Leakage and Stiffness Characteristics of Bionic Cluster Spiral Groove Dry Gas Seal
Spiral groove dry gas seal (S-DGS), the most widely used DGS in the world, encounters the problem of high leakage rate and inferior film stability when used in high-speed machinery equipment, which could not be well solved by optimization of geometrical parameters and molded line of spiral groove. A new type of bionic cluster spiral groove DGS (CS-DGS) is proved to have superior film stability than S-DGS at the condition of high-speed and low-pressure numerically. A bionic CS-DGS is experimentally investigated and compared with common S-DGS in order to provide evidence for theoretical study. The film thickness and leakage rate of both bionic spiral groove and common spiral groove DGS are measured and compared with each other and with theoretical values under different closing force at the condition of static pressure, high-speed and low-pressure, and the film stiffness and stiffness-leakage ratio of these two face seals are derived by the relationship between closing force and film thickness at the steady state. Experimental results agree well with the theory that the leakage and stiffness of bionic CS-DGS are superior to that of common S-DGS under the condition of high-speed and low-pressure, with the decreasing amplitude of 20% to 40% and the growth amplitude of 20%, respectively. The opening performance and stiffness characteristics of bionic CS-DGS are inferior to that of common S-DGS when rotation speed equals to 0 r/min. The proposed research provides a new method to measure the axis film stiffness of DGS, and validates the superior performance of bionic CS-DGS at the condition of high-speed and low-pressure experimentally.
KeywordsBionic cluster spiral groove Film stiffness Dry gas seal Leakage rate
Dry gas seals with unidirectional spiral groove (S-DGS) have encountered numerous applications in medium and high speed rotating equipment because of their low friction, long life time, such as centrifugal compressors, pumps and blowers [1, 2, 3, 4, 5, 6]. However, the abrupt failure of S-DGS occurs from time to time when used in high speed machinery with big vibration for the weak capacity of resisting disturbance of gas film between the two faces. Strict environmental protection laws require efficient sealing systems for preventing release of the poisonous and harmful fluids into atmosphere, which bring forward higher request for the sealing performance of S-DGS. Leakage rate and film stiffness are the two key sealing performance parameters of dry gas seal. The purpose of a seal is to prevent leakage; hence, the leakage path should be as small as possible, and the leakage rate should be as low as possible. The axial film stiffness is used to characterize the capacity of resisting disturbance of gas film. The higher of film stiffness, the stronger capacity of resisting disturbance of gas film; hence, it is of cardinal importance to enhance the stiffness of gas films to minimize vibration due to external excitations. The separation gap between the frictional surfaces deeply influence the leakage rate and film stiffness, which created by the opening force, and should be neither too large nor too small: if the film gap is too large, the process fluid leaks across the faces and lead to the large leakage rate and small film stiffness; and if too small, excessive wear is likely to occur .
In order to maximize the film stiffness of dry gas seal, and keep a low leakage rate at the same time, a great deal of numerical and experimental study on groove structure and parameter optimization of DGS under varying operation conditions have been carried out. The effect of geometry parameters of groove, such as spiral angle, groove-dam ratio and groove depth, on the performance of a S-DGS, such as opening force, film stiffness, leakage rate and stiffness-leakage ratio, are numerically analyzed by Zirkelback et al. , Liu et al. , Peng et al. . The optimum structural parameters of groove with different objective functions in different operating conditions are presented. In recent years, the effect of groove structural optimization on performance improvement of DGS has attracted more and more researchers’ attention because the limitation of performance enhancement by optimum design of groove geometry parameters of S-DGS simply. Recently, a parametric study and performance evaluation of typical bidirectional dry gas seals with modified surface were presented by Su et al. [11, 12] and Blasiaka and Zahorulko . Hashimoto and Ochiai [14, 15, 16] presented an entirely new optimum design methodology, in which the groove geometry can be flexibly changed by using the spline function, to maximize the stiffness of gas films for grooved thrust bearings. The film stiffness and load-capacity of the optimized bearing are higher than that of common spiral groove bearing and herringbone bearing, which were both theoretically and experimentally verified. The sealing performance of the spiral groove DGS with an inner annular groove (AS-DGS)  was numerically analyzed by Peng et al. , which was invented by Flowserve Corporation. The results show that the opening force and film stiffness of AS-DGS is larger than that of the common S-DGS at the condition of low-speed and high-pressure, while the leakage is a little larger. Dresser-Rand Corporation presented a patented pattern of short grooves in combination with the longer grooves to provide excellent lift-off effect and optimum gas film stiffness at low speed as well as high speed. During operation, the short grooves could create obvious high pressure to separate the mating rings at slow speed, while the longer grooves contribute to creating a stable gas film between the two rings at high speed. The uni-directional V groove with the combined radial-circumferential tapered groove bottom , which was invented by Burgmann Corporation, causes a reduction in the opening force and leakage rate. Despite that there are only few researchers pay attention to the experiment of the sealing performance test of DGS, most of the researchers only focus on the numerical simulation and analysis of groove structure and geometry parameters optimization as the subject to improve the gas film stiffness and reduce the fluid leakage of DGS.
Jiang et al. [20, 21] introduces a new type of bionic cluster spiral groove dry gas seal (CS-DGS) based on the coupling bionic theory, and presents the numerical study on the influence of groove geometry parameters on the sealing performance, such as opening force, axial film stiffness and seal leakage. The results shows that the sealing performance and operation stability of bionic CS-DGS are much better compared to common unidirectional S-DGS at high speed.
The aim of this study is to investigate the leakage and stiffness characteristics of a bionic cluster spiral groove DGS experimentally which could provide evidence for the theoretical study. The film thickness and leakage rate of CS-DGS and S-DGS were measured and the closing force was calculated at the condition of static pressure and high-speed, and the measured data were compared with the theoretical results. The relationship between film stiffness and film thickness was derived from the relationship between opening force and film thickness. The experimental results could provide a reference for the design and model selection of CS-DGS in engineering.
2.1 Experimental Apparatus
In order to measure the film thickness and leakage rate, a sensor holder is fixed on the test stationary ring. An eddy current displacement sensor is installed on the holder by thread and a stainless steel sheet is fixed on the test rotor by super glue. The on-line measurement of gas film thickness is carried out by measuring the displacement change between the eddy current sensor and the sheet during testing. The leaking gas goes out from the leakage rate measurement hold on the sensor holder, connected to leakage rate sensor, a digital MEMS gas mass flow meter, through pipe. Pressurized seal gas provided by a low pressure air compressor flows into the seal chamber after filtered (filter precision is less than 1 μm) and dehumidified by tertiary filters, and the pressure of which is adjusted by a pressure value and measured by a pressure sensor. All the data (including the data from the pressure sensor, the speed control system, the eddy current sensor and the gas mass flow meter) collected from the sensors are fed into a data acquisition system and interfaced with a computer.
2.2 Specimen Preparation
The specimens are made in pairs. The plain, rotating ring with an I.D. of 59 mm, O.D. of 73 mm is made of carbon because of its superior wear and corrosion properties. Silicon carbide has been chosen as the material of stationary ring for its high hardness and thermal conductivity, low coefficient of thermal expansion and good compatibility with carbon . The stationary ring is larger in size and textured with two different types of grooves: spiral groove and bionic cluster spiral groove which is made for comparison purpose, and the friction surfaces are polished. Figure 4 shows the photographs of the stationary rings textured with bionic cluster spiral groove (a) and spiral groove (b) where the textured areas appear as darker matt surface, and the 2D profile of grooves marked by the Optical Fiber Laser Marking machine, which are obtained by Dektakxt Stylus Profiler. The groove depth is controlled by the laser power, scanning speed and interval of the laser beam. In this experiment, when the laser power set to 30% of its full capacity, scanning speed of 100 mm/s and scanning internal of 0.05, the measured groove depth is about 9 μm. Roughness of untextured region is Ra = 0.05 μm, and the roughness of the bottom of grooves is Ra = 0.8 μm.
Specifications for cluster spiral groove seal face
Outside radius ro (mm)
Inside radius ri (mm)
Balance radius rb (mm)
Groove number Ng
Spiral angle β (°)
Groove-dam ratio αg
Groove-land ratio δ
2.3 Experimental Procedure
The experimental procedure for investigation of leakage and stiffness characteristics of bionic cluster spiral groove DGS and spiral groove DGS are provided as follows. Firstly, the rotating rings and stationary rings are cleaned in a bath ultrasonically with acetone for 5 min, the spring stiffness of the springs used is measured and eddy current sensor is calibrated before the experiment. Secondly, the stator and rotor specimens are installed in the holders, and the spring compression is measured. The desired gas pressure is set to 0.5 MPa and 0.4 MPa in the hydrostatic performance test and hydrodynamic performance test respectively; the speed of the motor is programmed to 8000 r/min in the dynamic pressure performance test. Spring force is changed by steps of 3 N, adjusted by the three uniformly distributed micro meters, and each force step is maintained for 1 min following the stabilization of the film thickness and leakage rate at a steady value. The test ends when a minimum film thickness of 1.5 μm is reached or if the leakage rate exceeds a value of 10 L/min. The gas pressure in the chamber under constant rotational speed and spring force is increased in a stepwise manner to examine the leakage and stiffness characteristics of these two types of DGSs at different gas pressure. In each step, the machine runs at a constant speed and gas pressure for 1 min. Each of the carbon rotating rings is used less than five times to ensure that the end faces are hardly worn. What’s more, experiments for the same seal ring under the given conditions are repeated twice to guarantee the repeatability of the results. At the same time, the film thickness, leakage rate, gas pressure and rotational speed are monitored and recorded by a high-speed data acquisition system with a sampling rate of 1000 Hz. At last, the collected data are output to an “.xls” file for graphic presentation.
3 Results and Discussion
3.1 Hydrostatic Characteristics Test
3.2 Hydrodynamic Characteristics Test
A new method to measure axial gas film stiffness of dry gas seals experimentally is proposed. The experimental results of opening force and leakage rate of bionic CS-DGS agrees well with that of theoretical values when film thickness is larger than 4 μm.
The opening performance and stiffness characteristics of bionic CS-DGS are inferior to that of common S-DGS when rotation speed equals to 0 r/min, which indicate that CS-DGS is not suitable for low speed or high pressure applications.
The bionic CS-DGS is a type of hydrodynamic seal for its significant hydrodynamic force generated under high-speed conditions. The leakage and stiffness characteristics of bionic CS-DGS are superior to that of common S-DGS under the condition of high-speed and low-pressure.
X-DP was in charge of the whole trial; J-BJ wrote the manuscript; J-YL and YC assisted with sampling and laboratory analyses. All authors have read and approved the final manuscript.
Jin-Bo Jiang, born in 1989, is currently a post-doctor at Engineering Research Center of Process Equipment and Its Re-manufacturing of Ministry of Education, Zhejiang University of Technology, China. He received his PhD degree from Zhejiang University of Technology, China, in 2016. His research interests include bionic design of mechanical face seal. Tel: +86-13819450477; E-mail: email@example.com.
Xu-Dong Peng, born in 1964, is currently a professor at Zhejiang University of Technology, China. He received his PhD degree from Xi’an Jiaotong University, China, in 1999. His research interests include fluid seal technology and design method of surface texture. Tel: +86-571-88871503; E-mail: firstname.lastname@example.org.
Ji-Yun Li, born in 1965, is currently a professor senior engineer at Zhejiang University of Technology, China. She received her master degree in China University of Petroleum. E-mail: email@example.com.
Yuan Chen, born in 1990, is currently a PhD candidate at Engineering Research Center of Process Equipment and Its Re-manufacturing of Ministry of Education, Zhejiang University of Technology, China. E-mail: firstname.lastname@example.org.
The authors declare no competing financial interests.
Ethics Approval and Consent to Participate
Supported by National Natural Science Foundation of China (Grant No. 51575490), National Key Basic Research Development Plan (973 Plan, Grant No. 2014CB046404), Key Program of Zhejiang Provincial Natural Science Fund Project (Grant No. LZ15E050002), and Zhejiang Provincial Natural Science Foundation of Youth Fund (Grant No. LQ17E050008).
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