Tank vs Vessel Classification
In industrial engineering, tanks and vessels are distinguished by their design pressure:
- Atmospheric tanks (API 650/620, EN 14015): Design pressure < 15 psig (1 bar)
- Low-pressure vessels: 15 psig to 150 psig
- Pressure vessels (ASME VIII, EN 13445): > 15 psig
- High-pressure vessels: > 1000 psig (special codes)
Atmospheric Storage Tanks (API 650)
Used for bulk storage of water, oil, chemicals at near-atmospheric pressure.
Types
| Type | Description | Typical Sizes |
|---|---|---|
| Fixed cone roof | Simplest, cheapest; for low-volatility liquids | Up to 100,000 m³ |
| External floating roof | Roof floats on liquid; minimal vapor space; for crude/gasoline | Up to 150,000 m³ |
| Internal floating roof | Fixed roof with internal pan; for clean products | Up to 50,000 m³ |
| Horizontal bullet | Cylindrical, for LPG/NGL (higher pressure) | 50-500 m³ |
Design Considerations
- Hydrostatic pressure: At tank bottom, P = ρgh (e.g., 10m water = ~1 bar)
- Wind loads: Must resist overturning; taller tanks need thicker shells or stiffening rings
- Seismic design: API 650 Annex E for earthquake zones
- Foundation settlement: Differential settlement causes shell distortion
- Corrosion allowance: Typically 1-3 mm added to calculated thickness
Where: t = thickness, ρ = liquid density, g = 9.81, H = liquid height, D = tank diameter, S = allowable stress, E = joint efficiency, CA = corrosion allowance.
Pressure Vessels (ASME Section VIII)
For process vessels operating above atmospheric pressure — reactors, separators, columns, heat exchanger shells.
Basic Design Formula — Internal Pressure
| Parameter | Meaning |
|---|---|
| t | Minimum required wall thickness |
| P | Design pressure (Pa or psi) |
| R | Inside radius of shell |
| S | Maximum allowable stress (material-dependent) |
| E | Joint efficiency (1.0 for full RT, 0.85 for spot RT, 0.7 for no RT) |
Common Vessel Configurations
| Type | Application |
|---|---|
| Vertical cylindrical | Process columns, separators, reactors |
| Horizontal cylindrical | Drum separators, heat exchangers, storage bullets |
| Spherical | High-pressure gas storage (LPG, NGL) |
Head Types
| Head Type | Description | Volume | Cost |
|---|---|---|---|
| Hemispherical | Strongest (half of sphere) | Highest | Most expensive |
| 2:1 Semi-elliptical | Most common for process vessels | Good | Moderate |
| Flanged & dished (F&D) | Shallow, low pressure | Lower | Cheapest |
| Flat head | Rectangular openings | Lowest | High stress — thick steel required |
Materials Selection
| Service | Material | Allowable Stress (100°C) |
|---|---|---|
| General carbon steel | SA-516-70 | 138 MPa |
| Low-temp carbon steel | SA-516-60 (normalized) | 118 MPa |
| High temp / hydrogen | SA-387-11 (1¼Cr-½Mo) | 118 MPa |
| Stainless steel | SA-240-304/316 | 138/115 MPa |
| Duplex | 2205 | 260 MPa |
Nozzles and Openings
Vessel shells require reinforcement at nozzle openings (per ASME UG-37). Typical nozzles:
- Process inlets/outlets
- Manways (for internal access, typically 20" minimum)
- Instrument connections (level, pressure, temperature)
- Relief valve connections
- Drain and vent connections
External Loads
Vessel design must consider:
- Weight: Shell + internals + liquid + insulation + ladders/platforms
- Wind: Tall columns act as vertical beams in wind
- Seismic: Horizontal force at center of gravity
- Piping loads: Nozzle loads from thermal expansion
- Vacuum: External pressure can collapse vessels (buckling analysis needed)
Tank Volume and Head
For a vertical cylinder with 2:1 elliptical heads:
Inspection and Testing
- Hydrostatic test: Fill with water at 1.3-1.5 × design pressure
- Pneumatic test: For vessels too large for hydrotest; uses air at lower pressure due to stored energy hazard
- Radiography: Weld inspection; full or spot per design joint efficiency
- In-service inspection: API 510 (pressure vessels), API 653 (tanks); periodic thickness testing
Summary
Atmospheric tanks follow API 650 and are designed primarily for hydrostatic load. Pressure vessels follow ASME VIII Div 1 or Div 2 and require certified design. Key design parameters: design pressure/temperature, material stress, joint efficiency, and corrosion allowance. Always engage qualified pressure vessel engineers — errors can have catastrophic consequences.