Dynamic tracking characteristic of high-speed dry gas seal considering centrifugal inertia effect and its interaction with complex special flow phenomena

A Comparative Study on the Performance of Typical Types of Bionic Groove Dry Gas Seal based on Bird Wing

To accurately ascertain the influence mechanism of real gas effect on the dynamic behaviors of spiral groove dry gas seal (S-DGS), the dynamic gas film characteristics and tracking properties under three degree of freedom (3-DOF) perturbation were investigated for carbon dioxide (CO 2 ) S-DGS by semi-analytical method. The results show that CO 2 real gas effect enhances the dynamic gas film stiffness and damping. Increasing entrance pressure and compressibility number are beneficial in improving the dynamic tracking capability of stationary ring. The real gas effect of CO 2 can effectively improve the dynamic tracking ability of stationary ring, and this improving effect gradually increases with an increase in entrance pressure or speed. In addition, the improvement of tracking ability which caused by entrance pressure rising under 3-DOF perturbation is slightly suppressed at high-speed condition.

The spiral groove dry gas seal is sealed by only a 3–5 µm gas film thickness between the faces of rotating and stationary rings. Considering the complexity and difficulty of measuring the gas film stiffness and system damping of a spiral groove dry gas seal, a test system is designed to study the stability of the dry gas seal. Vibration amplitude of the stationary ring and gas film is measured in this study by writing a LabVIEW test program, adopting a high-precision improved eddy current micro-sensor, obtaining a series of anti-interference measures, and so on. The velocity diagram of the stationary ring and frequency spectrum are processed. According to the data, the variation of gas film stiffness and system damping is analyzed under different operating conditions. The measurement results of gas film stiffness and system damping for the dry gas seal show that at the rotating speed of 1000–3000 r/min and a medium pressure of 0.2–0.6 MPa, the range of gas film stiffness values is 4.8×108−1.50×109 N/m and the range of system damping values is 7.2×104−2.15×105 N.s/m. Both variations of gas film stiffness and system damping are nonlinear. The gas film stiffness value decreases with an increase in rotating speed and increases with an increase in pressure. The system damping value remains stable despite a change in pressure and decreases with an increase in rotating speed. The results have solved the problem that the exact solution of system damping cannot be obtained through theoretical calculation, and such findings will be beneficial to further studies on bifurcation, and chaos of dry gas seals. The optimization of groove design in the future can be guided by the results.

The emergence of dry gas seals has revolutionized the form of fluid sealing. The traditional research and analysis of dry gas seals is carried out by considering the lubricating medium gas as an ideal gas, but at this stage, the sealing application environment is complicated, so it is necessary to consider the real gas effect of the lubricating medium gas to expand and break through the design system of dry gas seals. We choose seven common lubricating media in dry gas seal applications and screen the optimal density expression of the real gas using different real gas equations of state. Then, we study the extent to which the compression factors of different lubricating gases deviate from the ideal gas and analyze the errors of different real gas equations of state. These results can provide an optimal expression to clarify the mechanism by which the real gas effect affects the dry gas seal performance, which helps to grasp the nature of dry gas seals, predict the dry gas seal behavior, and guide the dry gas seal application.

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.

Centrifugal compressors used for transporting natural gas are usually equipped with dry gas seals. The working medium of the seal is usually the delivered gas, that is, natural gas. In this paper, the natural gas viscosity-pressure equation is derived from the Pederson mixed gas viscosity model and Lucas viscosity-pressure model, and the real gas property of natural gas is expressed by Redlich-Kwong equation. The gas film pressure governing equations proposed by Muijderman for narrow grooves are modified and solved for the seal faces. The influences of natural gas viscosity-pressure effect on the sealing characteristics, such as leakage rate and opening force, of spiral groove dry gas seal are analyzed. Results show that the viscosity-pressure effect has significant influence on spiral groove dry gas seal. This effect reduces the leakage rate but increases the opening force, compared to the situation without considering the viscosity-pressure effect. With the pressure up to 4 MPa, the viscosity-pressure effect of natural gas is weak and negligible. As the pressure increases, the viscosity-pressure effect increases. At 12 MPa, the relative deviations of leakage rate and opening force caused by the viscosity-pressure effect are respectively −30.6% and 1.65%. Therefore, the analyses indicate that the viscosity-pressure effect of natural gas needs to be considered when used in high pressure situation.

Purpose The purpose of this paper is to enhance film stiffness and control seal leakage of conventional spiral groove dry gas seal (S-DGS) at a high-speed condition by introducing a new type superellipse surface groove. Design/methodology/approach The steady-state performance and dynamic characteristics of superellipse groove dry gas seal and S-DGS are compared numerically at a high-speed condition. The optimized superellipse grooves for maximum steady-state film stiffness and dynamic stiffness coefficient are obtained. Findings Properly designed superellipse groove dry gas seal provides remarkable larger steady-state film stiffness, dynamic stiffness coefficient and lower leakage rate at a high-speed condition compared to a typical S-DGS. The optimal values of first superellipse coefficient for maximum steady and dynamic stiffness are 1.3 and 1.4, whereas the optimal values of second superellipse coefficient for which are 1.4 and 2.0, respectively. Originality/value A new type of molded line, namely, superellipse curve, is proposed to act as the boundary lines of surface groove of dry gas seal, as an alternative of typical logarithm helix. The conclusions provide references for surface groove design with larger stiffness and lower leakage rate at a high-speed condition.

Gas film disturbance characteristics analysis of high-speed and high-pressure dry gas seal

Finite difference method is used to solve modified Reynolds equation based on FK slip flow model. The startup performance of spiral groove dry gas seal (S-DGS), T-groove dry gas seal (T-DGS) and linear groove dry gas seal (L-DGS) in startup stage are analyzed. The results show that in the startup stage, the slip flow effect reduces the opening force, and the influence of slip flow effect on the opening force is the most obvious for S-DGS, while it is the weakest for L-DGS. Considering the slip flow effect, the hydrodynamic pressure effect of S-DGS is the largest and that of L-DGS is the smallest. Under the same conditions, the shaft speed range of S-DGS with obvious slip flow effect with the Knudsen number being 0.001 is the smallest, and that of T-DGS is the largest. The research provides theoretical guidance for the reasonable evaluation of the influence of slip flow effect on the startup performance of dry gas seals in the startup stage.KeywordsDry gas sealStartup performanceSlip flow effectHydrodynamic pressure effect

Purpose This study aims to study the gas film stiffness of the spiral groove dry gas seal. Design/methodology/approach The present study represents the first attempt to calculate gas film stiffness in consideration of the slipping effect by using the new test technology for dry gas seals. First, a theoretical model of modified generalized Reynolds equation is derived with slipping effect of a micro gap for spiral groove gas seal. Second, the test technology examines micro-scale gas film vibration and stationary ring vibration to determine gas film stiffness by establishing a dynamic test system. Findings An optimum value of the spiral angle and groove depth for improved gas film stiffness is clearly seen: the spiral angle is 1.34 rad (76.8º) and the groove depth is 1 × 10–5 m. Moreover, it can be observed that optimal structural parameters can obtain higher gas film stiffness in the experiment. The average error between experiment and theory is less than 20%. Originality/value The present study represents the first attempt to calculate gas film stiffness in consideration of the slipping effect by using the new test technology for dry gas seals.

PurposeThis 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/approachThe 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.FindingsThere 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/valueThe 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.

PurposeThis paper aims to acquire the phase distribution and sealing performance of supercritical carbon dioxide (SCO2) dry gas seals with phase transitions.Design/methodology/approachThe SCO2 spiral groove dry gas seal is taken as the research object. The finite differential method is applied to solve the governing equations. Furthermore, the phase distribution and the sealing performance are obtained. Compared to the ideal gas model, the effect of phase transitions on sealing performance is also explored.FindingsVaporization is likely to occur near the inner radius when SCO2 dry gas seals are operated near the critical point. Whether phase transitions are considered in the model affects the sealing performance seriously. When phase transitions are considered, the sealing performance depends significantly on the working conditions, and unexpected results are produced when inlet conditions approach the critical point.Originality/valueThe numerical model for SCO2 dry gas seals with phase transitions is established. The phase distribution and the sealing performance of SCO2 dry gas seals are explored.

Aiming at the serious problem of gap leakage between the stator and rotor of the supercritical carbon dioxide compressor, the dry gas seal scheme for centrifugal compressor of small S-CO2 power system is designed in this paper. The sealing characteristics of the single-stage spiral groove dry gas seal in the S-CO2 operating environment are studied by the large-scale commercial CFD numerical analysis software ANSYS CFX. The leakage characteristics of the S-CO2 dry gas seal at different film thicknesses are obtained. The results show that the S-CO2 dry gas seal leakage flow rate increases linearly with the increase of the film thickness. The dry gas seal leakage flow rate under different film thickness is 0.04%∼0.45% of the S-CO2 compressor flow rate, which can meet the leakage requirements of the centrifugal compressor shaft end seal in the small S-CO2 power system.

Supercritical carbon dioxide (SCO2) Brayton cycle has been widely used as a substitute for traditional steam cycles. As one of the key components of power-dense turbomachines, dry gas seal (DGS) has been considered one of the most suitable dynamic seal options due to its superior sealing performance. In this paper, considering the installation errors and external disturbances of the shaft end seal in a 14 MW supercritical carbon dioxide turbine, a comprehensive numerical study was conducted on the seal static and rotordynamic characteristics for four dry gas seal designs with different coning displacements. To account for the real gas effect of SCO2, a table lookup program was used to obtain the physics properties of CO2 under supercritical conditions based on the database of the National Institute of Standards and Technology (NIST). A frequency-dependent rotordynamic coefficients stiffness damping model (KC model) for SCO2 DGS and the mesh deformation technique were adapted to analyze the rotordynamic performance of the designed DGS. Steady-state numerical results of leakage rate, opening force, and steady film stiffness were analyzed and compared under four coning displacements. Transient numerical results of rotordynamic force coefficients were analyzed and studied under four coning displacement ratios and 10 vibration frequencies. Results show that the increase in coning displacement causes a 20% rise in leakage rate and a 2.4% increase in opening force. The tilted seal ring results in a maximum 30%–84% variation of the dynamic stiffness and greatly influences the damping coefficient. From a comprehensive view, the tilt of the seal rings brings undesired influences on both the static and rotor dynamic performance of SCO2 DGS. The distortion of the seal rings due to installation errors and other disturbances should be avoided as far as possible in SCO2 DGSs.

PurposeThe purpose of this paper is to investigate the influence rule and mechanism of three degrees of freedom film thickness disturbance on the transient performance of spiral groove, upstream pumping spiral groove dry gas seal (UP-SDGS) and double-row spiral groove dry gas seal (DR-SDGS).Design/methodology/approachThe transient performance of spiral groove, UP-SDGS and DR-SDGS are obtained by solving the transient Reynolds equation under different axial and angular disturbance coefficients. The transient and steady performance of the above-mentioned DGSs are compared and analyzed.FindingsThe film thickness disturbance has a remarkable impact on the sealing performance of DGS with different structures and the calculation deviations of the leakage rate of the UP-DGS will increase significantly if the film thickness disturbance is ignored. The axial and angular disturbance jointly affect the film thickness distribution of DGS, but there is no significant interaction between them on the transient sealing performance.Originality/valueThe influence mechanism of axial disturbance and angular disturbance on the transient performance of typical SDGSs behavior has been explained by theory. Considering small and large disturbance, the interaction between axial disturbance and angular disturbance on the transient performance have been studied.