A gate valve that seals “bubble‑tight” is ideal, but in real engineering, a certain amount of leakage is allowed. Knowing leakage classes avoids misjudging valve quality and helps you troubleshoot effectively.The main gate valves product names of China gate Valve Network include:High Temperature & pressure Power Station Gate Valve,Inside Screw Non-rising Stem Wedge Gate Valve,Insulation Jacket Gate Valve,Lined Gate Valve,MZ44W Rising Stem Gas Quick Open&close ParallelMZ48W Manual Non-rising Stem City Gas Quick Gate Valve,Manual Knife-shaped Gate Valve,Manual Slurry ValveManual Wedge Copper Gate Valve,Manual Vacuum Isolated Wedge Gate Valve

1. Leakage Class Overview (GB/T 13927 & API 598)

Class I: Design verification only – no production seal test. Used for non‑critical low‑pressure water lines.

Class II: Minor visible leakage allowed during hydrostatic test. Suitable for general water supply.

Class D (GB): No visible bubbles at 0.6 MPa air test. This is the basic requirement for industrial gate valves.

Class VI (ASME B16.104): Bubble‑tight sealing. Extremely low leakage rates – e.g., ≤0.15 mL/min for DN100. Required for toxic or expensive media.

Common mistake: Believing that “zero leakage” is necessary for all gate valves. Metal‑seated gate valves can achieve Class D at room temperature, but may allow slight leakage at high temperature. Resilient‑seated (EPDM) valves easily meet Class VI at ambient conditions.

2. Four Practical Field Leak Detection Methods

Visual & listening – Close the valve and watch the downstream pressure gauge. Put a screwdriver against the valve body and your ear to the handle – listen for a hissing sound.

Tissue paper test – Stick a dry tissue at the flange gasket. Moisture or movement indicates leakage. Beware of external condensation.

Ultrasonic detection – A handheld ultrasonic leak detector (≈$300‑700) captures the turbulent flow sound from micro‑leaks. Works even when the valve appears dry.

Isolation & pressure test – Isolate the valve, blind one end, and slowly pressurise with water to 1.1× design pressure. Look for water beads at the packing or seat. Most reliable, but requires shutdown.

3. Common Leaking Points & Fixes

Internal seat leakage – Caused by scratches or debris. Rapidly cycle the valve 2–3 times to flush particles. If it persists, dismantle and lap the sealing faces.

Stem packing leakage – Tighten gland nuts evenly. If still leaking, replace packing (valve fully open, depressurised).

Flange gasket weepage – Retighten bolts in a cross pattern, or replace the gasket.

4. Selection Recommendations

General water, oil, steam → Class D (no visible bubbles at air test).

Toxic, flammable, high‑vacuum → Class VI resilient‑seated or metal‑seated with bellows.

High temperature ( >400°C ) → Allow slight leakage, but record initial rate for trend tracking.

Bottom line: Understand the leakage class before judging a gate valve. Use a combination of listening, tissue, ultrasonic, and pressure‑test methods to pinpoint leaks. Choose the right class based on service severity – not too strict (wasteful) nor too loose (risky).

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