Improvement of Annular Boss Surfacing Method on Pressure Vessel Shell Inner Wall

Literature Overview

The technical paper by Wang Fuchen and Cheng Aiguo, published in Welding (2007, No. 10, pp. 57-58), presents an improved method for surfacing annular bosses on the inner wall of pressure vessel shells. The authors are affiliated with the Equipment Department of Dalian Hydrogenation Company under First Heavy Industries Group Corporation and Shengli Oilfield Administration's Subsurface Operations Third Company. This work addresses a specific and critical challenge in the fabrication of large-scale hydrogenation reactors used in petroleum refining.

Technical Context and Requirements

Large forged-welded hydrogenation reactors are essential equipment in petroleum refining, operating under extreme conditions of high temperature, high hydrogen pressure, and corrosive environments. The internal structure of these reactors often requires annular bosses on the shell inner wall for structural reinforcement, support of internal components, or flow distribution. These bosses must be surfaced with corrosion-resistant alloy material to withstand the harsh operating environment.

Typical Configuration

Conventional Method and Its Limitations

The traditional approach involves using a single surfacing machine to deposit one annular boss at a time, with propane gas flame heating to maintain the shell temperature above 150°C throughout the operation. This method has several significant drawbacks:

1. Extended cycle time - Each boss requires individual surfacing, with the entire shell needing to be maintained at temperature for the duration of each operation.
2. High propane consumption - Continuous flame heating over extended periods results in substantial fuel costs.
3. Thermal efficiency - Heat is applied to the entire shell circumference even though only a localized area is being surfaced.
4. Productivity limitation - Sequential processing of multiple bosses significantly extends total fabrication time.
5. Temperature uniformity - Maintaining consistent 150°C+ temperatures across the entire shell is challenging and energy-intensive.

Improved Method

The improved method addresses these limitations through the following innovations:

Key Improvements

1. Multi-equipment parallel processing - Using multiple surfacing machines simultaneously to process multiple bosses at the same time
2. Localized heating strategy - Applying heat only to the immediate welding area rather than the entire shell circumference
3. Optimized structural design - Redesigning the boss geometry to facilitate surfacing operations
4. Equipment modification - Adapting surfacing equipment for simultaneous multi-position operation

Process Comparison

Technical Analysis of the Improvement

Thermal Management

The shift from full-circumference heating to localized heating represents a fundamental change in thermal strategy. In the conventional method, the entire shell is heated to maintain the base temperature above 150°C, which is necessary to prevent cracking in the HAZ of low-alloy steels commonly used for hydrogenation reactors. However, this approach wastes significant energy heating large areas that are not being actively welded.

The improved localized heating approach recognizes that the critical temperature requirement exists only in the immediate vicinity of the weld. By concentrating heat input to the weld area, the improved method:

• Reduces propane consumption by 60-80%
• Maintains adequate HAZ temperature for crack prevention
• Reduces overall thermal distortion of the shell
• Shortens preheating and post-heating times

Structural Design Optimization

The paper also discusses structural design modifications to the annular bosses that facilitate the surfacing operation. Key design considerations include:

• Boss geometry optimized for accessibility of surfacing equipment
• Reduced wall thickness where possible to minimize heat input requirements
• Strategic placement to allow simultaneous processing without equipment interference
• Design features that minimize the number of passes required per boss

Equipment and Process Integration

The improved method requires coordination of multiple surfacing operations within the confined space of a reactor shell interior. Key equipment considerations include:

Quality Considerations

Despite the process improvements, quality requirements remain stringent:

1. Bond strength - The surfacing layer must achieve full metallurgical bonding with the base material
2. Crack freedom - No cracks in the overlay or HAZ, particularly important for hydrogen service
3. Corrosion resistance - The overlay must provide adequate protection against hydrogen attack and general corrosion
4. Dimensional accuracy - Boss dimensions must meet design specifications after surfacing and machining
5. NDE inspection - Complete non-destructive examination of all surfacing welds is mandatory

Study Insights and Implications

This paper exemplifies how process improvement in welding operations can yield significant economic and productivity benefits. The shift from sequential to parallel processing, combined with optimized thermal management, demonstrates that traditional approaches to welding large components can often be significantly improved through creative engineering. The methodology described here has direct applicability to other large pressure vessel fabrication operations where multiple surfacing operations are required on internal surfaces. The principles of localized heating and parallel processing can be extended to other applications in heavy equipment manufacturing. The paper also highlights the importance of structural design in facilitating manufacturing operations - a concept often overlooked in design but critical to production efficiency. For engineers involved in pressure vessel fabrication, this work provides a practical template for evaluating and improving existing surfacing processes.