Bubble Test Leak Detection
The bubble test is a method used to detect leak points based on visual observation. In this method, a pressure difference is created at the leak boundary, and the low-pressure side is exposed to a liquid. The location of the leak is determined by observing the bubbles that form as gas escapes due to the pressure difference.
Factors in bubble test detection:
- Rate of bubble formation: Indicates the severity of the leak.
- Bubble size and quantity: Help to estimate the leakage rate
The bubble test can detect leaks with a sensitivity between 10^-3 Pa.m³/s to 10^-4 Pa.m³/s. According to ASNT Chapter 7, the bubble test can be performed in two main ways:
Immersion Method:
The test chamber is pressurized with a light gas, and the equipment is submerged in water or another fluid. Leaks are detected by observing bubbles forming at the leak site as gas escapes. However, this method requires caution for vacuum systems or pressure-sensitive vessels that cannot withstand high pressures. Additionally, false bubbles may form at seams, making it necessary to distinguish them from actual leaks. This method can be time-consuming, sometimes taking hours to complete.
Foam Method
This method is used for detecting larger leaks. A foam-forming solution is applied to suspicious areas, and bubbles form at the site of a leak. This is a simpler method compared to immersion, particularly for large equipment and pipelines, but it has lower accuracy.
Challenges with bubble testing:
- Surface contamination: Oil, rust, oxide films, or other surface contaminants can interfere with bubble formation.
- Welding porosity: May lead to false positives by producing bubbles at non-leak areas.
- Temperature sensitivity: High or low surface temperatures make the method difficult to execute.
- Fluid blockage: The test fluid may temporarily block small leaks, preventing bubble formation.
- Excessive pressure: Too much pressure can prevent bubbles from forming at leaks in foam tests.
In systems under vacuum or with very low pressure relative to the test fluid, the fluid may boil, producing vapor bubbles that confuse the operator when identifying true leak bubbles.
Overall, the bubble test is highly dependent on the operator’s skill, the fluid used, and environmental conditions such as temperature and pressure at the test site.
Advantages of Helium Leak Detection over Bubble Testing
Helium leak detection using a mass spectrometer offers significantly higher accuracy, with a detection sensitivity of up to 10^-13 Pa.m³/s, compared to the bubble test.
For vacuum systems or pressure-sensitive equipment, helium leak detection is ideal because it does not require pressurization. Only a small amount of helium is needed to detect and locate leaks, making it the best method for low-pressure or vacuum systems. Additionally, helium is an inert gas, meaning it does not react with or contaminate the equipment or its contents.
Helium leak detection also works well for equipment with high or very low temperatures, providing accurate results even in challenging conditions where the bubble test would fail.
