Dry gas seal is commonly used for high pressure gas, and the gas of the dry gas seal is generally considered as an ideal-gas when the dry gas seal is investigated, designed and operated. However, the gas performance may be quite different from the ideal gas when the pressure is high. This paper presents a method in which the real-gas feature can be considered. The gas compressibility factor Z, a measure of deviation from ideal-gas behaviour, can be determined according to the virial equation of state, and the second virial coefficients can be calculated from the modified Pitzer equation. The spiral groove dry gas seal has been selected as an example. The real-gas equation has been used to modify the pressure-controlled equation stemmed from the Muijderman's spiral narrow groove theory. The modified equation has been resolved in order to find the effects of the real-gas. Some familiar gases, such as hydrogen, carbon dioxide, nitrogen, air, have been considered. The results show that the real-gas behaviour have evident effects on the leakage of the dry gas seal if the gas compressibility factor Z is far away from 1, but little effects on the pressure and the opening force.
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