In industrial operations, even a small leak can create big problems. Control valve leakage not only wastes product, but it can also compromise safety and lead to regulatory concerns. The challenge is that every industry defines “acceptable leakage” differently. So how can you determine if your valve leakage is within tolerance?

What Valve Leakage Means

Valve leakage is the unintended passage of fluid (liquid or gas) through a closed valve. While some standards permit a degree of leakage, the impact depends on the process and industry.

There are different types of control valve leakage:

1. Internal leakage: Fluid passes past the sealing surfaces inside the valve. (AKA through-seat leakage)
2. External leakage: Leakage occurs around valve stems, body seals, etc.

Leaks can result from wear, misalignment, thermal cycling, or materials that don’t match the service. In high-pressure steam, corrosive slurries, or other severe conditions, the risks are even greater.

Standards That Define Leakage Tolerance

To bring consistency, several industry standards establish leakage limits and testing procedures:

• API 598: Inspection and testing requirements for many valve types.
• API 6D: Pipeline valves, including leakage testing.
• ASME and ISO standards: Define leakage classes and testing procedures.
• MSS SP-61: Pressure testing of steel valves.
• ISO 5208: Defines test procedures for industrial valves.

Control valve leakage classifications typically range from Class I to Class VI. Below is a summaryfrom ANSI/FCI 70-2:

Metal-seated valves are handled differently since they are built for severe service where soft seats would not last. Some industries, such as nuclear or petrochemical refining, adapts more stringent requirements beyond these standards.

 “Acceptable” Leakage

A common belief is that “some leakage is okay.” While technically true under many standards, it can be costly:

• Lost product: Even small leaks can accumulate into large losses over time.
• Safety risks: Escaping hazardous fluids or gases can endanger personnel and equipment.
• Compliance challenges: Environmental and safety regulations are tightening, leaving less tolerance for leakage.

Another common idea is that “bubble-tight” means zero leakage. In fact, bubble-tight (Class VI) applies to soft-seated valves, not metal-seated severe service valves. Achieving true zero-leakage requires different design and manufacturing methods. A valve that passes the standard may still pose risks in high-stakes environments.

How to Judge if Leakage Is Within Tolerance

If you’re uncertain about control valve leakage, consider these steps:

1. Review OEM documentation: Start with the valve’s design specifications and manufacturer recommendations. Pay close attention to the stated leakage class and testing criteria.
2. Check against standards: Compare performance with API, ASME, ISO, or MSS classifications relevant to your application.
3. Test regularly: Conduct leak testing under operating conditions and keep detailed records. Additional tests may apply depending on the valve type.
4. Consider the service: For high-stakes operations such as nuclear, coking, or slurry applications, even “within tolerance” leakage may be unacceptable. Many operators set internal criteria stricter than industry norms.

By combining published standards with operating data, you can decide if a valve should be repaired, replaced, or upgraded.