Operating pressure
750 psi
Contain hydrostatic pressure with conservative safety margin.
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Mechanical design case study
Pressure Vessel Design
Design, analysis, machining, and validation of a 750 psi aluminum pressure vessel
Pressure 750 psi
Volume 58.7 cc
Seal 219 O-ring
Closure 4 bolts
Overview
A compact aluminum vessel designed around pressure containment, sealing reliability, and machinability.
This project focused on designing, analyzing, machining, and validating a cylindrical aluminum pressure vessel capable of containing water at 750 psig while holding 50-60 cc of fluid.
The final assembly used a 6061-T6 aluminum body, bored internal cavity, centered 0.25 in NPT threaded port, four-bolt end cap, and static O-ring sealing system. The design emphasized manufacturability on manual machines, reliable sealing, and conservative structural safety margins.
Hand calculations were completed before fabrication to validate hoop stress, O-ring groove sizing, fastener preload, joint stiffness, and separation resistance. Physical testing then confirmed the calculations by pressurizing the completed vessel to 750 psi without leakage.
Design Requirements
The vessel had to be safe, seal reliably, and remain practical to machine.
The constraints favored a simple pressure boundary with a controlled sealing interface and predictable fastener layout.
Fluid volume
50-60 cc
Hold the specified internal volume while keeping the vessel compact.
Seal
O-ring
Use a reliable static face seal for pressure containment.
Port
0.25 in NPT
Provide threaded fluid access for testing and filling.
Manufacturing
Manual machines
Keep the geometry practical for a lathe and mill workflow.
Budget
< $100
Stay within the material budget for the project.
Engineering Principles
The design was shaped by pressure loading, seal behavior, and manufacturability.
The final geometry was not the thinnest possible vessel. It was a practical part that could be machined, assembled, and tested repeatedly with acceptable margin.
Pressure vessels are driven by hoop stress.
The first screen check was a thin-wall style hoop stress calculation using the internal pressure, radius, and wall thickness.
Sealing matters as much as wall strength.
The end cap, gland, and O-ring had to work together to hold pressure without leakage or excessive assembly risk.
Geometry can simplify machining.
A cylindrical body reduced turning complexity compared with a flange-heavy concept and made the joint easier to inspect.
Fastener load sharing must be checked.
The separating force on the end cap was sized against bolt preload, joint stiffness, and separation margin.
Design margin can be intentional.
The final part was not optimized for minimum mass. It was sized for manufacturability, robustness, and repeatability.
Validation closes the loop.
The completed vessel was pressure tested, confirming the analytical and manufacturing decisions in the final design.
Design Evolution
The vessel evolved from a flange-heavy concept into a simpler cylindrical body.
The early design used a flange-based closure with a bolt and nut stack-up. That concept achieved the basic pressure vessel architecture, but it added unnecessary turning operations, more radial material removal, and more setup complexity on manual machines.
The design evolved into a uniform-diameter cylindrical body with threaded holes in the vessel. This removed extra lathe work, reduced the bolt count to four, kept the bolt pattern symmetric, and made the O-ring groove easier to machine between the internal bore and the fastener circle.
Bolt count 4
Material 6061-T6
Volume 58.7 cc
Final Design
The final vessel used a symmetric cylindrical body with a compact face-sealed end cap.
The final vessel was machined from aluminum 6061-T6 stock, selected for strength, machinability, and weight. The body has a 2.500 in outer diameter, 1.250 in inner diameter, 0.625 in wall thickness, and a 2.92 in internal cavity length, giving an internal volume of 3.58 in3, or 58.7 cc.
A single 0.25 in NPT port is centered at the top for fluid entry. The end cap sits flush against the vessel body and is clamped by four M6-1 bolts into threaded holes. The O-ring groove sits between the central cavity and bolt pattern, creating a compact static face seal.
Outer diameter 2.500 in
Wall thickness 0.625 in
Engineering Analysis
Hand calculations sized the vessel, the seal, and the bolted joint.
The analytical model combined pressure vessel stress checks, O-ring gland sizing, and bolted-joint separation analysis to validate the geometry before machining.
A
Pressure Vessel Stress Analysis
Thin-wall pressure vessel theory was used as a conservative screening calculation. With internal pressure P = 750 psi, inner radius ri = 0.625 in, and wall thickness t = 0.625 in, the hoop stress calculation gave sigma_hoop = 750 psi.
Material strength 6061-T6, 40 ksi yield and 50 ksi ultimate
Those values produced factors of safety of approximately 53.3 against yield and 66.7 against ultimate failure.
B
O-Ring Design
The final design used a silicone standard size 219 O-ring because the larger cross section fit between the 1.25 in cavity and bolt pattern while producing a practical gland.
Gland checks 19.4 percent compression, 72.5 percent fill
Groove geometry was derived using Parker radial seal design guidance and sized to keep the seal compressed without overfilling the gland.
C
Fastener Analysis
The internal pressure creates a separating force on the end cap that must be resisted by the bolted joint. Using the effective circular area of 1.2217 in2 and design pressure of 750 psi, the total separating force was calculated as 920.38 lbf.
Joint margins 16.8 separation factor and 18.7 overload factor
The joint model included bolt stiffness, member stiffness, preload, and torque, with the final stiffness ratio reported as C = 0.0906.
The calculations intentionally favored a robust, manufacturable geometry.
Those factors are intentionally high. The theoretical minimum wall thickness was far smaller, but a thin section would be impractical to machine, more likely to chatter, and less robust during assembly and pressure testing.
The final wall thickness was chosen for manufacturability and repeatable hardware quality as much as pure stress margin.
Manufacturing
The vessel was built around straightforward lathe and mill operations.
The final geometry was designed around straightforward manual lathe and mill operations. The main lathe work involved preparing the cylindrical stock and boring the internal cavity to the required diameter and depth.
Mill operations focused on drilling and tapping the threaded holes, machining the O-ring groove, and preparing the mating end cap. The simplified cylindrical body removed unnecessary flange turning operations and reduced the number of critical setups.
This manufacturability-driven approach made the pressure vessel easier to machine accurately, easier to assemble, and easier to inspect before testing.
Results
The completed vessel held 750 psi during testing with no observed leakage.
The completed pressure vessel successfully contained pressurized water at 750 psi during testing with no observed leakage. This validated the hand calculations, material selection, O-ring design, fastener design, and manufacturing decisions used throughout the project.
The final design met the project requirements for pressure containment, 50-60 cc capacity, manufacturability, sealing, threaded fluid access, and budget-conscious hardware selection.
Key Takeaways
Manufacturability is a primary design constraint.
The design was simplified so it could be machined, assembled, and inspected reliably on manual equipment.
Sealing systems deserve first-class analysis.
Pressure containment depends on the full closure system, not just on the vessel wall thickness.
Simple geometry can be the best answer.
The cylindrical body reduced setup complexity and made the design easier to inspect and build.
Validation closes the loop.
The pressure test confirmed that the calculations and fabrication choices worked in the real part.
Image Gallery
Extracted report visuals from CAD, evolution sketches, drawings, and the final vessel.
Click any image to open the extracted report asset at full size.