Air in-leakage is the hidden enemy of vacuum systems. Even a small leak — equivalent to a pinhole — can prevent a multi-stage ejector system from reaching design suction pressure. Leak testing is performed after mechanical completion and before process commissioning.
Pressure Rise Test (Decay Test)
The most common field test. Procedure:
- Isolate the vacuum system from the process vessel (blind flanges on all nozzles except vacuum pump connection).
- Evacuate the system to design suction pressure using the vacuum system.
- Isolate the vacuum pump from the system (close isolation valve).
- Record pressure at time zero and monitor pressure rise over 1–4 hours.
- Acceptance criterion: pressure rise rate ≤ specified limit (typically 0.1–0.5 mbar/hr for chemical plant vacuum systems, tighter for pharmaceutical).
Helium Mass Spectrometer Leak Detection
For tighter leak rate requirements or to locate specific leak points, helium leak detection is used. Helium (or a helium-nitrogen mix) is sprayed on suspected leak points — flange gaskets, valve stems, weld joints, instrument connections — while the mass spectrometer samples from inside the vacuum system. Detection limit: ~10⁻⁶ mbar·l/s for skilled operators.
Common leak locations in process vacuum systems: RFQ flange gaskets (wrong gasket material or insufficient bolt torque), sight glass seals, thermowell fittings, pressure gauge root valves left open, and welded joints with pinhole defects.
Specifying Leak Rate in Purchase Orders
Include in your vacuum system specification: maximum allowable leak rate (mbar·l/s or mbar/hr pressure rise), test method (pressure rise or helium), test duration, and whether test is at fabricator shop or site. For ejector systems supplied as packaged units, shop leak test before dispatch saves costly site troubleshooting.
Always leak-test the process vessel and the vacuum piping separately before connecting them. This isolates whether a leak is in the vessel or the vacuum system.
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