Guide to Analyzing External Leakage in Valves
5 min read
1. Hazards of External Leakage in Valves
In industrial production, the external leakage of valves poses significant hazards, especially when dealing with flammable, explosive, toxic, or corrosive media. The dangers of external leakage surpass internal leakage due to several reasons:
- Corrosive Media: Even small undetected leaks can cause corrosion to surrounding equipment over time, leading to potential accidents and equipment failure.
- Toxic or Carcinogenic Media: Small leaks might not cause immediate health issues but can result in long-term health damage to personnel.
- Flammable and Explosive Media: Small leaks can accumulate in localized areas, potentially reaching explosive limits. If ignition conditions are met, it can lead to serious accidents like explosions or fires.
To mitigate these risks, stringent quantitative control standards for external leakage should be established and monitored regularly, especially under harsh working conditions.
2. Common External Leakage Points in Valves
External leakage in valves, also known as fugitive emissions, typically occurs at three main points:
- Live Joints on the Valve Body: These include connections between the valve body and the bonnet or between different sections of the valve body (e.g., two-piece and three-piece ball valves). Some valves, such as butterfly valves and wafer check valves, do not have such connections. Most live joints use flange connections, with a few using threaded or welded connections. While flange and threaded connections theoretically allow for leakage unless sealed by welding, these static seals are generally more manageable and are not the main focus here.
- Valve Stem Packing: The seal between the valve stem and packing experiences relative up-and-down or rotational movement, especially in rising stem control valves, making it prone to leakage. Factors contributing to this include:
- Increased micro-gaps due to relative movement.
- Wear from the relative movement reduces sealing performance over time.
- Limited pressure on the packing to minimize stem wear and operational force, leading to inadequate sealing pressure.
Practice shows that leakage through the valve stem is significantly higher than through static seals on live joints. - Auxiliary Structure Openings: These include drain points, bypass connections, swing check valve lever shafts, and bottom shaft seals in fixed ball valves. These structures are not standard valve configurations and are typically small enough to meet sealing requirements. Therefore, they are not a primary focus. Measures to prevent or mitigate leakage from these structures include:
- Avoiding such structures for sensitive media.
- Redesigning structures to eliminate potential leak points, e.g., using internal lever shafts for check valves.
- Using welded connections where feasible.
- Employing seal welding for auxiliary structures requiring threaded connections.
3. Classification and Application of Valve External Leakage Standards
Common standards for testing valve external leakage include ISO15848-1/2, SPE77/312, and API622:
- ISO15848-1: Specifies type test requirements for valve external leakage.
- ISO15848-2: Specifies product test requirements for valve external leakage.
- SPE77/312: An enterprise standard that builds on ISO15848-1/2 with more operationally practical type and product test requirements.
Conclusion
Effective management of valve external leakage is crucial for industrial safety, especially in environments dealing with hazardous media. Identifying and addressing potential leak points, adhering to stringent testing standards, and employing appropriate sealing techniques are essential steps to ensure safe and reliable valve operation.
FAQs
- What is the primary cause of valve stem leakage?
- The primary cause is the relative movement between the valve stem and packing, which creates micro-gaps and wear, reducing sealing effectiveness.
- How can leakage through live joints on the valve body be minimized?
- Leakage can be minimized using welded seals, static seals, and ensuring proper flange and threaded connection techniques.
- Why are auxiliary structure openings not a primary focus for leakage control?
- These openings are typically small and not part of standard valve configurations, making them easier to seal effectively.
- What are the implications of long-term exposure to leaked toxic media?
- Long-term exposure can result in severe health issues, including chronic diseases and permanent health damage.
- What standards are used for testing valve external leakage?
- The standards include ISO15848-1/2, SPE77/312, and API622, which provide guidelines for both type and product testing requirements.
References
1 API STD 622: Type Testing of Process Valve Packing for Fugitive Emissions