**1. Introduction: Guangzhou Ethylene Cracking Unit**
Designed by the Italian company TCM, the ethylene plant utilizes switch ball valves manufactured by the Italian firm ALFA. These valves are installed in critical interlock systems of the cracking unit, particularly for fuel gas shut-off functions in six cracking furnaces. Given their role in maintaining safety, any leakage could lead to serious accidents. However, these valves have proven to be problematic.
During the initial test runs—such as purging and airtightness testing—it was discovered that most of the valves exhibited significant leakage, failing to close properly or maintain an effective seal. Although repairs could temporarily resolve the issue, the underlying problems were often not fully addressed. As a result, the valves were unreliable, with some functioning well and others not at all. This inconsistency has long been a major safety concern for the ethylene plant.
**2. Analysis of ALFA Switch Ball Valve Issues**
Upon investigation, it was found that the early models of ALFA’s switch ball valves suffered from design flaws. The valve actuators used a single-cylinder mechanism without a spring pressure adjustment feature. The sealing mechanism relied on PTFE gaskets, and the thickness of the gasket was manually adjusted to achieve a proper seal. However, this often led to excessive resistance on the sealing surface, making it difficult to adjust to the right thickness.
Additionally, the stress and thermal expansion from the process piping further complicated the sealing performance. Even after maintenance, many valves failed to close properly once reinstalled, leading to ongoing leakage issues.
**3. Proposed Transformation Program for ALFA Switch Ball Valves**
To address these issues, several transformation options were considered:
- **Option 1:** Replacing the valves with new products. While this would ensure better performance, the cost was extremely high—approximately 30,000 to 40,000 yuan per valve, totaling around 20 to 30 million yuan for 77 units. Moreover, due to size constraints, neither domestic nor foreign alternatives like KTM or NESS valves were suitable, making this option impractical.
- **Option 2:** Installing a spring washer in the seat groove to allow automatic sealing adjustment. Though similar to newer ALFA designs, this approach involved modifying the original structure, which could damage the valve. It also required advanced technical capabilities that the team did not currently possess. Additionally, sourcing suitable spring washers domestically was uncertain, and imported parts were too expensive. This option was deemed risky and not recommended.
- **Option 3:** 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 would allow instrument air pressure to increase the spring force, thereby enhancing the closing force and ensuring a tight seal. The modified structure is shown in Figure 2 below.
**Figure 2: Schematic of the Modified Structure**
The thin lines represent the original components (including redundant solenoid valves), while the bold lines show the added elements. Under normal operation, the spring side of the cylinder is connected to the atmosphere, while the other side is connected to the instrument air system. When instrument air is applied, the valve opens fully. With the pilot valve in place, instrument air is directed to the spring side, increasing the spring force and allowing the other side to vent to the atmosphere. This creates a larger pressure differential, enabling the valve to close tightly and maintain a secure seal.
This transformation required minimal changes to the original valve, keeping the investment low. Each valve renovation cost approximately 2,000 yuan, resulting in a total project cost of around 150,000 yuan for all 77 valves.
**4. Conclusion**
After implementing the transformation on all 77 ALFA switch ball valves, the system was successfully put into operation. Over the past two years, the project has performed exceptionally well, eliminating a long-standing safety hazard in the ethylene plant and delivering significant economic benefits.
**Author:** Liu Lizhong (born in 1969) is a male engineer from Guangzhou, Guangdong Province, holding a bachelor's degree in Production Process Automation from Shanghai China Textile University (now Donghua University). He currently works at the Instrument Control Center of Sinopec Guangzhou Branch, specializing in DCS, PLC systems, and field instrumentation applications, maintenance, and design management.
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Address: Workshop III, Instrument Control Center, Sinopec Guangzhou Branch
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