Hardfacing Repair of Air Compressor High and Low Pressure Cylinders

Literature Overview

This paper by Li Fangquan, published in Large Nitrogen Fertilizer (2022, Vol. 45, No. 1, pp. 9–12), describes the hardfacing repair of an air compressor's high and low pressure cylinders at a synthetic ammonia plant operated by Zhonghai Petroleum Chemical Co., Ltd. The repair was necessitated by erosion and oxidation corrosion damage that appeared on the cylinder heads and cylinder bodies after an oxygen-enrichment retrofit of the compressor unit. The paper documents a practical engineering solution using manual gas tungsten arc welding (GTAW) to restore the damaged components to serviceable condition within a tight maintenance schedule.

Background and Failure Analysis

The air compressor in question is part of a synthetic ammonia production train, which is a critical process in the production of nitrogen-based fertilizers. The original compressor was designed for a specific operating condition, and the oxygen-enrichment retrofit modified the process to improve efficiency. However, this modification altered the fluid dynamics and chemical environment within the compressor cylinders, leading to accelerated erosion and corrosion damage.

The failure analysis identified two primary damage mechanisms:

1. Fluid erosion: The increased gas velocity and pressure differential after the retrofit caused accelerated erosion of the cylinder surfaces, particularly at flow acceleration zones and around the cylinder heads where gas impingement is most severe.
2. Oxidation corrosion: The elevated oxygen content in the process gas, combined with the operating temperature, promoted oxidation of the cylinder surfaces, leading to pitting and surface degradation.

The severity of the damage posed a significant challenge, as the compressor was required to return to service within the planned maintenance window. The decision to repair rather than replace the cylinders was driven by both economic considerations and the time constraints of the maintenance schedule.

Repair Methodology and Welding Process

The authors selected manual gas tungsten arc welding (GTAW) as the repair process, which is a well-justified choice for this application:

• GTAW advantages: GTAW provides precise control over heat input, minimal dilution of the deposited metal with the base material, and excellent weld quality with low defect rates. These characteristics are essential for repairing thin-walled pressure vessels where excessive heat input could cause distortion or cracking.
• Filler metal selection: The filler metal must be compatible with the cylinder base material and resistant to the erosion and corrosion conditions that caused the original damage. For carbon steel cylinders in synthetic ammonia service, a low-alloy steel filler metal with good oxidation resistance is typically specified.
• Welding parameters: The GTAW parameters must be carefully controlled to minimize heat input while achieving complete fusion. Typical parameters for cylinder repair include: current 100–150 A, voltage 18–22 V, travel speed 5–10 cm/min, with argon shielding gas at 15–20 L/min flow rate.

The repair procedure involved several critical steps:

1. Surface preparation: The eroded and corroded areas were thoroughly ground to remove all damaged material and expose sound base metal. The weld preparation was designed to provide adequate fusion while minimizing the volume of deposited metal.
2. Pre-heat and interpass temperature control: For carbon steel cylinders, a pre-heat temperature of 100–150°C is typically applied to reduce residual stress and prevent cracking. Interpass temperatures should be maintained below 200°C.
3. Multi-pass welding: The repair welds were built up in multiple passes, with each pass carefully controlled to ensure complete fusion and proper weld profile.
4. Post-weld inspection: The repaired areas were inspected using magnetic particle testing (MT) to detect surface cracks and other discontinuities.

Engineering Practice Considerations

The repair of air compressor cylinders presents several important engineering considerations:

• Pressure vessel regulations: Repairs to pressure vessels are governed by strict codes and regulations (e.g., ASME Boiler and Pressure Vessel Code, Section VIII, or national equivalents). The repair must comply with applicable code requirements for weld quality, inspection, and requalification.
• Residual stress management: The high residual stresses introduced during welding can reduce the fatigue life of the repaired component. Post-weld stress relief treatment, if feasible, is strongly recommended.
• Long-term durability: The hardfacing repair addresses the immediate damage but does not eliminate the root cause. The oxygen-enrichment retrofit that caused the accelerated erosion and corrosion will continue to affect the cylinder surfaces. A monitoring program should be established to track the rate of surface degradation after repair.
• Welding procedure qualification: Before performing the repair, a welding procedure qualification (WPQ) should be performed to validate the proposed welding parameters, filler metal, and technique. This ensures that the repair welds will meet the required quality standards.

Study Insights and Reflections

This paper provides a valuable case study in pressure vessel repair engineering, demonstrating the practical application of GTAW for repairing erosion and corrosion damage in a critical process component. The decision to repair rather than replace the cylinders was driven by a combination of technical feasibility, economic considerations, and schedule constraints, which is a common scenario in industrial maintenance.

The paper highlights an important principle in equipment maintenance: when a process modification is implemented, the potential effects on equipment integrity must be thoroughly evaluated. The oxygen-enrichment retrofit, while intended to improve process efficiency, inadvertently created conditions that accelerated the degradation of the compressor cylinders. This underscores the importance of conducting a comprehensive impact assessment before implementing process changes.

The selection of GTAW for this repair is particularly noteworthy, as it demonstrates the versatility of this process for precision repair work on critical components. GTAW's ability to produce high-quality welds with minimal heat input makes it the preferred process for repairing thin-walled pressure vessels and other components where distortion must be minimized.

The broader lesson from this case is that effective maintenance engineering requires not only the ability to execute repairs but also the foresight to anticipate potential damage mechanisms and implement preventive measures. The hardfacing repair of the air compressor cylinders was a successful intervention, but the underlying issue of accelerated erosion and corrosion due to the process modification requires ongoing monitoring and potentially further engineering intervention to ensure long-term equipment reliability.