A Comprehensive Guide to Control Valve Leakage Classes
Ensuring Safety, Efficiency, and Compliance in Process Industries
1. Introduction
In the oil and gas, petrochemical, power, and process industries, control valves play a pivotal role in regulating fluid flow, pressure, temperature, and level. An often-overlooked but critical parameter in control valve performance is leakage class—a specification that defines how much leakage is permissible through a valve when it is in the closed position. This blog provides a comprehensive explanation of leakage classes, relevant standards, testing procedures, and how to select the right class for your application.
2. What Is Valve Leakage?
Leakage in control valves refers to the unwanted passage of fluid (gas or liquid) through the valve seat when it is fully closed. While many believe a shut valve stops flow completely, most control valves allow some permissible leakage unless specifically designed to be “bubble tight.” Leakage is influenced by:
- Valve seat design (metal vs soft seat)
- Valve trim condition
- Differential pressure
- Seal wear over time
3. Why Leakage Class Matters
The leakage class helps ensure:
- Safety: Reducing risk in critical services (e.g., toxic, flammable fluids).
- Process control: Preventing upset conditions.
- Regulatory compliance: Meeting environmental regulations (e.g., fugitive emissions).
- Reliability: Minimizing unplanned downtime.
- Cost efficiency: Avoiding energy and product loss.
4. Standards Defining Valve Leakage Classes
Control valve leakage classes are standardized under ANSI/FCI 70-2 (IEC 60534-4) – Control valve seat leakage classes, and API 598 – Valve inspection and testing (used for isolation valves). This blog will primarily focus on ANSI/FCI 70-2, which defines six leakage classes for control valves: Class I to Class VI.
5. Overview of Control Valve Leakage Classes (Class I–VI)
| Leakage Class | Description | Permissible Leakage Rate | Typical Valve Type |
|---|---|---|---|
| Class I | No test required (default) | Unspecified | General-purpose valves |
| Class II | Low leakage | 0.5% of full open capacity | Metal seated valves |
| Class III | Medium leakage | 0.1% of full open capacity | Higher integrity metal seat |
| Class IV | Tight shutoff (standard) | 0.01% of full open capacity | Common in process control |
| Class V | Very tight shutoff | 5 × 10⁻⁴ ml/min per inch per psi | High-pressure service |
| Class VI | Bubble tight shutoff | Based on bubbles/min | Soft seat valves |
6. Detailed Explanation of Each Leakage Class
Class I – No Test Required
- Definition: Intended for valves where leakage is not critical.
- Testing: No seat leakage test is performed.
- Usage: Low-cost, non-critical applications.
Class II – Low Leakage
- Permissible Leakage: ≤ 0.5% of valve's rated full open capacity.
- Test Conditions: With water at normal service pressure.
- Valve Type: Metal-to-metal seated valves with standard construction.
- Application: Low-pressure services where minor leakage is tolerable.
Class III – Moderate Leakage
- Permissible Leakage: ≤ 0.1% of rated capacity.
- Test Fluid: Water, same conditions as Class II.
- Design Requirements: Better seat and guide alignment than Class II.
- Applications: Moderate service where some leakage is allowed but tighter shutoff is preferred.
Class IV – Tight Shutoff
- Permissible Leakage: ≤ 0.01% of rated capacity.
- Valve Type: Precise metal-to-metal seating with lapped or high-integrity seats.
- Testing: With water or air under differential pressure.
- Common Use: Process applications with more stringent requirements.
- Note: This is the default class for most control valves unless otherwise specified.
Class V – Very Tight Shutoff
- Permissible Leakage: ≤ 5 × 10⁻⁴ ml/min of water per inch of orifice per psi differential pressure.
- Application: High-pressure drop services (e.g., steam, condensate).
- Valve Design: Specially engineered metal seats with tight tolerances.
- Usage: Power plants, critical service valves, steam control valves.
Class VI – Bubble Tight Shutoff
- Testing Method: Air or nitrogen at 50–125 psig, leakage is measured by the number of bubbles per minute.
- Application: Gas applications, where zero visible leakage is expected.
- Valve Type: Soft-seated control valves (e.g., PTFE, elastomer seat).
- Permissible Leakage (based on valve size):
Valve Size (in) Max Leakage (bubbles/min) 0.5 0 1.0 2 2.0 4 3.0 6 4.0 8 6.0 12
7. Testing Procedures and Considerations
Common parameters for leakage testing:
- Test Fluid: Water, air, or nitrogen.
- Differential Pressure: Typically 50–100 psig.
- Duration: 1 to 5 minutes.
- Valve Position: Fully closed.
- Measurement Method: Flow meters, bubble counters, graduated cylinders.
Environmental Conditions:
- Clean seat and plug
- Room temperature
- Specified actuator force applied
8. Factors Affecting Seat Leakage
- Seat and plug material
- Differential pressure across the valve
- Actuator thrust
- Trim wear or damage
- Valve size and design
- Operating temperature
Example: A valve with hardened trim in high-temperature steam service may degrade faster, leading to increased leakage even if it was Class IV at installation.
9. Selecting the Right Leakage Class
Key Considerations:
- Fluid Type: For gases, consider Class VI or bubble-tight valves. For liquids, Class IV or V may suffice.
- Toxic/flammable fluids: Use Class V or VI for safety.
- Critical process control: Class IV or higher.
- Shutoff requirement: Class VI for absolute shutoff.
- Seat material: Soft seat → Class VI; Metal seat → Class IV or V.
- Cost vs performance: Higher class → more expensive and maintenance-intensive.
Examples:
- Steam service in power plant: ➤ Use Class V, metal seat with hardened trim.
- Natural gas control valve: ➤ Use Class VI, soft-seat valve with bubble-tight requirement.
- Cooling water valve in non-critical application: ➤ Class II or III may be sufficient.
10. Soft-Seated vs. Metal-Seated Valves
| Feature | Soft-Seated Valve | Metal-Seated Valve |
|---|---|---|
| Seat Leakage Class | Typically Class VI | Class IV or V |
| Seal Material | PTFE, elastomers | Stainless steel, stellite |
| Shutoff Capability | Bubble tight | Minimal leakage |
| Temperature Limit | Lower | Higher |
| Wear Resistance | Lower | Higher |
| Applications | Gas, clean fluids | Steam, abrasive or hot fluids |
11. Limitations and Challenges
- Leakage classes are tested in ideal lab conditions, not real-life process conditions.
- Leakage can increase due to seat wear, erosion, thermal expansion, corrosion.
- Some valves may fail to maintain original leakage class over time.
- Soft seats may provide Class VI initially but degrade faster in aggressive services.
12. Maintenance and Inspection
To maintain valve performance:
- Periodic leakage testing
- Seat and trim inspection
- Actuator calibration
- Replacing worn gaskets and seals
- Keeping valve internals clean
13. Conclusion
Control valve leakage classes play a crucial role in defining the sealing performance of valves in their closed state. From basic Class I to tight Class VI shutoff, each class serves a specific purpose based on the application, medium, pressure, and safety requirements. For process engineers and instrumentation professionals, selecting the appropriate leakage class is not just a matter of compliance—it directly impacts process reliability, plant safety, and operational cost.
14. References
- ANSI/FCI 70-2: Control Valve Seat Leakage Standard
- IEC 60534-4: Industrial Process Control Valves
- API 598: Valve Inspection and Testing
- ISA Handbook of Control Valves
- Emerson, Fisher, Samson, and Spirax Sarco product catalogs
In the oil and gas, petrochemical, power, and process industries, control valves play a pivotal role in regulating fluid flow, pressure, temperature, and level. An often-overlooked but critical parameter in control valve performance is leakage class—a specification that defines how much leakage is permissible through a valve when it is in the closed position.
Understanding control valve leakage classes is vital for instrumentation and control engineers, maintenance professionals, and procurement teams to ensure process safety, efficiency, and environmental compliance. This blog provides a comprehensive explanation of leakage classes, relevant standards, testing procedures, and how to select the right class for your application.
- What Is Valve Leakage?
Leakage in control valves refers to the unwanted passage of fluid (gas or liquid) through the valve seat when it is fully closed. While many believe a shut valve stops flow completely, most control valves allow some permissible leakage unless specifically designed to be “bubble tight.”
Leakage is influenced by:
- Valve seat design (metal vs soft seat)
- Valve trim condition
- Differential pressure
- Seal wear over time
- Why Leakage Class Matters
The leakage class helps ensure:
- Safety – Reducing risk in critical services (e.g., toxic, flammable fluids)
- Process control – Preventing upset conditions
- Regulatory compliance – Meeting environmental regulations (e.g., fugitive emissions)
- Reliability – Minimizing unplanned downtime
- Cost efficiency – Avoiding energy and product loss
- Standards Defining Valve Leakage Classes
Control valve leakage classes are standardized under:
- ANSI/FCI 70-2 (IEC 60534-4) – Control valve seat leakage classes
- API 598 – Valve inspection and testing (used for isolation valves)
This blog will primarily focus on ANSI/FCI 70-2, which defines six leakage classes for control valves: Class I to Class VI.
- Overview of Control Valve Leakage Classes (Class I–VI)
| Leakage Class | Description | Permissible Leakage Rate | Typical Valve Type |
| Class I | No test required (default) | Unspecified | General-purpose valves |
| Class II | Low leakage | 0.5% of full open capacity | Metal seated valves |
| Class III | Medium leakage | 0.1% of full open capacity | Higher integrity metal seat |
| Class IV | Tight shutoff (standard for metal seat) | 0.01% of full open capacity | Common in process control |
| Class V | Very tight shutoff | 5 × 10⁻⁴ ml/min per inch of orifice diameter per psi | High-pressure service |
| Class VI | Bubble tight shutoff | Based on number of bubbles/min at set pressure | Soft seat valves, gas applications |
- Detailed Explanation of Each Leakage Class
Class I – No Test Required
- Definition: Intended for valves where leakage is not critical.
- Testing: No seat leakage test is performed.
- Usage: Low-cost, non-critical applications.
Class II – Low Leakage
- Permissible Leakage:
- ≤ 0.5% of valve’s rated full open capacity.
- Test Conditions: With water at normal service pressure.
- Valve Type: Metal-to-metal seated valves with standard construction.
- Application: Low-pressure services where minor leakage is tolerable.
Class III – Moderate Leakage
- Permissible Leakage:
- ≤ 0.1% of rated capacity.
- Test Fluid: Water, same conditions as Class II.
- Design Requirements: Better seat and guide alignment than Class II.
- Applications: Moderate service where some leakage is allowed but tighter shutoff is preferred.
Class IV – Tight Shutoff
- Permissible Leakage:
- ≤ 0.01% of rated capacity.
- Valve Type: Precise metal-to-metal seating with lapped or high-integrity seats.
- Testing: With water or air under differential pressure.
- Common Use: Process applications with more stringent requirements.
Note: This is the default class for most control valves unless otherwise specified.
Class V – Very Tight Shutoff
- Permissible Leakage:
- ≤ 5 × 10⁻⁴ ml/min of water per inch of orifice per psi differential pressure.
- Application: High-pressure drop services (e.g., steam, condensate).
- Valve Design: Specially engineered metal seats with tight tolerances.
- Usage: Power plants, critical service valves, steam control valves.
Class VI – Bubble Tight Shutoff
- Testing Method: Air or nitrogen at 50–125 psig, leakage is measured by the number of bubbles per minute.
- Permissible Leakage (based on valve size):
| Valve Size (inches) | Max Leakage (bubbles/min) |
| 0.5 | 0 |
| 1.0 | 2 |
| 2.0 | 4 |
| 3.0 | 6 |
| 4.0 | 8 |
| 6.0 | 12 |
- Application: Gas applications, where zero visible leakage is expected.
- Valve Type: Soft-seated control valves (e.g., PTFE, elastomer seat).
- Testing Procedures and Considerations
Common parameters for leakage testing:
- Test Fluid: Water, air, or nitrogen.
- Differential Pressure: Typically 50–100 psig.
- Duration: 1 to 5 minutes.
- Valve Position: Fully closed.
- Measurement Method: Flow meters, bubble counters, graduated cylinders.
Environmental Conditions:
- Clean seat and plug
- Room temperature
- Specified actuator force applied
- Factors Affecting Seat Leakage
- Seat and plug material
- Differential pressure across the valve
- Actuator thrust
- Trim wear or damage
- Valve size and design
- Operating temperature
Example: A valve with hardened trim in high-temperature steam service may degrade faster, leading to increased leakage even if it was Class IV at installation.
- Selecting the Right Leakage Class
Key Considerations:
| Factor | Recommendation |
| Fluid Type | For gases, consider Class VI or bubble-tight valves. For liquids, Class IV or V may suffice. |
| Toxic/flammable fluids | Use Class V or VI for safety. |
| Critical process control | Class IV or higher. |
| Shutoff requirement | Class VI for absolute shutoff. |
| Seat material | Soft seat → Class VI; Metal seat → Class IV or V. |
| Cost vs performance | Higher class → more expensive and maintenance-intensive. |
Examples:
- Steam service in power plant:
➤ Use Class V, metal seat with hardened trim. - Natural gas control valve:
➤ Use Class VI, soft-seat valve with bubble-tight requirement. - Cooling water valve in non-critical application:
➤ Class II or III may be sufficient.
- Soft-Seated vs Metal-Seated Valves and Leakage
| Feature | Soft-Seated Valve | Metal-Seated Valve |
| Seat Leakage Class | Typically Class VI | Class IV or V |
| Seal Material | PTFE, elastomers | Stainless steel, stellite |
| Shutoff Capability | Bubble tight | Minimal leakage |
| Temperature Limit | Lower | Higher |
| Wear Resistance | Lower | Higher |
| Applications | Gas, clean fluids | Steam, abrasive or hot fluids |
- Limitations and Challenges
- Leakage classes are tested in ideal lab conditions, not real-life process conditions.
- Leakage can increase due to seat wear, erosion, thermal expansion, corrosion.
- Some valves may fail to maintain original leakage class over time.
- Soft seats may provide Class VI initially but degrade faster in aggressive services.
- Maintenance and Inspection
To maintain valve performance:
- Periodic leakage testing
- Seat and trim inspection
- Actuator calibration
- Replacing worn gaskets and seals
- Keeping valve internals clean
- Conclusion
Control valve leakage classes play a crucial role in defining the sealing performance of valves in their closed state. From basic Class I to tight Class VI shutoff, each class serves a specific purpose based on the application, medium, pressure, and safety requirements.
For process engineers and instrumentation professionals, selecting the appropriate leakage class is not just a matter of compliance—it directly impacts process reliability, plant safety, and operational cost.
When specifying or evaluating a control valve, always refer to ANSI/FCI 70-2 or IEC 60534-4 standards, understand the nature of the process fluid, and weigh the trade-offs between leakage class, valve type, cost, and maintenance.
Further Reading and References
- ANSI/FCI 70-2: Control Valve Seat Leakage Standard
- IEC 60534-4: Industrial Process Control Valves
- API 598: Valve Inspection and Testing
- ISA Handbook of Control Valves
- Emerson, Fisher, Samson, and Spirax Sarco product catalogs