You have an API 600 Class 150 Gate Valve mounted on the test bench. The shell hydro-test at 450 psi passed perfectly. But when you switch to the high-pressure hydrostatic seat test (typically 315 psi for Class 150 WCB), the downstream side starts dripping faster than the allowable API 598 limits. Why?
1. Misunderstanding "Zero Leakage"
A metal-seated gate valve (like Stellite Trim 8) is almost never "bubble-tight" or zero leakage under high pressure water. API 598 allows a specific amount of drops per minute (e.g., 2 drops / minute / inch of NPS). Before rejecting the valve, confirm you are actually exceeding the API 598 Table 5 leakage allowances.
2. Debris or Weld Spatter on the Seats
The number one cause of failure on the test bench is microscopic debris. Even a single grain of sand or a speck of weld spatter trapped between the wedge face and the body seat ring will create a leak path. Flush the valve thoroughly with clean water and retest.
3. Improper Lapping / Mating
In API 600 cast steel gate valves, the solid or flexible wedge must be perfectly "lapped" to match the angles of the body seat rings. If the manufacturer rushed the assembly without applying lapping compound and grinding the wedge into the seats, the seal will fail. The Fix: Disassemble the valve, apply a fine-grit lapping paste, and hand-lap the wedge to the seats until a continuous, uniform contact ring appears.
4. Actuator/Handwheel Insufficient Torque
Sometimes the valve is fine, but the operator simply hasn't applied enough torque to the handwheel to properly wedge the gate between the seats. Use a cheater bar or torque wrench to hit the manufacturer's specified seating torque.
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