Control Valve Leakage Classes and Standards: Classification, Limits, and Applications

Control valves, as critical actuators in process control systems, must not only deliver precise regulating performance but also ensure a certain degree of tight shutoff capability when closed. If leakage exceeds the allowable range, it can result in energy losses, safety hazards, or even environmental pollution. Therefore, clearly understanding the leakage classes and related standards of control valves is essential for engineering design, equipment selection, and operation and maintenance.

Basic Concept of Valve Leakage

Valve leakage refers to the flow of medium through the seat and plug (or disc) of a valve even when it is fully closed. The leakage rate depends on factors such as sealing structure design, manufacturing accuracy, materials of the sealing pair, and the nature of the medium.

It is important to note that some degree of leakage is permissible, provided it remains within the limits defined by international or national standards. Both domestic and international organizations have established classifications for valve leakage levels.

Classification of Leakage Classes

Currently, leakage classes are commonly defined by ANSI/FCI 70-2 (American Control Valve Standard) and GB/T 4213 (Chinese standard equivalent to IEC 60534-4). These standards categorize leakage into six classes (Class I to VI):

1. Class I (Basic Leakage Class)
   • No special leakage testing required.
   • Permits minor leakage.
   • Suitable for non-critical applications.
2. Class II
   • Permissible leakage: about 0.5% of rated valve capacity.
   • Used in general industrial processes.
3. Class III
   • Permissible leakage: about 0.1% of rated capacity.
   • Suitable for control valves requiring moderate sealing performance.
4. Class IV (Metal-to-Metal Shutoff)
   • Permissible leakage: about 0.01% of rated capacity.
   • Most common class for metal-seated valves.
   • Widely applied in energy and process industries.
5. Class V (High Integrity Shutoff)
   • Permissible leakage: 0.0005 ml per minute per inch of port diameter per psi differential.
   • Applied in high-pressure drop services where leakage must be minimized.
6. Class VI (Soft Seat Shutoff, “Bubble-Tight”)
   • Tested by bubble count method; essentially “zero leakage.”
   • Typical for soft-seated valves.
   • Applied in critical services such as flammable or toxic gas control.

Comparison of Standards

• ANSI/FCI 70-2: Widely used in petrochemical and power industries.
• IEC 60534-4 / GB/T 4213: Equivalent international and Chinese standards, covering broader applications.
• JB/T 7387, API 598: Additional references for specific industries.

In practice, metal-seated control valves typically meet Class IV–V, while soft-seated valves usually comply with Class VI requirements.

Application and Selection Guidelines

1. General services (water, steam regulation)
   • Class III or IV metal-seated valves are usually sufficient.
2. High-pressure and high-temperature applications
   • Class V is recommended for enhanced reliability.
3. Hazardous media (flammable or toxic gases)
   • Class VI soft-seated valves should be used to ensure minimal leakage.
4. Balancing safety and cost
   • Higher leakage class means higher manufacturing precision and cost. Selection should match process requirements without excessive overdesign.

Leakage classification of control valves is a critical parameter that directly affects both safety and efficiency of process systems. Selection must be based on the nature of the medium, operating pressure, temperature, and safety requirements. A solid understanding of ANSI/FCI, IEC, and GB standards ensures optimal reliability, cost-effectiveness, and compliance in industrial applications.