Control Valve Seat Leakage Classes: ANSI/FCI 70-2 Class I to VI

Ask five people on a project what "Class IV shutoff" means and you will usually get one confident answer and four shrugs. Yet the leakage class written on a control valve data sheet drives real money: it determines seat material, lapping requirements, test time, and in many cases whether the valve gets a metal or a soft seat at all. ANSI/FCI 70-2 is the standard that defines those classes.

What the Standard Actually Does

ANSI/FCI 70-2 (harmonized with IEC 60534-4) does not measure leakage the way a leak test on a pipeline does. It defines allowed leakage through the seat of a control valve under defined test conditions, expressed either as a percentage of rated capacity or as a count of bubbles per minute. The key idea is that every control valve leaks when closed — the question is how much is acceptable for the service.

The standard specifies:

• The test medium: air or water, depending on the class.
• The pressure differential applied across the seat.
• Whether the test is performed with flow under the plug, over the plug, or as specified by the purchaser.
• The leakage limit and how it is measured.

Because the test conditions are fixed, two valves from different makers that both claim "Class IV" can be compared on a like-for-like basis. That is the entire point of the standard, and it is also why a vague requirement like "bubble-tight shutoff" on a data sheet causes so much trouble — it is not a defined term.

The Six Classes in Brief

The standard defines six leakage classes, and they are not evenly spaced. The jumps between them are enormous.

• Class I — effectively a designation with no quantitative test requirement. It is used when the purchaser and manufacturer agree that leakage testing is not required, or when the valve's construction makes a meaningful test impractical. It does not mean "some leakage is fine"; it means the standard says nothing measurable.
• Class II — maximum leakage of 0.5% of rated capacity, tested with air at the maximum differential or with water at defined conditions. Historically associated with double-acting piston actuators and older trim designs; rarely specified today.
• Class III — maximum leakage of 0.1% of rated capacity, tested with air or water at the same differential conditions as Class II. Sometimes applied to double-seated balancing valves.
• Class IV — maximum leakage of 0.01% of rated capacity with air at the maximum service differential (or water at defined conditions). This is the workhorse class and the industry default, described in more detail below.
• Class V — maximum leakage of 5 × 10⁻⁴ mL per minute per inch of seat orifice per psi differential, tested with water at the maximum service differential. Specified where even small continuous leakage is costly, such as high-pressure letdown or boiler feedwater bypass.
• Class VI — the "bubble-tight" class, applicable to soft-seated valves (resilient elastomer or PTFE seats) or elastomer-lined constructions. Leakage is specified in bubbles per minute of air or nitrogen at the maximum rated differential — roughly one bubble per minute for a small seat orifice, rising with seat diameter. Metal-to-metal seats generally cannot achieve Class VI; this is a soft-seat class by construction.

Notice the geometry of the scale: from Class III to Class IV the allowed leakage drops by a factor of ten, and the step to Class V drops it further by orders of magnitude. Each step up is not an incremental improvement — it usually requires a different seat technology.

Why Class IV Is the Default

Class IV dominates specifications for a simple reason: it is what a well-made metal-seated globe or rotary control valve with a single seat achieves naturally. A single-seat, unbalanced trim that is lapped and tested will comfortably hold 0.01% of rated capacity. Double-seated and balanced trims, which seal with two seating surfaces or rely on a small allowable path for pressure balancing, typically cannot — which is why balanced trim valves are often Class II or III unless a soft seat is added.

Practical consequences of the default:

• If your data sheet says Class IV, expect a metal seat, a single-seat unbalanced trim (or a soft seat on a balanced design), and a leakage test at the rated differential.
• If your service is clean and shutoff is a process nicety rather than a safety or cost requirement, Class IV is almost always the right answer; specifying anything higher adds cost without adding value.
• If the valve is oversized such that it rarely operates, leakage rates in service will be far below the tested maximum; the class then matters less than seat material compatibility.

Class VI and the Limits of Metal Seats

Class VI deserves its own discussion because it is the most misapplied class. It is achievable only with resilient seating: PTFE or similar polymer seat inserts, elastomer seats, or lined constructions. The bubble-count criterion sounds forgiving — a handful of bubbles per minute — but against a metal seat it is unattainable, because metal-to-metal contact always leaves microscopic leakage paths, especially at high differential.

Three cautions when Class VI is specified:

• Temperature and chemical limits of the soft seat now govern the valve. PTFE seats cap the continuous temperature well below what metal seats tolerate, and soft seats deform under high differential, raising seat loading and actuator size.
• Class VI at the rated differential, not the operating differential, is the meaningful claim — but the soft seat must survive the rated differential mechanically, which is not always true at high pressure.
• Abrasive or dirty media destroy soft seats quickly. In slurry or erosive service, a Class VI soft seat may last weeks; a metal seat at Class IV will outlive several soft-seat replacements. Total cost of ownership usually favors the metal seat.

A related distinction: for severe isolation duty — as opposed to modulating control — many engineers step away from ANSI/FCI 70-2 entirely and specify seat leakage per API 598 or ISO 5208 on an isolation valve instead. Mixing these standards on one data sheet is a common source of confusion.

Mapping to ISO 5208

Since most projects now run international supply chains, the mapping between ANSI/FCI 70-2 and ISO 5208 rate A through rate G matters. The correspondence is approximate but widely used:

• ANSI/FCI 70-2 Class IV roughly corresponds to ISO 5208 rate D for metal-seated valves tested with liquid.
• Class V has no clean single equivalent; it is tighter than most ISO rate categories for metal seats and is usually declared directly.
• Class VI is generally aligned with ISO 5208 rate A for soft-seated valves ("no visible leakage" during the test duration).

The mapping is direction-dependent — a valve meeting a tighter ISO rate will satisfy the looser ANSI class, but the reverse needs verification because the test media and differential definitions differ.

The Cost of Over-Specifying

Every step up in leakage class costs money, and the cost is not linear:

• Higher classes require longer production tests at higher differentials, which increases test time and scrap rates on the test bench.
• Class V water testing requires precise flow measurement equipment and stable temperature conditions; some workshops cannot perform it at full rated differential and will quietly test at reduced differential unless the purchase order is explicit.
• Class VI forces soft seat selection, which then cascades into temperature limits, fire-safety questions, and shorter maintenance intervals.
• On large actuators, tighter shutoff can raise required seat loading and push actuator sizing up a frame, sometimes changing the instrument air supply requirement.

A useful discipline: write the leakage class on the data sheet only where shutoff has a defined process consequence — cross-contamination, catalyst loss, safety isolation, or energy cost — and default to Class IV everywhere else.

Choosing the Class with Manufacturer Support

Selecting a leakage class is ultimately a service decision, not a catalog decision. The seat material, differential, temperature, and media cleanliness interact in ways that a single line on a specification cannot capture. An experienced manufacturer that produces the full valve range in-house — ball valves, control valves, gate valves, globe valves, check valves, strainers, knife gate valves, plug valves and butterfly valves — can review each line and confirm which class is achievable with which seat construction, and can provide the leakage test reports to back the claim. Send your Valve Schedule or the key parameters for each position to the manufacturer's applications team for a per-line proposal before the leakage classes are frozen into the specification.