What to do with a vacuum leak?
November 9, 2020
3 MIN READ
Unforeseen issues like vacuum leaks can become catastrophic for vacuum systems, operators, and the environment. It is crucial to develop a mitigation plan ahead of these events so vacuum levels can be restored as quickly as possible.
What is a vacuum leak?
Vacuum leaks are small holes that can occur in parts of the system that allow unwanted exit or entry of gas. They vary in severity based on the hole's size, temperature, the type of gas being pumped through the system, and inside vs. outside pressure differentials.
When a vacuum system develops a leak, efficiency suffers, and operators and products are put at risk. Several negative results can occur due to a vacuum leak, including:
- Toxic leaks into the environment
- Contaminant entry
- Explosions
- System damage
Note that vacuum systems don't need to be vacuum-tight to run safely and efficiently. They do, however, need to maintain proper operating pressure, gas balance, and an ability to reach and maintain ultimate pressure.
Vacuum leak testing methods
There are four standard tests to detect vacuum leaks:
Bubble tests
Here, the tester places a leaking item underwater and marks where any bubbles originate. Another method is placing detergent around a leaking water or gas-filled item and marking it to indicate where any bubbles form.
Pressure decay tests
Pressure decay tests require the tester to evacuate a closed vacuum vessel to the point where a specific pressure is reached. Then, the tester closes the pump's inlet valve and reopens it after a set amount of time.
The tester repeats this process several times to determine how long it takes for the vacuum to return to its original level. If the time remains constant, a leak is likely present. However, a decrease in time doesn't mean no leak is present.
Pressure rise tests
The pressure rise test is essentially the flip side of the pressure decay test. Here, the tester marks the vacuum level against the time taken to achieve that level. If a leak is present, the curve will straighten out.
Helium tests
Helium testing is the only method for detecting a leak smaller than 1x10-6 mbar *l/s. Testers use helium as a tracer gas due to its low mass, low cost, and inertness.
Related: Unwanted leaks in any system can easily damage expensive equipment and machinery reducing the effectiveness of the process. The primary reasons for helium leak testing are quality, reliability, personnel protection, and protecting the environment.
Fixing a vacuum leak
Once a tester has discovered a vacuum leak and determined its leakage rate, it's time to move on to the repair stage. Repairs can’t create a leak-tight system, but they can restore vacuum performance to the level required to achieve the system's purpose.
Most leaks will fall into one of two categories: seal failure or issues with view glasses/feedthroughs.
Seal failure vacuum leak repair
The majority of vacuum system leaks occur at component joints — the system's seals. Leaks are common around pump lines, chamber doors, and electrical feedthrough areas. After a few years of wear and tear, all seals will become brittle.
Repairs here are relatively straightforward: replace the leaky seal.
View glass and feedthrough vacuum leak repair
View glasses and feedthroughs are glued or welded. Over time, mechanical force or temperature cycling can crack at the junction between these materials and others, creating a leak.
Unfortunately, repairing view glass and feedthrough vacuum leaks is often impossible. The components will typically need to be exchanged. Sometimes, however, view glasses and feedthroughs can be patched with low-degassing resins or Vacseal® (a type of cement).
For vacuum vessel bodies or port welding, the best solution is to weld the leak to ensure the system's tightness. In high-vacuum vessels, users should weld from the inside to prevent degassing.
