How to classify the leakage grades of control valves

As a key executive component in industrial automation control systems, the performance of the control valve directly affects the control accuracy and operational safety of the system. Among many performance indicators, the leakage grade is an extremely important parameter. The classification of leakage grades not only relates to the sealing performance of the control valve but also directly affects the energy-saving effect of the system, environmental pollution control, and production safety. Therefore, understanding and mastering the classification standards of control valve leakage grades is of great significance for selection, installation, and maintenance.

Firstly, the basic concept of control valve leakage

In the closed state, there is no possibility of complete non-leakage between the valve core and the valve seat of the control valve. The term 'leakage' refers to the phenomenon where the fluid continues to flow through the tiny gap between the valve core and the valve seat even when the valve is fully closed. Although this leakage amount is usually small, it must be strictly controlled in certain situations with extremely high sealing requirements (such as high temperature, high pressure, toxic or flammable medium systems).

Secondly, the standard classification of leakage grades

Currently, internationally, the IEC 60534-4 standard formulated by the International Electrotechnical Commission (IEC) and the ISA-75.05.01 standard of the American Instrumentation Association (ISA) are widely used. These two standards provide a detailed classification of the leakage grades of control valves. According to these standards, the leakage grades of control valves are usually divided into six levels, from level I to level VI, with level VI being the highest grade, requiring the minimum leakage amount.

| Leakage Grade | Allowable Leakage Amount (with water as the medium) | Sealing Type |
|----------|-----------------------------|----------|
| I Grade | No Requirement | Non-metallic Sealing |
| II Grade | ≤0.5% Rated Capacity | Metal Sealing |
| III Grade | ≤0.1% Rated Capacity | Metal Sealing |
| IV Grade | ≤0.01% Rated Capacity | Metal Sealing |
| V Grade | ≤0.001% Rated Capacity | Metal Sealing |
| VI Grade | ≤Bubble Grade (minuscule) | Soft Sealing (such as polytetrafluoroethylene)|

Note: Rated capacity refers to the maximum flow rate through the valve under the specified pressure difference.

III. Application Scenes of Different Grades

1. I Grade: Generally used in applications with low sealing requirements, such as ordinary water treatment systems.
2. II to IV Grades: Suitable for most industrial control systems, such as steam, air, water, etc.
3. V Grade: Used in applications with strict leakage control, such as the chemical industry, pharmaceutical industry, etc.
4. VI Grade: Used in high-sealing requirements applications, such as natural gas transmission, toxic medium systems, nuclear power equipment, etc.

IV. Factors Affecting the Leakage Grade

1. Structural Design: The structural form of the valve (such as single seat, double seat, angular valve, etc.) has a direct impact on the leakage amount.
2. Material Selection: Whether the materials of the valve core and the valve seat match and whether they have good wear resistance and corrosion resistance.
3. Manufacturing Precision: The higher the fit precision between the valve core and the valve seat, the smaller the leakage amount.
4. Installation and Maintenance: Improper installation or wear caused by long-term use can also affect the sealing effect.

V. Conclusion

In summary, the leakage grade of the control valve is an important indicator for measuring its sealing performance. In practical engineering applications, the leakage grade should be reasonably selected according to the process flow, medium properties, and safety requirements. At the same time, regular maintenance and inspection of the control valve should be strengthened to ensure stable operation within the design grade, thereby ensuring the safety, efficiency, and environmental protection of the entire system.

By scientifically and reasonably dividing and applying the leakage grades of control valves, not only can the reliability of system operation be improved, but also the maintenance costs can be effectively reduced, enhancing the economic benefits and social responsibility level of the enterprise.