Introduction to Pressure Vessels

Introduction to Pressure Vessels: Types, Uses, Design Considerations & Codes

A pressure vessel is any enclosed container designed to hold fluids—whether liquids or gases—at a pressure significantly different from the ambient pressure. These vessels play a crucial role in various industries, including chemical processing, power generation, oil and gas, and manufacturing. They are engineered to withstand either internal or external pressure while ensuring safety, efficiency, and durability.

One of the most common types of pressure vessels is the cylindrical vessel, which is typically designed using the principles of thin-walled cylinder theory. These vessels can be oriented either horizontally or vertically, depending on the application and space constraints.

                                     

Key Factors in Pressure Vessel Design

Before constructing a pressure vessel, engineers must consider several critical factors to determine the most suitable type for its intended function. Some of the main considerations include:

1. Purpose and Function: Whether the vessel is meant for fluid storage or a process-related operation.
2. Operating Pressure and Temperature: The vessel must be able to handle specific pressure and temperature conditions without failure.
3. Type of Fluid Stored: The nature of the fluid (liquid, gas, corrosive, flammable, etc.) determines the material selection and safety requirements.
4. Location and Environmental Conditions: Outdoor vs. indoor placement, exposure to extreme temperatures, and potential seismic or wind loads.
5. Required Storage Capacity: The volume of fluid the vessel needs to hold impacts its shape, size, and structural integrity.

Classification of Pressure Vessels

Pressure vessels can be categorized based on their function, shape, and operating conditions. The two primary classifications are:

1. Based on Function

• Storage Vessels: These are used to store fluids under pressure and are commonly found in industries such as petroleum refining, chemical processing, and water treatment.
• Process Vessels: These are used in industrial processes where fluids undergo changes in pressure, temperature, or chemical composition. Examples include heat exchangers, distillation columns, reactors, and separators.

2. Based on Shape

• Cylindrical Pressure Vessels: These are the most widely used due to their ease of manufacturing and cost-effectiveness. They can be oriented horizontally or vertically and may include additional features like heating or cooling coils.
• Spherical Pressure Vessels: These are primarily used for storing large volumes of pressurized fluids. Their spherical shape is ideal for distributing internal pressure evenly, reducing stress concentrations and material usage. However, they are more expensive to manufacture due to complex fabrication and welding requirements.

Why Spherical Vessels for Storage?

Spherical pressure vessels are often preferred for storing gases and liquids under high pressure because their shape naturally distributes stress evenly, minimizing weak points. This makes them safer and more efficient for long-term storage. However, their production cost is significantly higher compared to cylindrical vessels, which is why cylindrical tanks remain the more common choice in industrial applications.

Pressure Vessel Parts and Geometry Definition

Pressure vessels come in various shapes and orientations, with the most common being vertical and horizontal vessels. Regardless of the type, each pressure vessel consists of several critical components that contribute to its structural integrity and functionality.

Main Components of a Pressure Vessel

1. Shell: The main cylindrical body that contains the pressurized fluid.
2. Heads (End Closures): These are the curved sections at both ends of the vessel that close the cylinder. Common types include ellipsoidal, hemispherical, and torispherical heads.
3. Welding Line: The point where the shell and heads are joined through welding. This is a crucial area for structural integrity.
4. Tangent Line: The location where the curvature of the head begins, marking the transition between the cylindrical shell and the end closure.
   
   
   
   
5. Nozzles and Openings: These provide entry and exit points for fluids, as well as access for maintenance and instrumentation.
6. Supports: Depending on whether the vessel is horizontal or vertical, supports such as saddles, skirts, or legs are used to provide stability.

Understanding Pressure Vessel Geometry

When defining the geometry of a pressure vessel, two key measurements are taken into account:

• Inner Diameter: This is a critical measurement because it directly relates to the vessel’s capacity and the process requirements.
• Distance Between Tangent Lines: This refers to the length of the vessel, measured between the points where the head curvature begins. Unlike the overall length, this measurement is independent of the head fabrication method, making it a more standardized reference for vessel design.

Importance of the Tangent Line in Vessel Design

The tangent line is significant because it marks the transition between the cylindrical shell and the curved head. Since different manufacturing processes can result in heads with slight variations, using the distance between tangent lines ensures consistency in the vessel’s length, regardless of the specific head type.

In most cases, the welding line and tangent line do not coincide, as the heads often include a straight skirt section before the curvature begins. This minor detail is crucial for manufacturing precision and structural analysis.

Manufacturing Sequence of a Pressure Vessel

The production of a pressure vessel follows a structured sequence to ensure safety, durability, and compliance with industrial standards. The typical steps include:

Material Selection: Choosing high-strength metals capable of withstanding the intended pressure and temperature conditions.

ASME Codes for Pressure Vessels – Detailed Explanation

Pressure vessels are governed by the ASME Boiler and Pressure Vessel Code (BPVC), specifically Section VIII, which ensures the safety, design, fabrication, inspection, and testing of pressure vessels.

1. ASME BPVC Section VIII – Pressure Vessel Code

This section is divided into three divisions, each covering different types of pressure vessels based on design complexity, pressure levels, and safety requirements.

1.1 Division 1 – Standard Pressure Vessel Design

• Most widely used division for designing pressure vessels.
• Follows simpler design rules with a focus on safety factors.
• Uses Allowable Stress Design (ASD) instead of advanced stress analysis.
• Application: Low to medium-pressure vessels in industries like oil & gas, refineries, and chemical plants.

Key Features:

• Design by Rules (DBR): Simple calculations based on formulas.
• Factor of Safety: Higher than Division 2 for added safety.
• Testing: Requires hydrostatic or pneumatic testing.

1.2 Division 2 – Alternative Rules for Pressure Vessels

• Used for higher-pressure applications or when weight savings are critical.
• Uses Design by Analysis (DBA) along with Finite Element Analysis (FEA).
• Lower safety factor (compared to Division 1) but more accurate stress calculations.
• Application: High-pressure vessels in nuclear, aerospace, and offshore industries.

Key Features:

• Advanced Material Strength Calculations: Uses higher allowable stresses.
• More Rigorous Inspection & Testing: Requires non-destructive testing (NDT).
• Fatigue Analysis: Includes cyclic loading and fatigue life assessment.

1.3 Division 3 – High-Pressure Vessels

• Designed for extreme high-pressure applications (above 10,000 psi or 69 MPa).
• Used in rocket propulsion, deep-sea research, and high-pressure chemical reactors.
• Requires advanced FEA methods and specialized materials.

Key Features:

• Higher Strength Materials & Weld Design.
• Strict Quality Control & Inspection.
• Extensive FEA and Computational Stress Analysis.

2. Related ASME Codes for Pressure Vessels

Apart from Section VIII, several other ASME codes apply to pressure vessels:

3. Key Design Considerations in ASME Section VIII

1. Material Selection – Must comply with ASME Section II standards.
2. Pressure Design – Calculations include internal/external pressure, wind, and seismic loads.
3. Thickness Calculation – Based on operating pressure, temperature, and corrosion allowance.
4. Welding & Fabrication – Follows Section IX for welder qualification and procedures.
5. Inspection & Testing – Hydrostatic, pneumatic, and NDE testing per Section V.
6. Marking & Certification – ASME U-Stamp required for compliance.