Valve Leakage: Domestic & International Standards, Sealing Classes, Selection & Acceptance Key Points

Valve leakage is the core indicator for sealing performance of industrial valves. It directly determines process system safety & stability, energy consumption management and environmental compliance. For equipment operation‑maintenance and procurement selection, it is critical to master relevant standards and technical specifications. Combined with mainstream standards including GB/T 13927‑2022, API 598‑2023, ANSI/FCI 70‑2 and ISO 5208, this article sorts out valve leakage definition, sealing grades, test specifications and acceptance principles.

1. Basic Definition of Valve Leakage

Valve leakage means the volumetric flow of media passing through sealing pairs under specified test pressure, test medium and pressure holding duration. Sealing pairs cover seat‑closure member, body‑bonnet, stem‑packing and other sealing positions. There are two types of leakage:

• Visible leakage: Directly observable by naked eyes, such as liquid dripping or continuous gas bubbling;
• Micro‑leakage: Extremely low leakage volume which requires special testing equipment for quantitative measurement.

Scope of each standard: GB/T 13927, API 598 and ANSI/FCI 70‑2 are mostly used for control valve leakage classification. ISO 5208 is the global universal pressure‑test standard for metallic valves. These standard systems can be cross‑referenced for component selection, factory inspection and incoming acceptance.

2. Sealing Leakage Classes (Class Ⅰ‑Ⅵ, performance from low to high)

• Class Ⅰ: No quantitative leakage requirement, only basic sealing. Applied for non‑critical conditions such as atmospheric air and ordinary cooling‑water pipelines; dedicated leakage test is not required.
• Class Ⅱ: Minor leakage permitted; liquid ≤0.1×DN drops per minute, gas ≤30×DN mm³/s. Suitable for general gate valves and globe valves for common working conditions.
• Class Ⅲ: Better sealing performance than Class Ⅱ, widely adopted for low‑pressure chemical pipelines and general water‑treatment systems.
• Class Ⅳ: Mainstream grade for metal hard‑seal valves. Allowable leakage equals 0.01% of rated flow. For harsh conditions including high‑temperature high‑pressure steam, oil & gas services.
• Class Ⅴ: Leakage limit is 1/10 of Class Ⅳ. Designed for pipelines carrying toxic, flammable‑explosive and high‑risk media.
• Class Ⅵ: Highest sealing class, mostly for soft‑seal valves. Zero bubbles for small size DN≤50mm; trace bubbles permitted for large bore sizes. Applied in pharmaceutical & food processing, high‑purity gas, LNG and other strict high‑cleanliness scenarios.

3. Sealing Leakage Test Specifications

Leakage test shall be carried out only after valve body shell strength test passes

• Test medium: Clean water for liquid‑phase test; dry air or nitrogen for gas‑phase test. Actual process media can be adopted for special applications with full test records kept.
• Test pressure: High‑pressure sealing test at 1.1 × nominal pressure; low‑pressure sealing test set at 0.4‑0.7MPa for micro‑leakage detection. For valves with back‑seat structure, back‑seat test pressure is also 1.1 × nominal pressure.
• Pressure holding time: ≥60s for liquid test; ≥30s for gas test. For large bore DN＞100mm valves, hold pressure ≥120s, collect leakage data after pressure stabilizes.
• Detection methods: Measure liquid dripping volume via graduated container; gas is checked by immersion bubble method or flow meter. Bubble method is mandatory for Class Ⅵ sealing verification.

4. Component Selection & On‑site Acceptance Principles

• Class Ⅲ or Class Ⅳ for general industrial pipelines; Class Ⅳ is recommended for metal hard‑seal valves.
• Select Class Ⅴ / Class Ⅵ for toxic, flammable, corrosive media and high‑purity process pipelines. Soft‑seal valves easily achieve Class Ⅵ while limited by maximum service temperature; material trade‑off is required for high‑temperature services.
• Acceptance rule: Measured leakage shall not exceed allowable limit of corresponding class. No visible dripping for liquid service; bubble counting or flow meter for gas service.
• Disposal for non‑conforming products: Disassemble, lap & repair sealing surfaces or replace sealing parts then retest. Scrap directly if performance cannot be restored after repair.

Proper selection of leakage class avoids safety and environmental risks caused by medium leakage, meanwhile prevents over‑specified sealing configuration, reduces procurement and maintenance cost, and guarantees long‑term reliable operation of process equipment.

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