ANSI/FCI 70-2 Leakage Classes for Control Valves

In industrial process control systems, the selection of control valves directly affects the safety and economic performance of a facility. Many engineers focus primarily on valve body material, pressure rating, and flow coefficient during selection, yet often overlook a critical parameter—valve seat leakage class. If the selection specifications do not clearly define the allowable leakage, the system may experience fluid leakage, product contamination, or even safety incidents during operation. From an engineering application perspective, this article provides a systematic introduction to the ANSI/FCI 70-2 leakage class standard and its coordination with the IEC 60534 flow calculation standard, helping technical personnel establish a comprehensive control valve selection methodology.

Before delving into specific leakage classes, it is necessary to understand why a standardized leakage class system is required. From an operator’s intuition, a control valve should completely block fluid flow upon receiving a close command. However, in practical engineering, absolute zero leakage is rarely achievable due to factors such as manufacturing precision, sealing structures, and material properties. Moreover, different operating conditions have vastly different leakage tolerances. Therefore, a standardized method is needed to specify the maximum allowable leakage for various applications.

On industrial sites, operators often assume that fluid flow has completely stopped once a valve is closed. Mechanically, however, “closed” does not mean absolute zero leakage. For example, a small leakage in a cooling water bypass system may be acceptable, whereas even minimal leakage of toxic chemicals, high-pressure steam, or flammable media could result in batch scrapping, equipment damage, or severe safety hazards. Thus, the engineering field requires a standardized, quantifiable method to define maximum permissible leakage under different operating scenarios.

The ANSI/FCI 70-2 standard, jointly developed by the American National Standards Institute (ANSI) and the Fluid Control Institute (FCI), addresses this need. The standard establishes uniform test methods and allowable leakage rates for control valves, providing manufacturers and users with clear acceptance criteria.

Before introducing each leakage class, it is important to understand the overall framework of ANSI/FCI 70-2. The standard defines six classes, from Class I to Class VI. Higher classes impose stricter sealing requirements. It is important to note that these classes represent leakage levels achievable by new valves under standardized factory test conditions—they do not guarantee long-term performance after years of operation. The following sections systematically describe the technical requirements and application scenarios for each class.

The six leakage classes in ANSI/FCI 70-2 range from Class I to Class VI, with increasingly strict requirements for shutoff performance. Importantly, these ratings only reflect the leakage achievable under factory-standard testing for new valves, not the ability to maintain the same sealing performance after years of exposure to corrosion, erosion, or wear.

Each of the six leakage classes corresponds to specific allowable leakage limits and suitable operating conditions. From the minimally regulated Class I to the bubble-tight Class VI, each plays a role in industrial applications. The following section introduces these classes in ascending order.

Class I is nearly obsolete in modern industrial applications. It does not require formal leakage testing, and allowable leakage is negotiated between user and manufacturer. Therefore, it rarely appears in current project specifications.

Class II permits a maximum leakage of 0.5% of the valve’s rated flow. This class is typically used for standard double-seated globe valves or balanced single-seated valves with metal seats. Due to thermal expansion differences and structural limitations, such valves cannot achieve strict shutoff, making them suitable for applications where tight closure is not critical or for continuously throttled systems rarely operating in fully closed positions.

Class III reduces the maximum leakage to 0.1% of rated flow. Achieving this requires higher seat grinding precision and tighter manufacturing tolerances, often implemented via precision-ground seats or spring-loaded sealing mechanisms. Class III is commonly applied to conventional globe control valves requiring moderate shutoff capability.

Class IV is the most widely used standard in industrial control valves and serves as the industry benchmark for metal-to-metal single-seated valves. It allows a maximum leakage of 0.01% of rated flow. Valves achieving Class IV typically require high-precision machining and sufficient actuator closing force. As such, Class IV is often the default choice for most industrial control valves, applied across water treatment, petrochemical, public utilities, and general process control systems.

Class V represents a substantial increase in sealing requirements. Its allowable leakage is no longer expressed as a percentage of rated flow but is defined by a formula: 0.0005 milliliters of water per minute per inch of seat diameter per psi of pressure differential.

Class V is primarily used in high-pressure steam throttling, high ΔP applications, and severe-service control valves. Excessive leakage at high velocity can cause “wire drawing” erosion between the seat and plug, quickly damaging the sealing surfaces. Achieving Class V typically requires precision grinding and large actuator closing forces, often exceeding 100 lb per linear inch of seat circumference. These applications include boiler feedwater systems and other high-demand control valve scenarios where soft seals would fail under high temperature or erosive flow.

Class VI, commonly referred to as “bubble-tight,” is the strictest leakage class in ANSI/FCI 70-2. Testing is performed using air or nitrogen, and leakage is measured by the number of bubbles generated per minute.

Achieving Class VI usually requires elastomeric or PTFE soft seats rather than traditional metal-to-metal sealing. Materials such as PTFE, PEEK, and Viton are commonly used, making this class suitable for toxic media isolation, environmental discharge control, fuel gas systems, oxygen systems, and safety interlocks requiring absolute shutoff. Soft-seated materials are generally temperature-limited, typically not exceeding 204°C (400°F).

Leakage testing varies significantly between classes. To ensure comparability across manufacturers, ANSI/FCI 70-2 specifies test media, pressure, and evaluation methods for each class. The following sections describe common testing conditions for Classes II–IV and stricter requirements for Classes V and VI.

For Classes II, III, and IV, the test medium can be air or water, typically at 10°C–52°C. Test pressure is either 50 psig or the maximum operating pressure, whichever is lower. After testing, the actual leakage is compared to the allowed percentage to determine compliance.

Class V tests use water at the maximum design differential pressure. Due to the extremely low allowable leakage, testing requires higher precision and longer duration.

Class VI tests use air or nitrogen, with the gas introduced into water in a standard test apparatus. Leakage is evaluated by counting bubbles per minute, demanding higher standards for airtight test setups and operational rigor.

Leakage class primarily addresses sealing performance in the closed state, while valve flow capacity in the open state is equally important. IEC 60534 addresses this through Cv calculations, assisting engineers in valve sizing. Leakage class and flow capacity are interrelated and must be considered together.

IEC 60534-2-1 and ANSI/ISA-75.01.01 use essentially equivalent methods for Cv-based sizing. Correct sizing and Cv calculation are critical because even a valve meeting leakage requirements may perform poorly if undersized or oversized, affecting control precision and service life.

Oversized valves operate near the closed position, reducing control resolution and causing local high-velocity erosion, ultimately damaging the seat and exceeding leakage limits. Undersized valves cannot meet system flow demand, creating process bottlenecks. IEC 60534 considers fluid density, pressure drop, vapor pressure, and gas compressibility to optimize valve operation, ensuring accuracy and longevity.

Control valves and isolation valves serve different functions and have distinct leakage standards. Confusing them may lead to severe safety risks. API 598 governs isolation valves, often requiring zero visible leakage for soft seats and extremely low leakage for metal seats, while ANSI/FCI 70-2 applies to control valves, allowing higher leakage due to frequent operation and seat wear.

Unless specifically designed for tight shutoff (TSO), control valves should not substitute for emergency shutdown (ESD) or safety isolation valves. Using ordinary Class IV control valves in critical fuel gas or flammable systems could lead to gas accumulation, burner trips, or product contamination.

Valve sealing depends not only on body, seat, and plug design but also on actuator closing force. Insufficient force prevents proper seat compression, making even fully compliant valves fail leakage tests. For Class IV metal seats, 20–40 lb per inch of seat circumference is typical; Class V may require over 100 lb/in. Actuator sizing is therefore critical alongside seat design.

Even correctly selected and installed valves may leak over time. Systematic diagnosis identifies causes for timely remediation. Common reasons include misaligned valve zero points, seat or plug surface damage, high-velocity wire drawing erosion, or debris such as welding slag, metal chips, or sealing tape residues.

Metal-seated valves can be refurbished by fine-grit lapping to restore mating surfaces. Routine inspection, maintenance, and operational management extend valve life and maintain specified leakage performance.

Selecting an appropriate leakage class balances safety and cost. A decision framework based on media hazard and operating conditions helps:

• By Media Hazard: Critical or toxic fluids require Class VI with compatible soft seals. High-pressure or erosive media require Class V for metal seats.
• By Operating Conditions: Typical industrial loops (cooling water, low-pressure gases, lubricants) are suited to Class IV for cost-effectiveness and reliability. Bypass or continuously flowing lines may only require Class II or III.
• Economic Considerations: Over-specifying Class VI increases cost due to expensive, wear-prone soft seats; under-specifying in critical applications compromises safety. Correct leakage class selection ensures safety, reliability, and economic efficiency.

Control valve selection should consider not only Cv, body material, and pressure rating but also leakage class and IEC 60534 flow sizing. ANSI/FCI 70-2 provides six clear leakage classes as quantitative guidance, while IEC 60534 ensures proper valve sizing. Accurate determination of both valve size and leakage class guarantees safe, reliable, and cost-effective operation over the valve lifecycle. Engineers must define leakage class during design, considering media characteristics, operating conditions, and safety regulations to avoid operational risks and economic losses.