Valve leakage is one of the most important performance requirements for industrial valves, particularly in oil and gas, petrochemical, power generation, chemical processing, and other critical applications.

However, terms such as “zero leakage,” “bubble-tight,” “Rate A,” and “Class VI” are sometimes used without clearly defining the applicable test standard. These terms are not necessarily interchangeable.

Understanding valve leakage classes and test standards helps engineers and buyers specify the correct valve and avoid unnecessary testing requirements.

What Is Valve Seat Leakage?

Valve seat leakage refers to the amount of fluid that passes through the valve closure system when the valve is in the fully closed position and subjected to a specified test pressure.

The allowable leakage depends on several factors, including:

• Valve type and size

• Seat design and material

• Test medium

• Test pressure

• Applicable product standard

• Project or purchaser requirements

For this reason, simply specifying “zero leakage” may not be sufficient. A proper valve specification should identify the applicable test standard and acceptance criteria.

ISO 5208 Leakage Rates

ISO 5208 specifies pressure testing requirements for metallic industrial valves. It covers tests used to verify pressure boundary integrity, closure tightness, and the structural adequacy of the closure mechanism. ISO 5208 is intended to be used together with the applicable valve product standard.

ISO 5208 uses a series of leakage rates, including Rate A, AA, B, C, CC, D, E, EE, F and G.

In general, Rate A represents the most stringent closure tightness requirement, with no visually detectable leakage during the specified test period. The permitted leakage increases as the rate progresses toward the higher-numbered categories.

The actual allowable leakage depends on the test medium and valve nominal size. Therefore, the leakage rate should always be evaluated together with the complete test conditions rather than by the leakage class alone.

API 598 and Valve Closure Testing

API 598, Valve Inspection and Testing, is widely used for industrial and API valves, including gate, globe, plug, ball, butterfly and check valves. It establishes requirements for inspection and pressure testing, including closure tests and allowable leakage.

For API valves, the required testing is normally determined by the applicable product standard, such as API 6D, together with the purchaser’s specification.

API 598 provides specific allowable leakage criteria for different valve configurations, including resilient-seated and metal-seated valves. Therefore, metal-seated valves should not automatically be judged by the same leakage criteria as soft-seated valves.

This distinction is particularly important when specifying ball valves for high-temperature, abrasive, or severe-service applications.

EN 12266-1 for European Industrial Valves

EN 12266-1 specifies pressure tests, test procedures, and acceptance criteria for production testing of industrial valves made from metallic materials.

Its leakage-rate system includes Rates A through G, which are closely related to the corresponding leakage-rate system used in ISO 5208.

For projects based on European standards, EN 12266-1 may therefore be specified for valve production testing and closure tightness verification.

As with ISO 5208 and API 598, the applicable valve product standard and project specification should be checked before determining the final acceptance criteria.

What About ANSI/FCI 70-2?

ANSI/FCI 70-2 is different from the standards discussed above.

It is primarily used for control valve seat leakage classification, rather than as a general pressure-testing standard for industrial isolation valves.

Its leakage classes range from Class I through Class VI, with higher classes generally representing more stringent seat leakage requirements.

Therefore, a specification such as “Class VI leakage” should not automatically be applied to every industrial ball, gate, globe, or check valve.

The valve type and applicable standard must first be identified.

Key Differences Between the Standards

These standards address related aspects of valve performance, but they should not be treated as completely interchangeable. The applicable product standard, valve type, service conditions, and purchaser requirements must be considered together.

Metal-Seated Valves and Leakage Requirements

For metal-seated ball valves, achieving reliable shut-off performance requires more than simply selecting a metal seat.

The final sealing performance can be influenced by:

• Ball and seat machining accuracy

• Surface finish

• Seat and ball material

• Hardfacing or surface treatment

• Seat design

• Assembly accuracy

• Operating conditions

• Test medium and test pressure

Metal-seated valves are commonly selected for demanding applications where temperature, erosion, abrasion, or process conditions may limit the use of conventional soft-seat materials.

However, metal-seated does not automatically mean zero measurable leakage. The required leakage acceptance level must be defined according to the applicable standard and project specification.

How Should Buyers Specify Valve Leakage?

When preparing an RFQ or purchase specification, avoid using only general terms such as:

“Zero leakage required.”

A clearer specification should identify:

1. Applicable test standard
For example, API 598, ISO 5208, or EN 12266-1.

2. Test medium
Such as water, air, nitrogen, or another specified medium.

3. Test pressure

4. Allowable leakage rate

5. Test duration

6. Applicable valve product standard

This information allows the manufacturer and purchaser to evaluate the same acceptance criteria and reduces the possibility of misunderstandings during inspection and testing.

Valve leakage classification is not simply a matter of choosing the highest possible class. The correct requirement depends on the valve type, seat design, service conditions, applicable product standard, and project requirements.

ISO 5208, API 598, and EN 12266-1 provide important frameworks for industrial valve pressure and closure testing, while ANSI/FCI 70-2 is primarily associated with control valve seat leakage classification.

For buyers and engineers, the best approach is to define the applicable standard and measurable acceptance criteria rather than relying on general terms such as “zero leakage.”

For valve manufacturers, consistent machining, material control, assembly, inspection, and pressure testing are essential to achieving reliable and repeatable sealing performance.