What are ANSI/FCI 70-2 leakage classes for control valves?

If you've ever asked, "What are ANSI/FCI 70-2 leakage classes for control valves?" you're really asking how the industry defines acceptable shutoff leakage when a control valve is commanded closed. Because control valves are built to modulate, not to be perfect isolation devices, some leakage past the seat is normal-yet it must be predictable, measurable, and specifiable. That's exactly what ANSI FCI 70-2 leakage classes provide: a practical control valve shutoff classification guide used by engineers, OEMs, and maintenance teams to align performance, risk, and cost.

This ansi fci 70 2 control valve seat leakage classes overview explains what each class means, why valves leak, how testing works, and how to choose the right class for real services like steam and gas.

Why control valves leak in shutoff (and why it matters)

Understanding why control valves leak in shutoff helps you specify the right class instead of over-specifying (and overpaying).

Common causes include:

Seat and plug surface finish limits (especially on metal seating)

Wear, erosion, and debris embedded in seating surfaces

Thermal distortion (hot/cold cycling changes geometry)

Insufficient actuator thrust → not enough seat load

Pressure/flow-induced forces and misalignment

Soft goods damage (nicks, extrusion, swelling)

A key practical idea is seat load impact on leakage class: higher shutoff thrust generally improves seat contact, but only up to the point where geometry, material, and damage dominate.

What the ANSI/FCI 70-2 standard actually defines

ANSI/FCI 70-2 leakage classes (often written as ansi fci 70-2 leakage classes or ANSI FCI 70-2 leakage classes) specify:

Test medium (air, water, etc., depending on class)

Test differential pressure across the closed valve (typically based on valve rating or specified ΔP)

Maximum allowable leakage rate for each class

In short, it's one of the most widely used control valve seat leakage standards for defining acceptable leakage for control valves.

ANSI FCI 70-2 leakage classes: the quick meaning of each

Below is a practical control valve leakage class chart in words (rates are defined by the standard, and manufacturers publish the exact numbers/tables).

Class I - "As is"

No formal test required (or minimal verification).

Rarely used for critical services.

Class II - Low-performance shutoff

Intended for non-critical applications where some visible leakage is fine.

Class III - Moderate shutoff

Tighter than Class II; common in general process control where shutoff isn't a safety function.

Class IV - Standard metal-seat shutoff

Often the default request for metal-seated globe valves.

Good balance of cost vs shutoff.

When people ask "ansi fci 70 2 control valve seat leakage classes", Class IV is frequently the baseline.

Class V - Very tight metal-seat shutoff (with defined calculation)

Higher shutoff expectation than Class IV.

Uses water testing and a formula-based allowable leakage.

Engineers often search for Class V leakage rate calculation because it's derived from valve size and pressure differential.

Class VI - "Bubble-tight" (typically soft seated)

Tested with air; very low leakage.

Common for soft seat vs metal seat leakage discussions because Class VI is usually achieved with resilient seats (PTFE, elastomers) or special sealing designs.

Often described as "gas tight," though the standard defines the allowable bubble rate rather than absolute zero.

Class IV vs Class VI leakage is one of the most common comparisons: Class IV is typical for metal seats in process throttling; Class VI is chosen when you need extremely low air/gas leakage, often with soft seats.

Leakage testing procedure for control valves (what happens in practice)

A typical leakage testing procedure for control valves under ANSI/FCI 70-2 involves:

Installing the valve on a test stand and ensuring proper alignment

Closing the valve with the specified actuator or test fixture load

Applying the specified test differential pressure

Using the required test medium

Measuring leakage (by volumetric method, bubbles, or collection)

Two real-world notes:

A valve that meets Class VI in the shop may leak more in service if debris, wear, or temperature cycling damages soft goods.

Actuator sizing matters: insufficient thrust can fail even a modest leakage class because the valve never develops the required seat load.

How to specify valve leakage class (without guessing)

Use this checklist when deciding how to specify valve leakage class:

Define the consequence of leakage

Environmental release? Safety hazard? Product contamination? Energy loss?

Identify the fluid and phase

Gas services usually drive tighter classes than liquids.

Consider temperature and cycling

High-temperature steam can punish soft seats.

Clarify if the valve is a control valve or an isolation valve

Don't force a control valve to act like a block valve unless you accept the tradeoffs.

Specify test standard + class explicitly

Write "ANSI/FCI 70-2 Class ___" in the data sheet.

This approach turns ansi fci 70 2 control valve seat leakage classes overview into an actionable spec, not a vague requirement.

Selecting the right class for common services (tips that save money)

Leakage class selection for steam service

For leakage class selection for steam service, be careful with "bubble-tight" expectations:

Steam is hot, can flash, and can erode seating.

Many steam control applications use Class IV or Class V with hardened trims, then rely on downstream isolation for true tight shutoff.

Best leakage class for gas tight shutoff

If you need the best leakage class for gas tight shutoff, many specs point to Class VI, but confirm:

Maximum temperature and chemical compatibility of soft seats

Required cycle life

Whether a special seal (e.g., energized PTFE) is needed

Also remember: "gas tight" in practice may require system design (double block, purge, or downstream isolation), not just a single valve class.

IEC 60534-4 vs ANSI FCI 70-2 (what's the difference?)

People often compare IEC 60534-4 vs ANSI FCI 70-2 because both address seat leakage classification. The biggest practical takeaway:

Both standards define leakage limits and test methods, but the class names, test media, and limit expressions aren't always one-to-one equivalent.

If your project spans regions or suppliers, specify the exact standard required and avoid assuming "Class IV" means the same thing under IEC.

Final takeaway

So, What are ANSI/FCI 70-2 leakage classes for control valves? They're a standardized way to define and test control valve seat leakage standards, from loosely acceptable leakage to near "bubble-tight" performance. Use the standard to balance risk, operability, and cost-then document the exact class in your datasheet. When in doubt, align leakage class with the consequence of leakage, confirm seat load impact on leakage class, and remember that shutoff performance depends as much on actuator sizing and service conditions as it does on the trim design.

Estimated word count: ~980 words.