What Is a Pressure Vessel? Design Basics
A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Pressure vessels are found throughout oil and gas processing, petrochemical, power generation, and chemical plants. They include separators, reactors, columns, drums, heat exchangers, and accumulators.
Pressure vessels operating above 15 psig (1.03 barg) are designed, fabricated, inspected, and stamped in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII (Divisions 1, 2, or 3), which is the most widely used international standard for pressure vessel design.
When Pressure Vessels Are Used
Pressure vessels are required whenever a process demands containment of fluids above atmospheric pressure. Common applications include:
- Separation of oil, gas, and water in production facilities
- Chemical reactions under elevated pressure and temperature
- Storage of compressed gases (air receivers, nitrogen bottles)
- Heat transfer duties (shell-and-tube heat exchangers are pressure vessels)
- Surge and pulsation dampening (accumulators, knock-out drums)
Pressure Vessel Design Specifications
| Parameter | Typical Range / Standard |
|---|---|
| Design code | ASME VIII Div.1 (most common), Div.2 (higher allowable stress), PD 5500 (UK) |
| Design pressure | 15 psig to 10,000+ psig |
| Design temperature | -196 degrees C (cryogenic) to 900+ degrees C |
| Shell materials | SA-516 Gr.70, SA-240 (stainless), SA-387 (alloy) |
| Head types | Ellipsoidal (2:1), torispherical, hemispherical, flat |
| Corrosion allowance | Typically 3 mm (carbon steel), 0 mm (CRA) |
| Radiography | Full (RT1), spot (RT2), or none—per joint category |
| PWHT | Required above certain thresholds per UCS-56 |
| Hydrostatic test | 1.3 x MAWP (Div.1) or 1.25 x MAWP (Div.2) |
| Nameplate | ASME U-stamp with MAWP, temperature, serial number |
Pressure Vessel vs. Storage Tank
| Feature | Pressure Vessel | Storage Tank |
|---|---|---|
| Design code | ASME Section VIII | API 650 (atmospheric), API 620 (low pressure) |
| Pressure range | Above 15 psig | Atmospheric to 15 psig |
| Wall thickness | Calculated by pressure formulas | Often governed by minimum thickness rules |
| Shape | Cylindrical with dished heads | Cylindrical with flat or cone roofs |
| Inspection | ASME U-stamp, authorized inspector | API 653 (in-service), manufacturer QC |
| Typical sizes | Up to ~6m diameter | Up to 100m+ diameter |
Pressure vessels are integral to most oil and gas processing facilities, from wellhead separators in upstream operations to distillation columns and reactors in downstream refining.
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