Valve Leakage Classification Explained (ANSI/FCI 70-2 Class IV vs Class VI)

Quick Answer: ANSI/FCI 70-2 is the standard that classifies control valve seat leakage into six classes (I through VI). Class I has no leakage test requirement. Class IV allows a maximum leakage of 0.01% of rated valve capacity – suitable for most general industrial service. Class VI (also called “bubble-tight”) allows zero visible bubbles during a specified test period using air or nitrogen at low pressure – required for critical applications like toxic gas isolation, oxygen service, and safety shutdown valves. The higher the class number, the tighter the shut-off.

If you have ever received a valve spec sheet from an EPC contractor, you have almost certainly seen a line that reads something like “Leakage Class: FCI Class IV” or “Shut-off: Class V per ANSI/FCI 70-2.” And if you are like most procurement engineers encountering this for the first time, your immediate question was probably: “What do these classes actually mean, and does it really matter whether I specify Class IV or Class VI?“

The short answer: yes, it matters enormously. Specifying the wrong leakage class can mean the difference between a valve that performs perfectly and one that constantly leaks, wastes product, creates safety hazards, or fails environmental compliance.

This guide breaks down everything you need to know – in plain English, with practical examples and product recommendations from Cair Euromatic Automation, one of India’s most experienced valve manufacturers.

What Is ANSI/FCI 70-2?

ANSI/FCI 70-2 (titled “Control Valve Seat Leakage“) is a standard published by the Fluid Controls Institute (FCI), an American trade association. It is also harmonised with the international standard IEC 60534-4. Together, these standards define:

1. The leakage classes (how much leakage is acceptable)
2. The test procedures (how the valve is tested)
3. The test media (water or air/nitrogen, depending on the class)
4. The test pressure (maximum rated differential pressure or a specified lower pressure)

The standard applies to control valves and isolation valves in industrial process applications. It does not apply to safety relief valves (which follow API 527) or pipeline valves (which follow API 598), although many procurement engineers apply FCI 70-2 leakage classes as a specification tool across all valve types.

The Six Leakage Classes – Explained One by One

Here is what each class actually means, with the allowable leakage rates. Pay close attention to the test conditions – they matter as much as the numbers.

Class I – No Test Required

Class I simply means no production leak test is required. The manufacturer guarantees that the valve is built to a design that provides some level of shut-off, but there is no quantified leakage limit and no test to verify it.

When is this acceptable? Almost never in modern practice. Class I is essentially a legacy classification. You will rarely see it specified today.

Class II – Metal Seat, 0.5% of Rated Capacity

• Maximum allowable leakage: 0.5% of rated valve capacity (Cv)
• Test medium: Air or water at 45–60 psig (3–4 bar)
• Typical valve types: Single-seat globe valves with metal seats

Class II allows a relatively large amount of leakage. It is acceptable for non-critical throttling applications where some bypass leakage during shut-off is tolerable – for example, cooling water temperature control where a small amount of bypass flow does not affect the process.

Class III – Metal Seat, 0.1% of Rated Capacity

• Maximum allowable leakage: 0.1% of rated valve capacity
• Test medium: Air or water at 45–60 psig
• Typical valve types: Balanced-plug globe valves, some double-seated globe valves

Class III is five times tighter than Class II. It suits applications where moderate leakage control is needed but bubble-tight shut-off is not critical.

Class IV – Metal Seat, 0.01% of Rated Capacity

• Maximum allowable leakage: 0.01% of rated valve capacity
• Test medium: Air or water at maximum rated differential pressure
• Typical valve types: Single-seated globe valves with precision-lapped metal seats, high-performance butterfly valves

Quick Answer – What is Class IV? Class IV allows 0.01% of rated capacity as leakage – meaning for a valve with a Cv of 100, the maximum allowable leakage at full differential pressure is 0.01 Cv = 0.001 gallons per minute (about 4 ml/min). This is the most commonly specified class for general industrial control valves in oil & gas, petrochemical, and power plant applications.

Class IV is what most engineers mean when they say “tight shut-off” in general industrial service. The globe-type control valve with precision-lapped metal plug and seat is the classic valve design that achieves Class IV. The motorised control valve with electrical actuator from Cair delivers this performance with automated position control.

High-performance butterfly valves with metal seats also typically achieve Class IV, making them a cost-effective alternative to globe valves for larger line sizes. The motorised high-performance butterfly valve combines this shut-off capability with electric actuation for BMS/DCS integration.

Class V – Metal Seat, 5 × 10⁻⁴ ml/min per inch of seat diameter

• Maximum allowable leakage: 0.0005 ml per minute per inch of port diameter, per psi differential
• Test medium: Water at maximum rated differential pressure
• Typical valve types: Single-seated globe valves with stellite-lapped seats, triple-offset butterfly valves, trunnion-mounted ball valves with metal seats

Class V is dramatically tighter than Class IV. To put it in perspective: for a 4-inch valve at 100 psi differential, Class V allows roughly 0.1 ml/min of leakage – about two drops per minute.

This class is specified for applications where leakage has direct economic or safety consequences – steam isolation, hydrocarbon shut-off, compressor bypass, and turbine inlet valves.

The triple-offset butterfly valve is one of the most elegant valve designs for achieving Class V. Its cone-in-cone seating geometry provides metal-to-metal bi-directional shut-off without the friction and wear associated with conventional butterfly valves. The motorised triple-offset butterfly valve adds automated operation for critical process control.

For ball valve applications, the metal-seated ball valve for high temperature achieves Class V through precision-ground metal seat rings – essential for high-temperature services where soft seats cannot survive.

The motorised high-pressure globe control valve with stellite-hardened trim also delivers Class V shut-off for high-pressure throttling applications.

Class VI – Soft Seat, Bubble-Tight (Zero Visible Leakage)

• Maximum allowable leakage: Zero visible bubbles during a specified observation period (test duration depends on valve size – 15 seconds for ≤1 inch, up to 2 minutes for 8-inch and larger)
• Test medium: Air or nitrogen at 50 psig (3.4 bar)
• Test method: Downstream end submerged in water; count bubbles during observation period
• Typical valve types: Soft-seated ball valves (PTFE/RPTFE), rubber-lined butterfly valves, diaphragm valves

Quick Answer – What is Class VI (Bubble-Tight)? Class VI is the tightest shut-off class in ANSI/FCI 70-2. It requires zero visible leakage (no bubbles) when tested with air at 50 psig. This is achieved using soft (elastomeric or polymer) seat materials – PTFE, RPTFE, EPDM, FKM, or PFA. Class VI is required for toxic/lethal service, oxygen isolation, safety shutdown valves (ESD/SDV), and environmental compliance (fugitive emission regulations).

Class VI is where soft-seated valves shine. The motorised 2-way ball valve with PTFE seats achieves bubble-tight Class VI shut-off as standard – making it the go-to choice for isolation duty in critical applications. For larger sizes, the motorised rubber-lined butterfly valve and the motorised rubber-lined double-flanged butterfly valve deliver Class VI performance at a fraction of the cost of a ball valve.

The motorised diaphragm valve inherently provides Class VI shut-off – the flexible diaphragm conforms completely to the weir seat, leaving no leakage path. This makes it ideal for pharmaceutical, semiconductor, and ultra-pure water applications where even micro-leakage is unacceptable.

For chemical services requiring both Class VI shut-off and chemical resistance, the motorised PFA-lined ball valve and the motorised PFA-lined butterfly valve combine bubble-tight sealing with full chemical isolation.

Class IV vs Class V vs Class VI – The Practical Comparison

The Key Trade-Off Engineers Must Understand

Here is the nuance that most procurement engineers miss: Class VI sounds better because the number is higher and it means “bubble-tight,” but it is not always the right choice.

Class VI relies on soft seat materials – PTFE, rubber, polymer. These materials degrade at high temperatures, swell in certain chemicals, and can be damaged by abrasive media. If your application involves superheated steam at 450°C, specifying Class VI is physically impossible because no soft seat material survives that temperature.

Conversely, if your application is ambient-temperature nitrogen isolation, specifying Class IV (metal seat) when Class VI (soft seat) is achievable means you are accepting unnecessary leakage for no engineering benefit.

The right leakage class depends on:

1. Process fluid – toxic, flammable, expensive, or benign?
2. Temperature – can soft seats survive?
3. Cycle frequency – how often does the valve operate? (soft seats wear faster in high-cycle applications)
4. Consequence of leakage – safety, environmental, or economic impact?

Which Leakage Class for Which Industry?

Common Mistakes Engineers Make When Specifying Leakage Class

Quick Answer – Top 3 Mistakes:

1. Over-specifying Class VI for high-temperature service – soft seats cannot survive; you will get Class VI on the test bench but field failure within months.
2. Under-specifying Class II/III for hydrocarbon isolation – saving money on the valve but paying for it in product loss, environmental fines, and safety incidents.
3. Confusing FCI 70-2 with API 598 – API 598 is a production pressure-test standard (pass/fail), not a leakage classification standard. A valve can pass API 598 and still not meet FCI Class IV. Always specify both: the leak class AND the production test standard.

How to Specify Leakage Class in Your Valve Data Sheet

When creating a valve specification or enquiry, include the following in your data sheet:

Leakage Class: ANSI/FCI 70-2 Class [IV / V / VI]

Production Test: API 598 / BS EN 12266-1

Test Medium: [Water / Air / Nitrogen – as applicable]

Test Pressure: [Max rated ΔP / Specify if reduced]

Acceptance Criteria: [Per FCI 70-2 Table 2 / Zero visible leakage]

Certificate Required: [Yes – individual valve test certificate]

For critical applications (ESD, toxic service, oxygen), always request an individual valve test certificate – not a batch certificate. Cair Euromatic’s testing facilities can provide individual hydrostatic and pneumatic test certificates with full traceability documentation.

Need Help Choosing the Right Leakage Class?

Selecting the correct leakage class is a decision that balances safety, performance, temperature, and cost. If you are unsure which class to specify – or which valve type and seat material combination delivers the performance you need – Cair’s engineering team can help.

With 20+ years of experience supplying valves to power plants, refineries, chemical plants, pharma facilities, and defence establishments, Cair Euromatic Automation understands leakage classification not as an abstract standard – but as a real-world performance requirement that must be met, documented, and maintained throughout the valve’s service life.

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