linearization and classification under ASME Section VIII Division 2, Part 5 (Design-by-Analysis).
For a medical chamber that will carry human beings, “the stresses are probably fine” is not an acceptable verdict. The vessel requires documented proof that every stress category, primary local membrane, primary-plus-bending-plus-secondary, and general membrane, stays below allowables at every door, under every load case, for both design pressure and hydrotest.
2. Why This Vessel Matters
Hyperbaric Oxygen Therapy (HBOT) delivers medical-grade oxygen at elevated ambient pressure. It is a recognised treatment for a growing list of clinical indications, decompression sickness, carbon monoxide poisoning, non-healing diabetic ulcers, radiation tissue injury, severe skin grafts, necrotising soft-tissue infections, and several others. Patients enter the chamber, the chamber is pressurised to therapeutic pressure (typically 2.0–3.0 ATA absolute), and they breathe high-concentration oxygen for a defined treatment interval.
Life-Critical Equipment
Unlike an industrial reactor, this vessel contains humans. Structural failure at pressure is not a containment incident, it is a direct threat to patient life.
Cyclic Duty, Long Life
Chambers pressurise and depressurise multiple times per day, for decades. Every treatment cycle adds to fatigue demand at the doors, which are the stress-concentration sites.
Rectangular Doors = Non-Standard Geometry
Access doors must accommodate a patient on a stretcher. Circular doors are impractical. That forces rectangular openings, which Div. 1 code cannot validate directly.
3. Detailed View - the Three Rectangular Door Openings
The chamber has three patient positions, each with its own rectangular door and a 65 kg live-load representing an occupant. These are the structural features that govern the analysis:
Figure 2: 3D CAD geometry – horizontal chamber with two dish heads, twin saddle supports, and three rectangular patient doors (A, B, C) on the lower shell.
3D CAD geometry of a multiplace hyperbaric treatment chamber pressure vessel, horizontal cylindrical chamber with two dish-head ends, twin saddle supports (fixed and sliding), three rectangular door openings labelled A, B, C on the bottom of the vessel for patient access, multiple instrumentation nozzles on the top, SA 516 Gr 70N shell construction, modelled in ANSYS Workbench for ASME VIII Div 2 Part 5 Design-by-Analysis
Figure 3: Live-load application – three 65 kg point masses (A, B, C) represent the three patient occupants at each rectangular door position during treatment.
ANSYS boundary-condition view of the multiplace hyperbaric treatment chamber showing three point masses (labelled A, B, C) of 65 kg each applied at the rectangular door openings, representing three simultaneous patient occupants during HBOT treatment, applied live loading for the ASME Div 2 Part 5 stress analysis
Figure 4a: Door 1 – peak local membrane 88.16 MPa (LC1), 141.9 MPa (LC2 hydrotest)
Close-up ANSYS equivalent von Mises stress contour at rectangular Door 1 of the hyperbaric treatment chamber under internal design pressure, peak stress concentration at the corners of the rectangular opening, primary local membrane PL stress 88.16 MPa (LC1) against 252.4 MPa allowable per ASME Div 2 Part 5
Figure 4b: Door 2 – peak local membrane 60.18 MPa (LC1)
Close-up ANSYS equivalent von Mises stress contour at rectangular Door 2 of the hyperbaric treatment chamber under internal design pressure, stress distribution at the second patient-door opening, all stresses within ASME allowable limits
Figure 4c: Door 3 – peak local membrane 72.20 MPa (LC3)
Close-up ANSYS equivalent von Mises stress contour at rectangular Door 3 of the hyperbaric treatment chamber, stress distribution at the third patient-door opening under design pressure, all stresses within ASME Div 2 Part 5 Part 5.2 allowable limits
The FEA model was built from the as-designed geometry. Every door, every corner, every hinge, every gasket face, resolved at the mesh level. The model captures what hand calculations fundamentally cannot: the three-dimensional stress field at non-circular openings.
4. Codes & Standards
Why Design-by-Analysis?
ASME Div 1 rules assume circular openings with standard reinforcement geometry. This chamber has rectangular doors — a geometry outside Div 1’s closed-form evaluation envelope. Div 2 Part 5 Design-by-Analysis exists precisely for this kind of case: when the geometry is valid, the service is defined, but the stresses can only be resolved numerically. The output is the same code-defensible verdict, produced through FEA with stress linearization.
5. Why FEA + FFS Was the Only Defensible Path
Classical pressure-vessel hand calculations assume simplifying geometry that this chamber violates in two ways:
Life-Critical Equipment
- Stress concentration at rectangular opening corners – Div. 1 reinforcement rules apply only to circular nozzles; rectangular openings have no closed-form solution
- Interaction between three adjacent doors – when three rectangular cut-outs sit on the same shell, their stress fields overlap in ways that single-opening formulae cannot resolve
- Live load at each door – the 65 kg patient mass at each opening adds a local bending demand on the door frame during cyclic pressurisation
- Stress categorization – ASME requires separation of primary, bending, and peak stress components, which is only meaningful from a full FEA result
- Hydrotest + static head – the hydrostatic pressure gradient along the chamber height creates non-uniform loading that formulae treat as uniform
This is exactly the case ASME Div 2 Part 5 was written for, valid construction, well-defined service, geometry outside closed-form rules, and a stress state that must be resolved by FEA.
6. Verdict at the Deviation Locations
The three rectangular doors are the governing stress locations on the chamber. The table below pulls every ASME Div 2 Part 5 stress check at every door, across all four load cases, into one view:
Bottom line: the worst stress at any door across any load case is 141.9 MPa (PL at Door 1 under LC2 hydrotest), 38% of its 373.35 MPa allowable, with 2.6× margin remaining. The chamber has significant structural reserve at its most demanding locations.
7. Headline Results
ALL PASS
2.6×
Worst-case margin on primary local membrane
1.1%
Hand-calc hoop stress vs FEA match (48.11 vs 48.64 MPa)
0.6%
Global equilibrium validation error
Figure 5: Full-chamber equivalent stress under LC1 – peak at Door 1 rectangular opening
ANSYS equivalent von Mises stress contour for the full multiplace hyperbaric treatment chamber under Load Case 1 (internal design pressure plus self-weight plus nozzle thrust plus 65 kg live load at each of three rectangular doors), peak stress at Door 1 rectangular opening, colour legend in MPa, used for ASME Div 2 Part 5 plastic collapse screening
Figure 6: Stress linearization at Door 1 – membrane/bending/peak decomposition per ASME Div 2 Part 5
ANSYS stress linearization Stress Classification Line (SCL) plot at maximum stress location on Door 1 of the hyperbaric treatment chamber, membrane, bending and peak stress components decomposed per ASME Div 2 Part 5 stress categorization methodology
8. Lessons Learned
Finding 1 - Rectangular openings in pressure vessels are FEA-mandatory, not FEA-optional
There is no closed-form code solution for a rectangular opening in a pressurised shell. When a design requires non-circular openings for functional reasons (patient access, inspection ports, viewing windows), Design-by-Analysis under ASME Div 2 Part 5 is the only defensible validation path. Skipping it means the vessel is undocumented for its most critical geometry.
Finding 2 - Door 1 governs and it's always the same door
Across every load case, Door 1 carries the highest stress. The three doors are geometrically similar but their positions along the chamber create slightly different local boundary conditions. For in-service inspection planning, Door 1 is the priority NDT location — the others are effectively monitored by the Door 1 reference.
Finding 3 - Medical pressure vessels need structural margin, not just structural compliance
Passing ASME is necessary; it is not sufficient. This chamber was not designed to sit exactly at its allowable limits, it was designed to sit at 38% of allowable PL under hydrotest, with 2.6× margin. That margin is the difference between “compliant” and “trustworthy.” For any vessel that carries patients, trustworthy is the only acceptable standard.
Request the Complete Technical Report
This public case study summarises the stress validation.
- Complete equivalent-stress contour plots for all 4 load cases
- Full primary (PL), primary+secondary (PL+Pb+Q), and general membrane (Pm) comparison tables for every door and every load case
- Stress linearization plots at every maximum-stress location, Door 1, Door 2, Door 3, and general membrane
- Loading and boundary-condition diagrams for each load case
- Hoop stress FEA-vs-hand-calc validation
- Global equilibrium check (applied load vs. support reactions)
- Mesh quality metrics and geometry corrosion-allowance details
- Materials of construction properties at operating temperature