Control of tightness – is the most common method of testing welds on vessels used for storing liquids or gases under pressure for presence of leakage. This method is used for objects that are not under pressure, but which high demands of tightness. Vacuum testing is determination of total degree of leakage of closed and open structures. The tightness testing are exposed the products which should safe the pressure of working or controlling substance for a specified time, or the working substance must not exceed allowed meaning. These values are set in the technical conditions (TC) for the products manufacture. The tightness destruction of the products are due to the leakage of the material from which made their elements and components, as well as looseness in the joints of these elements and components with each other. The required tightness of connections provided by improving their structures and processes of assembly, welding, etc. For vessels under overpressure, the requirements for the tightness are defined by their volume and permissible pressure changes in them over time. Tightness is a necessary condition for operability of various items so the reliability of testing must be high.
The tightness of the objects may be compromised due to several reasons:
- the conversion of non-through defects under the influence of residual voltage in the structure as a result of various physical and chemical influences;
- the disappearance and appearance of leakage as a result of deformation of shells, especially thin-walled, under the influence of mechanical or thermal stresses, in connection with which the test membranes articles critical applications should be conducted under conditions as close as possible to the workers;
- the random overlap of the cavity due to contact of dust or liquids, as well as atmospheric moisture.