Key welding techniques for pressure vessel nozzles and reinforcement pads

In the design and fabrication of pressure vessels, nozzle-to-shell and reinforcement pad welding play a critical role in ensuring structural integrity and long-term operational safety. These welded joints are subjected to complex loading conditions, and any compromise in their execution may lead to premature failure or safety incidents. This article explores the key welding techniques, structural considerations, and quality control methods essential for pressure vessel nozzle and pad welding.

pressure vessels

Welded Joint Types: Fillet Welds and Lap Welds

In pressure vessel manufacturing, fillet welds and lap joints are the two primary types used to connect nozzles to vessel shells and reinforcement pads. The selection and design of these welds must consider multiple factors:

• Operating pressure and temperature
• Process medium characteristics (e.g. corrosivity, phase state)
• Base material properties
• Load distribution and stress concentration

The weld geometry and penetration depth significantly influence the sealing performance and structural reliability of the vessel. Special attention must be paid to groove shape, weld throat thickness, and welding accessibility during both design and fabrication stages.

Insertion-Type Nozzle Welding Techniques

For small-diameter nozzles on low- to medium-pressure vessels, insertion-type nozzle welding is commonly adopted. This design keeps the insertion gap within ≤3 mm, ensuring proper fit-up and reduced misalignment.

Typical structures include:

• Single-side fillet weld without groove: Often used in non-reinforced, low-pressure systems
• Double-sided weld without groove: Provides both sealing and partial reinforcement

Single-side beveled groove with full penetration: Recommended for low-temperature applications, ensuring high toughness and crack resistance

These configurations are simple, economical, and suitable for moderate operating conditions, but must be evaluated carefully when applied to cyclic or corrosive environments.

Special Considerations for Large Nozzles and Thick-Wall Vessels

When dealing with large-diameter nozzles and thick-walled pressure vessels, standard insertion welds may be inadequate. In such cases, U-type grooves are recommended for improved weld quality and accessibility.

Key design parameters include:

• Groove angle between 45° and 50°
• Single-side bevel for better weldability
• Rounded transition at pipe end: radius ≤ wall thickness/4 and ≤ 19 mm

This approach ensures sufficient weld penetration and strength while reducing stress concentration at the joint.

Welding Techniques for Reinforcement Pads

Reinforcement pads are used to compensate for the reduced wall thickness around nozzle penetrations. Their welding and sealing must be executed precisely to achieve three-layer protection:

• Inner side fillet weld with wide gap and small groove angle for base sealing
• Root weld with complete penetration to ensure structural continuity
• Outer side lap weld in a step-lap configuration for extra reinforcement and sealing

These welds must be free from cracks, incomplete fusion, or undercuts, as they serve both a sealing and load-distributing function.

Pressure testing of pipeline product

Inspection and Testing Methods

To guarantee the quality of these critical welds, multiple quality verification techniques are employed:

• Pneumatic pressure testing: Compressed air (0.4–0.5 MPa) is introduced through an M10 test port to check for leaks
• Soap bubble leak test: Simultaneous inspection of both inner and outer welds
• Sealing with threaded plugs: After inspection, test holes are closed with sealant-coated plugs

These methods ensure real-time detection of defects and minimize the risk of in-service leakage.

Critical Parameters and Engineering Guidelines

Design and fabrication must conform to specific dimensional and performance criteria, such as:

• Pad height ≥ 1/2 of nozzle wall thickness and ≥ 6 mm
• Lap weld height δ: If δ ≤ 8 mm, use δ; if δ > 8 mm, use 0.7δ
• Inspection pressure: 0.4–0.5 MPa
• Pipe-end fillet radius: ≤ δ/4 and ≤ 19 mm

These guidelines help standardize fabrication while ensuring compliance with relevant codes such as ASME BPVC or EN 13445.

Practical Engineering Advice

From a practical standpoint, successful nozzle and pad welding goes beyond just following standards:

• Detailed welding drawings are mandatory for critical nodes
• For dissimilar metal welding, post-weld heat treatment (PWHT) requirements must be clearly specified
• In fatigue-prone applications, opt for integral reinforcement instead of separate pads
• In corrosive environments, avoid designs prone to crevice corrosion

Welding design is essentially an exercise in stress optimization. The goal is to achieve a balance between manufacturing feasibility and structural reliability.

Welded Safety Starts at the Design Table

The safety of a pressure vessel over its lifecycle begins with intelligent welding design and precise execution. At Haihao Group, we advocate the development of a welding procedure qualification database, coupled with finite element analysis (FEA) for special joints. These tools support engineers in making informed decisions and ensuring each weld contributes to the integrity and longevity of the pressure vessel.

Whether you’re dealing with high-pressure reactors or cryogenic LNG tanks, proper nozzle and reinforcement pad welding is not optional—it’s essential. Email:sales@haihaogroup.com