Tightness

Tightness 

the ability of the shell of a body or of its separate elements and joints to block the exchange of gases or liquids between the environments separated by the shell. Tightness is a necessary working condition for many systems, apparatus, and instruments. The required tightness of a covering (the permissible exchange) is determined from the conditions necessary for the normal course of the process for which the tightness is created. The degree to which a covering is hermetic is characterized by the quantity of a substance passing through it per unit of time and is measured for liquids in liters per sec (l/sec) or grams per sec and, for gases and vapors, in grams per sec or liters · mm Hg/sec (l · mm Hg/sec). A covering is considered hermetic if the liquid or gas exchange through it does not exceed the permissible level.

Tightness is an important property of objects and must be considered in planning sealed objects meant for prolonged keeping and use. The quantitative characteristics of an object’s hermetic properties determine its reliability and durability. Calculations are made to establish the probable period of time within which, given certain usage conditions (fluctuations in pressure, temperature, load, and so on), liquids or gases may flow through the entire covering or parts of it and destroy the hermetic properties.

Gas or liquid exchange—that is, the reason for disruption of the hermetic state—can be caused by the permeability of the material of the covering or of a joint with intact structure, which is taken into consideration during construction, or leaks in the structure of the material or joints, which are detected by vacuum technology. There is large-scale production of so-called hermetic castings for parts of motors, turbines, and water-supply and heating systems.

Other objects that must be hermetic include the bodies of aircraft and spacecraft, submarines, diving suits, and caisson chambers. A high degree of tightness is necessary to maintain a superhigh vacuum in the cavities of thermonuclear devices, accelerators, and space simulators. The normal operation of such systems can be disrupted by leaks as slight as 10^-7 to 10^-8 /· mm Hg/sec. Even more stringent tightness requirements exist for the casings of electrovacuum and gas-filled miniature instruments, in which the pressure or composition of the gaseous medium must not change perceptibly during prolonged storage or in the process of use. For example, to maintain working pressure in an electrovacuum instrument with a volume of 100 cm³ that does not contain a getter, the leak in its shell must not exceed 3 x 10^-12 1·mm Hg/sec during a one-year storage period.

The hermetic properties of oil and gas boreholes play an important part during the drilling process in deposits with high formational pressure. Surfaces are sealed while opening oil strata when there is a danger of blowout or open gushing. It is also necessary for surfaces to be hermetic while working oil and gas deposits to prevent small leaks of the mineral between the place where it is drilled and the refinery. The maintenance of hermetic seals is obligatory during many technological processes in the chemical, food, pharmaceutical, and canning industries when it is necessary to maintain high pressure, a vacuum, or sterile conditions. It is necessary to seal industrial machinery and instruments that are exposed to the action of moisture, gases, dust, dirt, aggressive chemical substances, or instruments emitting radioactive rays. If temperature-moisture and sanitary conditions must be observed in places of work, hermetic production buildings are constructed.

REFERENCES

Smirnov, A. I., and A. A. Sotnikov. Liteinoe proizvodstvo, 1964, no. 4, p. 25; no. 6, p. 27.
Lanis, V. A., and L. E. Levina. Tekhnika vakuumnykh ispytanii, 2nd ed. Moscow-Leningrad, 1963.

L. E. LEVINA