Every control valve leaks
A control valve is designed to modulate, not to seal. Even the tightest standard class permits measurable flow when fully closed, and the leakage classes of FCI 70-2 (mirrored in IEC 60534-4) exist to say how much is acceptable rather than to promise none.
This is why "the control valve is closed" is never the same as "the line is isolated." Where positive isolation matters — maintenance, safety, or preventing a reactor from being fed — a dedicated block valve does that job.
How the classes work
The classes use two different bases, which is the part that confuses people:
- Classes II, III and IV express leakage as a percentage of rated capacity. A bigger valve is allowed to leak more, proportionally.
- Class V uses a formula proportional to seat diameter and differential pressure: 0.0005 mL/min of water per inch of port diameter per psi of ΔP.
- Class VI is a fixed table of bubble counts in mL/min of air by port size — the soft-seat class.
Worked example
A 4-inch (102 mm) port valve, Cv 50, tested at 6.9 bar (100 psi):
- Class IV = 0.01% of rated capacity — the shutoff most metal-seated valves are supplied to
- Class V = 0.0005 × 4 in × 100 psi = 0.20 mL/min of water
- Class VI = 1.70 mL/min of air, or about 11 bubbles per minute
Note that Class V and Class VI are measured in different media, so the numbers are not directly comparable. In practice Class VI soft seats give the tightest real shutoff, while Class V is what a well-lapped metal seat can achieve at high temperature where soft seats cannot go.
Field notes
- Specify the class you need, not the tightest available. Each step up costs money and constrains the design — Class VI usually means a soft seat, which caps operating temperature around 200 °C and rules out abrasive service.
- Tight shutoff needs actuator force. Seat load comes from the actuator, so a Class VI requirement can drive a larger actuator or higher spring range than the control duty alone would need. Check it against the sizing from the liquid Cv calculator.
- Leakage classes degrade in service. They are factory acceptance figures on a new valve. Erosion, cavitation damage and particulate all open the seat up — a valve that passed Class IV at FAT may be Class II after two years on dirty service.
- Direction matters. Testing is done in the specified flow direction; a valve installed backwards may not meet its class at all.
- For safety shutoff duty, seat leakage feeds into the functional safety assessment — a leaking final element is a partial failure of the safety function. See the SIL and PFD calculator.
Frequently asked questions
What is ANSI Class IV seat leakage?
A maximum leakage of 0.01 percent of the valve’s rated capacity. It is the standard expectation for a metal-seated control valve in good condition and is the most commonly specified class in general process service.
What is the difference between Class V and Class VI?
Class V is a liquid test with an allowance proportional to seat diameter and differential pressure, typically achieved by lapped metal seats. Class VI is a soft-seat bubble test with a fixed allowance in millilitres per minute of air by port size, and is roughly ten times tighter in practice.
Does a higher leakage class mean a better shutoff?
Yes. Class I is effectively no requirement and Class VI is the tightest of the standard series. Each step up costs money and usually trades away something else — soft seats limit temperature, and tighter classes need higher actuator seating force.
Is a control valve suitable for isolation duty?
Generally no. Even Class VI permits measurable leakage, so a control valve is not a substitute for a certified isolation or shutoff valve. Where positive isolation is required, a dedicated block valve is the correct answer.
Based on the FCI 70-2 / IEC 60534-4 leakage classes, for reference and education. Acceptance testing must follow the standard and the purchase specification. See our disclaimer.