Industrial Control Valve
Control Valve Leakage Classes Explained: ANSI/FCI 70-2 Class I through VI
“Bubble-tight” is a marketing phrase; Class VI is a test result. When a specification says the valve must be bubble-tight, the first thing to establish is which standard, at what differential pressure, and with which test fluid — because the allowable leakage changes by three orders of magnitude between classes.
The class definitions
| Class | Allowable leakage | Test medium / ΔP | Typical seat construction |
|---|---|---|---|
| Class I | Not tested (by agreement) | — | Metal-to-metal, general purpose |
| Class II | 0.5% of rated full-open Cv | Air or water at 3.5 bar (50 psi) or operating ΔP if lower | Metal seat, unbalanced plug |
| Class III | 0.1% of rated Cv | Air or water at 3.5 bar | Metal seat, lapped, tighter machining |
| Class IV | 0.01% of rated Cv | Air or water at 3.5 bar | Metal seat with precision lapping; most common industrial spec |
| Class V | 5 × 10⁻⁴ ml/min of water per mm of seat diameter per bar of ΔP (≈ 5 × 10⁻¹² m³/s per mm per bar) | Water at full operating ΔP | Metal seat, precision-lapped, high seat load |
| Class VI | Bubble-tight — tabulated ml/min of air (or bubbles/min) by port diameter, e.g. 1 ml/min at DN25, 4 ml/min at DN50, 11 ml/min at DN100 | Air or nitrogen at 3.5 bar | Soft seat — PTFE, PEEK, RTFE, or an elastomer-inserted metal seat |
What the numbers mean in practice
Take a DN100 valve with a rated Cv of 250. At Class IV the permitted leak is 0.01% of 250 Cv — which converts to roughly 0.9 l/min of water at the test pressure. That is fine for a cooling-water circuit and unacceptable for a fuel-gas header. At Class VI the same valve is limited to about 11 ml/min of air — three orders of magnitude tighter.
Which class for which service?
| Service | Recommended class | Why |
|---|---|---|
| Cooling water, general utility | III or IV | Leakage is not a safety or product issue |
| Steam, condensate, boiler feedwater | IV (V on HP bypass) | Continuous leakage wastes energy and erodes seats |
| Hydrocarbon, flammable fluid, flare | IV minimum, V on critical isolation | Emissions and safety |
| Toxic, lethal or high-value fluid | V or VI with bellows seal | Fugitive emission limits (ISO 15848-1) |
| Food, beverage, pharma (hygienic) | VI on the product-contact shut-off | Cleanability and batch separation; see our 316L range |
| Vacuum service | VI | In-leakage ruins the vacuum |
Class V and VI are not a durability guarantee
Leakage class is measured on a new, clean valve at ambient temperature after a defined break-in. It is not a guarantee after 50 000 cycles, after a thermal cycle to 400 °C, or after the line has carried a slurry. If the duty is erosive, specify a hardened trim (Stellite overlay, tungsten carbide, ceramic) and accept that leakage class will degrade — plan the seat as a maintenance item rather than a lifetime component.
How to write the specification
- State the standard explicitly: “Seat leakage to ANSI/FCI 70-2 (IEC 60534-4) Class IV.”
- State the test pressure if you want it above the default 3.5 bar.
- State whether Class V is to be tested with water at full operating ΔP — this materially changes the actuator thrust needed.
- Add the fugitive-emission requirement separately (ISO 15848-1 tightness class AH / BH / CH and endurance class) — leakage class says nothing about stem sealing.
Our control valve range is available with metal seats to Class IV/V and soft seats to Class VI; send the fluid, temperature and ΔP and we will confirm the achievable class before you commit.
Frequently Asked Questions
Is Class VI the same as zero leakage?
No. Class VI permits a small, tabulated amount of air leakage measured in millilitres per minute or bubbles per minute, scaled to port diameter. It is the tightest class defined by ANSI/FCI 70-2 but it is not absolute zero, and it is tested with air or nitrogen, not with the process fluid at operating temperature.
Which leakage class do I need for steam service?
Class IV is the usual specification for steam control valves. Class V is specified on high-pressure bypass and turbine bypass duties where continuous leakage would cause significant energy loss and seat erosion. Class VI requires a soft seat, which is generally not suitable for superheated steam temperatures.
Can a metal-seated valve meet Class VI?
Generally no. Class VI is achieved with a soft seat material such as PTFE, RTFE or PEEK, or with a metal seat incorporating an elastomer insert. A conventional all-metal seat is realistically Class IV, or Class V with precision lapping and high seat load.
How does leakage class relate to fugitive emissions?
They are different tests. Leakage class measures flow past the seat when the valve is closed. Fugitive emissions measure leakage past the stem packing to atmosphere, covered by ISO 15848-1 or EPA Method 21. A Class VI valve can still have a leaking stem if the packing is wrong.
Send us the process data — fluid, inlet pressure, differential pressure, temperature, required flow and pipe size — and our engineers will return a sized selection with Cv calculation, leakage class and material recommendation, plus documentation packs for FAT / IQ-OQ.
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