**1. Introduction: Guangzhou Ethylene Cracking Unit**
The ethylene plant in Guangzhou was designed by TCM Company from Italy, and it uses switch ball valves manufactured by the Italian company ALFA. These valves are specifically installed in the interlock system of the cracking unit, where they serve as critical shut-off valves for fuel gas lines. Each system typically includes more than five such valves, with one example being the fuel gas shut-off valve for the six cracking furnaces. Any leakage from these valves could lead to serious accidents, making their proper function essential for safety.
However, the ALFA switch ball valves have repeatedly shown issues during operation. During the initial testing phase (including purging and airtightness checks), most of the valves were found to have significant leakage, failing to close properly or maintain a tight seal. Although some of these issues can be resolved through maintenance, the root causes often remain unaddressed, leading to recurring problems. The valves are inconsistent in performance, with some working well and others not, which makes them unreliable and poses a long-standing safety risk for the ethylene plant.
**2. Analysis of the ALFA Switch Ball Valve Issues**
Upon investigation, it became clear that the problems stem from the design of the early ALFA valves. The valve actuators are single-cylinder type, but lack a spring pressure adjustment mechanism. The sealing is achieved using PTFE gaskets, and the thickness of the gasket is adjusted to ensure a tight seal. However, this method often results in excessive resistance on the sealing surface, making it difficult to achieve the correct balance.
Moreover, the influence of mechanical and thermal stresses from the process piping further complicates the issue. Even after repairs during overhauls, many valves fail to close properly when put back into service. This indicates that the original design has fundamental flaws that cannot be easily corrected through simple maintenance.
**3. Proposed Transformation Program for ALFA Switch Ball Valves**
Based on the structural shortcomings of the ALFA valves, several transformation options were considered:
- **Option One**: Replacing the valves with new products. While effective, this option would cost between 30,000 to 40,000 yuan per valve, resulting in an estimated investment of 2 to 3 million yuan for all 77 units. However, due to size constraints, neither domestic nor foreign brands like KTM or NESS offer compatible alternatives, making this option impractical.
- **Option Two**: Installing a spring washer in the seat groove to allow automatic sealing adjustment. This approach mimics the design of newer ALFA valves but presents challenges, including damage to the original structure, high technical requirements, and the lack of suitable domestic spring washers. Additionally, imported parts are too expensive, making this option unsuitable at this stage.
- **Option Three**: After extensive testing, it was determined that the main issue was insufficient closing force, which made it difficult to overcome the resistance on the sealing surface. The best solution was to install a pilot valve on the cylinder’s spring side. This allows the instrument air pressure to increase the spring force, ensuring the valve closes completely and maintains a tight seal.
The modified structure is shown in Figure 2. In normal operation, the spring side of the cylinder is connected to the atmosphere, while the other side is connected to the instrument air supply. When the pilot valve is activated, instrument air is introduced to the spring side, increasing the closing force and overcoming the sealing resistance. This ensures full closure and a reliable seal.
The materials required for each valve modification are listed in the table below. The total cost per valve is approximately 2,000 yuan, with the entire project (modifying 77 valves) costing around 150,000 yuan. This approach does not alter the original valve structure, making it a cost-effective and practical solution.
**4. Conclusion**
After implementing the transformation program for all 77 ALFA switch ball valves, the system was successfully put into operation. Over the past two years, the modified valves have performed reliably, eliminating a long-standing safety hazard in the ethylene plant. The project has also generated considerable economic benefits by reducing downtime and maintenance costs.
**Author**: Liu Lizhong (born in 1969), male, from Guangzhou, Guangdong Province. He holds a bachelor's degree in Production Process Automation from Shanghai China Textile University (now Donghua University). Currently, he works at the Instrument Control Center of Sinopec Guangzhou Branch, specializing in DCS, PLC systems, and field instrumentation applications, maintenance, and design management.
Phone: (020) (O) (H)
Address: Workshop III, Instrument Control Center, Sinopec Guangzhou Branch
ZIP Code: 510725
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