Zhuojin Pipe Fitting Co., LtdSurfacing of Flow Orifice Plates in Heavy Oil Catalytic Cracking Units
Literature Overview
In heavy oil catalytic cracker (HOC) units, flow orifice plates are used to measure and control the flow of process fluids through various sections of the plant. These orifice plates are subjected to severe operating conditions, including high temperatures (up to 500 °C), abrasive fluid streams, and corrosive environments. To extend service life, the contact surfaces of orifice plates are often protected by surfacing (overlay welding) with wear-resistant and high-temperature alloys. This study note examines the technical challenges, welding procedures, and quality control measures associated with surfacing flow orifice plates in HOC units.
Core Technical Challenges
The surfacing of flow orifice plates presents several unique technical challenges:
- High-temperature wear resistance: The surfacing alloy must maintain its hardness and wear resistance at operating temperatures up to 500 °C, which is significantly higher than typical wear-resistant alloy performance limits.
- Thermal stress management: The orifice plate geometry, with its thin cross-section and sharp edges, is susceptible to cracking during the heating and cooling cycles of surfacing. Preheating and interpass temperature control are critical.
- Dimensional accuracy: The orifice bore diameter must be maintained within tight tolerances to ensure accurate flow measurement. Distortion during surfacing can alter the bore geometry, affecting measurement accuracy.
- Material compatibility: The surfacing alloy must be compatible with the base metal of the orifice plate to avoid cracking, porosity, or other welding defects.
Typical Welding Procedures and Parameters
| Parameter | Recommended Value | Notes |
|---|---|---|
| Base metal | 15CrMo or 12Cr1MoV | Typical orifice plate materials |
| Surfacing alloy | Co-Cr-W (e.g., Stellite 6) | High-temperature wear resistance |
| Welding process | SMAW or GTAW | For precise control |
| Electrode type | Ni-based or Co-based | For high-temperature service |
| Preheat temperature | 150–250 °C | To reduce thermal stress |
| Interpass temperature | <250 °C | To prevent excessive softening |
| Heat input | 0.5–1.5 kJ/mm | Low heat input to minimize distortion |
| Surfacing layers | 2–3 passes | For adequate build-up |
| Post-weld heat treatment | 600 °C × 2h | To relieve residual stress |
Quality Control and Defect Prevention
The quality of the surfacing is critical to the performance and service life of the orifice plate. Key quality control measures include:
- Visual inspection: Examination of the surfacing for cracks, porosity, undercut, and excessive reinforcement.
- Penetrant testing (PT): Detection of surface-breaking cracks and defects.
- Ultrasonic testing (UT): Detection of subsurface defects, including lack of fusion and porosity.
- Hardness testing: Verification that the surfacing alloy has achieved the required hardness (typically 35–45 HRC for Co-Cr-W alloys).
- Dimensional inspection: Measurement of the orifice bore diameter and plate thickness to ensure compliance with specifications.
Common defects and their countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | Excessive thermal stress | Preheating, low heat input |
| Porosity | Inadequate shielding | Proper gas flow, clean surfaces |
| Lack of fusion | Insufficient penetration | Increase current, proper technique |
| Excessive dilution | High heat input | Reduce current, use low-dilution electrode |
| Distortion | Uneven heating | Symmetrical welding, clamping |
Engineering Practice and Case Study
A practical case involves the surfacing of a critical velocity nozzle orifice plate operating at 500 °C in an HOC unit. The original orifice plate, made of 15CrMo steel, experienced rapid wear after only 6 months of service. To extend service life, the contact surface was surfaced with a Co-Cr-W alloy (Stellite 6 equivalent) using GTAW with a Co-based filler wire. The surfacing was performed in three passes, with careful control of heat input and interpass temperature. Post-weld heat treatment at 600 °C for 2 hours was applied to relieve residual stresses. The surfaced orifice plate achieved a service life of over 24 months, a fourfold improvement over the uncoated condition.
Key Questions and Reflections
A significant question is the optimal selection of surfacing alloy for specific operating conditions. While Co-Cr-W alloys offer excellent high-temperature wear resistance, they are expensive and difficult to weld. Alternative alloys, such as Ni-based or Fe-Cr-C alloys, may offer adequate performance at lower cost, but require careful evaluation of their performance at the specific operating temperature and flow conditions.
Another consideration is the effect of surfacing on the flow characteristics of the orifice plate. The surfacing adds material to the bore surface, which can alter the flow profile and affect measurement accuracy. This effect must be accounted for in the calibration of the flow measurement system.
Study Insights and Implications
The literature underscores that the surfacing of flow orifice plates is a critical maintenance activity that requires careful attention to welding procedure, material selection, and quality control. For engineers involved in HOC unit maintenance, the development of standardized surfacing procedures, including qualified welding procedures and rigorous inspection protocols, is essential for ensuring reliable service life and accurate flow measurement. The integration of material science knowledge with practical welding expertise is key to successful surfacing operations.