Control Valve Seat Leakage Class Calculator
Find the maximum seat leakage allowed for a control valve in each leakage class, for setting up a shop leak test.
- Rated flow = Kv × √ΔP = 63 × √3.5 = 117.9 m³/h
- Leakage = 0.01 % of rated flow = 0.1964 L/min
Leakage Class Limits
Class III: 0.1 % Class IV: 0.01 %
Class V: 5 × 10⁻⁴ mL/min per inch seat per psi ΔP (water)
Class VI: table by seat diameter (air)
Classes II to IV are tested with water at 3 to 4 bar ΔP (or maximum operating ΔP if lower) and expressed as a fraction of rated flow at that ΔP.
Worked example
A Kv 63 valve tested at 3.5 bar has a rated flow of 117.9 m³/h; Class IV allows 0.01 % of that, about 0.20 L/min.
Frequently Asked Questions
What is Class IV leakage?
0.01 % of the valve’s rated capacity at the test pressure drop, typical for metal seated globe valves.
What does Class VI mean?
Bubble tight shutoff measured with air, usually requiring a soft seat. The allowance depends on seat diameter.
Is shop leakage the same as in service?
No. Wear, erosion and lower seat load can increase leakage in service; the class applies to the new valve under test conditions.
Learn more on the blog
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