For a formal liquid bubble leak test using a detector like Swagelok Snoop, the industry-standard minimum pressure governed by ASME Section V, Article 10 and ASME B31.3 is 15 psig or 25% of the system design pressure, whichever is less. While SNOOP itself will bubble at virtually any positive pressure where gas escapes, engineering codes specify pressure thresholds to ensure sufficient velocity and volume for micro-leaks to form visible indications within a practical inspection window.
ASME B31.3 (Process Piping — Sensitive Leak Test, Para. 345.8):
Mandates that a sensitive bubble leak test must be conducted at a pressure of at least 15 psig or 25% of the design pressure, whichever is lower. This ensures consistent sensitivity across process piping joints without exceeding safe stored-energy limits for pneumatic testing.
ASME Section V, Article 10 (Mandatory Appendix I — Bubble Test, Direct Pressure Technique) mirrors the 15 psig or 25% design pressure minimum threshold for pressure vessels and mechanical containment, while capping standard direct-pressure bubble tests at a maximum of 25% of design pressure unless specified otherwise.
For fragile systems, low-pressure ductwork, atmospheric tanks, or specialized containers (Low-Pressure Exception Standards (ASTM E515 / DOT) where 15 psig exceeds structural limits, standard operating procedures typically establish a minimum testing pressure of 3 psig to 8 psig (or 1.5 times operating pressure).
Manufacturer Specifications & Sensitivity
Swagelok does not dictate a minimum required line pressure for Snoop Liquid Leak Detector to function. Because of its specialized surfactant blend and sustained bubble action, it will generate foam or bubbles at any positive pressure differential.
FYI… Leak detection sensitivity is directly tied to differential pressure. A pinhole or micro-channel leak (e.g., around 10−4 std cm3/s) that takes several minutes to generate a visible white foam mushroom at 3 psig will produce clear, rapid bubbling in seconds at 15 psig.
Best Practices for Execution
When testing higher-pressure systems, codes recommend ramping up to a preliminary check pressure of no more than 25 psig or 50% of the target test pressure, to locate and isolate gross leaks before proceeding to full test pressure. A major leak at high pressure will blast the soap solution away rather than forming bubbles.
Once the minimum leak test pressure is reached, hold the pressure for at least 10 to 15 minutes before applying Snoop. This soak period allows piping strains to equalize and ensures a steady-state gas migration through complex paths like threads or flange gaskets.
Allow the surfactant to dwell for at least 15 to 30 seconds on each joint. Watch for the formation of tiny, growing foam clusters rather than large individual bubbles, which indicate very slow micro-leaks.