Control Valve Leakage Class and Rangeability: The "Limits" of Valve Performance

If the Cv value determines how much work the valve can do, then the leakage class (Leakage Class) and rangeability (Rangeability) determine the "quality of the work" the valve performs.

       Leakage Class is the lower limit of performance: How tightly can the valve close?

       Rangeability is the upper limit of performance: How wide can the valve adjust?

       Many field incidents happen not because the valve cannot pass the flow, but because the valve cannot close properly (causing high-pressure gas leaks, material waste) or cannot adjust properly (causing instability at low flow and saturation at high flow).

       In this article, we will explain these two key indicators that determine the "level" of a valve's performance.

01 Leakage Class: The Art of Closing the Valve

There is no absolute "zero leakage" in the world. Even metal atoms have gaps between them.

The industry standard followed is ANSI/FCI 70-2 (corresponding to IEC 60534-4). This standard divides leakage into 6 classes.

Here’s a detailed explanation of the commonly used classes:

Class IV: The Standard for Metal Hard Seal

• Definition: Leakage does not exceed 0.01% of the rated Cv value.
• Application: Most ordinary single-seat valves and cage valves.
• Intuitive Understanding: For a valve with Cv=100, a small leak might not be audible to the human ear, but instruments can detect it.

Class V: A Tough Step to Cross

• Definition: Extremely low leakage, with a complex calculation formula (depending on pressure differential and orifice size), roughly 1/100 of Class IV.
• Application: Situations requiring extremely high metal sealing, usually requiring precise grinding of the valve seat and disc.

Class VI: The World of Soft Seals

• Definition: Bubble-tight seal
• Testing Method: Air is blown through, counting how many bubbles leak per minute. For example, a 1-inch valve should not leak more than 1 bubble per minute.
• Material: Can almost only be achieved with soft materials such as PTFE (Teflon) or rubber.
• Limitations: Soft seals do not perform well at high temperatures (usually < 230°C).

💡 Selection Pitfall:

Do not blindly pursue Class VI. If you are working with high-temperature and high-pressure steam and demand Class VI, manufacturers will only be able to provide expensive special metal structures, leading to skyrocketing costs and uncertain service life. Typically, Class IV is sufficient for control valves.

02 Rangeability: Ideal vs. Reality

Rangeability, also known as Turndown Ratio, is defined as:

The ratio between the maximum controllable flow and the minimum controllable flow of the valve.

• Linear Valves: Theoretically, the rangeability is about 30:1.
• Equal Percentage Valves: Theoretically, the rangeability is about 50:1 or even 100:1.

Why the "100:1" on samples is misleading:

The rangeability indicated on samples is called Inherent Rangeability.

But in the field, we are dealing with Installed Rangeability.

Remember the valve authority, S?

Pipe resistance will "eat up" the pressure difference of the valve

• S = 1 (Ideal): Installed Rangeability equals Inherent Rangeability.
• S = 0.1 (Common): A valve rated for 50:1 might only have an actual installed rangeability of 5:1!

What does this mean?

It means that when the flow rate drops to 20%, the valve may already be near its closed position, becoming unstable.

✅ Engineering Rule:

Do not trust sample data blindly. In systems with low S values, the installed rangeability must be calculated. If the actual flow range is wide (e.g., minimal flow during startup, maximal flow during normal operation), one valve alone might not be sufficient. A "split range" solution, using multiple valves in parallel, might be needed.

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