Pressurized equipment is used in many industrial processes, for example in petrochemical plants, offshore oil rigs, gas storage and control systems. In each case the pressure vessel must be carefully designed to manage with the operating temperatures and pressure. With increasing service demands, quality requirements and safety legislation it has vitally become important for designers to understand the fundamental principles underlying the methodologies of the design standards and codes. Design usually means fitness for service. Pressure vessel designs are intended to reduce all possible means of failures like bursting of vessel wall, buckling, excessive deformation, corrosion fatigue, vibration and damage, tearing at a discontinuity and excessive deformation. Safety is designers responsibility and the designers design pressure vessel to overcome these failures.

Material selection factors:

- Strength
- Corrosion resistance
- Resistance to hydrogen attack
- Fracture toughness
- Fabric ability.

Strength determines the required component strength. The overall strength of the pressure vessel is determined by the yield strength, ultimate tensile strength, creep strength and rupture strength. Corrosion resistance is the most important factor to consider in pressure vessel. Corrosion allowance supply additional thickness to the pressure vessel. Alloying elements provide additional resistance to corrosion. Above 600癋 hydrogen attack causes irreparable damage through component thickness. Fracture toughness is the ability of material to withstand conditions that could cause brittle fracture. Brittle fracture occurs typically at low temperature, can occur below design pressure and no yielding before complete failure.

Conditions required for brittle fracture:

- High enough stress for crack initiation and growth
- Low enough material fracture toughness at temperature
- Critical size defect to act as stress concentration.

Fracture toughness is influenced by many factors like temperature, type and chemistry of steel and manufacturing and fabrication process. Other factors influencing fracture toughness are arc strikes especially over repaired area and stress raisers or scratchers in cold formed thick plate.

Stress is force per unit area that resists loads induced by external force. Pressure vessel components are designed to keep stress within safe operational limits

Maximum allowable stress: Includes safety margin and varies with temperature and material.

Design for external force and compressive stress is also an important factor to consider. Compressive force may be caused by dead weight, wind and internal vacuum. This may cause elastic instability. Vessel maximum allowable working pressure is based on weakest component

- Originally based on new thickness less corrosion allowance
- Later based on actual thickness less future corrosion allowance needed.

Finally after all designs the important step is inspection. Without proper inspection the pressure vessel design is not complete.