By taking carbon dioxide and hydrogen as lubricating gas, respectively, this paper presents an analysis on the pressure characteristics and temperature distribution of spiral groove dry gas seal which influenced by real gas effect under choked flow condition. Numerical results show that the deviation between real gas and ideal gas, which expressed by the deviation degree between compressibility factor Z and 1, is the main reason for real gas effect affecting sealing performance. Compared with ideal gas model, real gas effect raises exit pressure, opening force, leakage rate, Mach number in dam region, and temperature for carbon dioxide ( Z < 1), while it decreases those characteristics for hydrogen ( Z > 1) under the same operating conditions. In addition, choked flow effect increases opening force and reduces leakage rate and temperature-drop between entrance and exit of sealing clearance. Meanwhile, it may cause an unstable behavior for the seal.
Purpose
This study aims to acquire the influence mechanism of gas film adaptive adjustment (GFAA) acted on the dynamic characteristics of spiral groove dry gas seal (S-DGS) and then propose a sealing stability enhancement measure.
Design/methodology/approach
The gas film dynamic stiffness and damping of S-DGS are obtained by numerically solving the transient Reynolds equation based on perturbation method and finite difference method. The dynamic coefficients in GFAA model and constant gas film thickness (CGFT) model are compared and analyzed.
Findings
There is the risk to misestimate the instability of DGS with rotational speed or medium pressure grows under the condition of CGFT assumption. Based on GFAA model, increasing balance ratio B properly is an effective measure to improve the stability of DGS. The balance ratio can stimulate the sensitivity of gas film dynamic coefficients to the variation of rotational speed. Increasing medium pressure in small balance ratio range will be conducive to reducing the risk of angular instability.
Originality/value
The influence mechanism of GFAA on S-DGS dynamic characteristics is analyzed. The interactions between rotational speed and balance ratio, medium pressure and balance ratio acted on gas film dynamic characteristics are explored based on the GFAA model.
The gas film of the carbon dioxide (CO2) spiral groove dry gas seal (S-DGS) is less than 3 μm during the startup process, and its opening stability is directly related to the operating performance of S-DGS. The finite difference method is employed to solve numerically the pressure distribution of S-DGS considering the slip flow and the real gas effect. The influence of both effects on the startup characteristics of S-DGS is discussed at different structural parameters. The results show that the slip flow effect inhibits the opening ability of CO2 S-DGS, whereas the real gas effect enhances its opening ability. Within the range of working conditions investigated, the seal processes a lowest startup rotational speed when the spiral angle is 7.5°, and the highest gas film stiffness occurs at small spiral angle when the film thickness is 0.6 μm. However, the relationship between groove number and gas film stiffness is complex which relates to the balance film thickness of the startup process. A higher opening ability can also be achieved by reasonably increasing the balance coefficient.
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