7.1.1 Leaks and leak detection

In non-destructive testing, a leak is defined as a hole, a porous area, a permeable area for gases or a different structure in the wall of a test specimen through which a gas can escape from one side of the wall to the other due to a difference in pressure or concentration [33]. Expressed in simpler terms, leaks are small holes through which gases or liquids flow from the side of higher pressure to the side of lower pressure. The geometry of the holes is not known. This means that the tester does not know whether the leak is a smooth-walled round pipe or occurs in the form of a crack or gap, for instance. Assumptions and calculations can only be made for ideal geometries. Since the real geometry of a leak channel is usually unknown, only calculated values can be assumed as an upper limit for a leakage rate. NOTE: The European standard DIN 1330-8 referred to previously uses the term "leakage rate". In the interests of readability we will continue to use the more common term "leak rate" in this book.

A leak can be a harmless leak such as a dripping water faucet. Leaks involving the escape of aggressive media or toxic substances can have more serious consequences. The accident suffered by the US space shuttle Challenger in 1986 was also due to the failure of an O-ring on the solid fuel rocket and the leakage of hot combustion gases.

Any number of technical products will not function, or will not function for an adequate period of time, if they have leaks.
Examples include:

  • The refrigerant circulation system in refrigerators
  • Air conditioning systems in cars
  • Automobile tires
  • Automotive fuel tanks or heating oil tanks
  • Processing systems in the chemical or pharmaceutical industries.

In many cases, the leak-tightness of machines and systems in the production process is an indispensable prerequisite for the quality of the manufactured products.

Returning to the original definition of a leak, we thus find that it is impossible to completely prevent substances from flowing through a wall. The term ”tight“ therefore refers to the requirements of the respective machine, plant or vessel, and must be quantified accordingly.