Static Seal Research Articles

Design of a frictionless magnetorheological damper with a high dynamic force range

This article discusses an increase in dynamic force range in a spring–damper unit achieved by elimination of sealings’ friction. This friction is a part of damping force that cannot be controlled; therefore, it is undesirable in magnetorheological dampers. A new design of a magnetorheological damper with no friction force is described and compared with a traditional magnetorheological damper consisting of a piston and piston rod seals. In the traditional design, fluid is forced to flow by a hydraulic cylinder with high friction caused by sealings. In order to eliminate this friction, a frictionless unit made of metal bellows was designed. Elastic metal bellows can be sealed only by static seals. The measurement of force–velocity dependency was carried out for the original and the new damper with the same magnetorheological valve. The results indicate that the frictionless unit exhibits a significant improvement in the dynamic force range. In the case of adaptive-passive damping control, the increase in the dynamic force range enables the improvement of vibration elimination in the entire frequency range.

Turbine thermomechanical modelling during excessive axial movement and overspeed

ABSTRACTThis manuscript discusses the numerical (finite element) and analytical modelling of structural interactions between gas turbine components in case of excessive axial movement and overspeed. Excessive axial movement, which may occur after a shaft failure, results in contact between rotating and static turbine components under high forces. These forces create friction which can act as a counter torque, potentially retarding the ‘free-rotating’ components. The study is based on a shaft failure scenario of a ‘three-shaft’, high ‘bypass’ ratio, civil ‘large-fan’ engine. Coupled analytical performance and friction methods are used as stand-alone tools to investigate the effect of rubbing between rotating and stationary components. The method is supported by ‘high-fidelity’, ‘three-dimensional’, thermomechanical finite element simulations using LS-DYNA software. The novelty of the work reported herein lies in the development of a generalised modelling approach that can produce useful engine design guidelines to minimise the terminal speed of a free running turbine after an unlocated shaft failure. The study demonstrates the advantage of using a fast analytical formulation in a design space exploration, after verifying the analytical model against finite element simulation results. The radius and the area of a stationary seal platform in the turbine assembly are changed systematically and the design space is explored in terms of turbine acceleration, turbine dislocation rate and stationary component mass. The radius of the friction interface increases due to the increasing radius of the nozzle guide vane flow path and stationary seal platform. This increases the frictional torque generated at the interface. It was found that if the axial dislocation rate of the free running turbine wheel is high, the resulting friction torque becomes more effective as an overspeed prevention mechanism. Reduced contact area results in a higher axial dislocation rate and this condition leads to a design compromise between available friction capacity, during shaft failure contact and seal platform structural integrity.

Performance verification of elastomer materials in corrosive gas and liquid conditions

Elastomer seals are essential for zonal isolation in vertical and deviated wells. They are often used either as O-rings (static seals) or as energized seals (packers). For elastomers to perform their functions effectively over an extended period, it is essential that the elastomer withstand the deleterious effects of pressure, prolonged loading effects, severe temperature, corrosive fluids, and acidic gases. As such it is imperative to understand the various elastomer degradation mechanisms and develop methods to reduce the degradation. The objectives of this study are: to investigate if elastomers are “fit for service” for well construction applications and to evaluate elastomer performances under downhole corrosive conditions. To achieve this goal, some dynamic wellbore conditions which close-to-real field conditions are were simulated in a controlled laboratory setup. The following properties were investigated: elastomer hardness, elastomer compression, and elastomer volumetric swelling. In the experimental setup, four frequently used elastomers in the oil and gas industry were studied: Nitrile butadiene rubber (NBR), Ethylene propylene diene monomer (EPDM), Fluoroelastomer (FKM) commonly known as Viton, and Polytetrafluoroethylene (PTFE). The results indicate that NBR exhibited the most significant deterioration. Viton showed the least amount of deterioration but has poor decompression resistance. The aging condition that proved to be the most damaging is CO2. Analysis of Variance (ANOVA) was also performed on the data from the samples as a statistical analysis indicator. Some other observations and conclusions are that the degree of chain growth and chain rupture depends on the temperature and the degree of chemical exposure.

Design and Analysis of 10 MW Class HTS Exciting Double Stator Direct-Drive Wind Generator With Stationary Seal

The superconducting (SC) generators are expected to have a promising application for offshore wind generation due to the merits of high power density and high efficiency over the conventional generators. In this paper, a 10-MW conceptual high temperature superconducting (HTS) exciting double stator direct-drive wind generator (HTS-DSDDG) with the stationary seal is proposed and investigated. The proposed HTS-DSDDG has two spatially independent stators: to place HTS field windings and copper armature windings, respectively. Thus, it realizes the stationary seal of the cooling system and removes brushes and slip rings and the other additional excitation device; hence offering the advantages of the high reliability and the low operation & maintenance cost. The main design considerations and the initial design specifications of the 10 MW HTS-DSDDG are given in detail. Furthermore, the electromagnetic characteristics of the HTS- DSDDG under no-load, rated load, and three-phase short-circuit fault conditions are analyzed by the finite element method (FEM). Subsequently, its size, weight, and cost are compared with the existing conceptual 10 MW wind generators. The results reveal that the HTS-DSDDG has excellent electromagnetic performances, such as good sinusoidal voltage, low cogging torque, and torque ripple; besides, its output power per volume is twice than that of the permanent magnet direct-drive generator (PMDDG), being comparable to that of the existing conceptual MgB2-HTS direct-drive generators.

Procedure to Determine Friction Torque in a Mechanical Membrane Contact Seal

The main types of available mechanical seals are considered briefly. The operating conditions of a membrane contact mechanical seal with a stationary secondary seal are analyzed. The design procedure of friction torque of the seal under consideration is presented for the example of a pumping unit feeding the fuel components of the steering assembly of LRE.

Numerical Analysis and Experimental Study on Magnetic Fluid Reciprocating Seals

Magnetic fluid is a new type of functional materials. The seal of reciprocating shaft is a typical application of the magnetic fluid. However, there is little research on the magnetic fluid reciprocating seals, thus improving the sealing life and seal capacity is the key problem of studying the reciprocating seals. This paper presents a structure of the seal to test the sealing life and other characteristics. The magnetic field of the seal is estimated using the finite-element method. The thickness of the magnetic fluid on the surface of reciprocating shaft and the flow rate of magnetic fluid are obtained theoretically. The experimental result shows that the static seal capacity is related to the fluid volume. The static seal capacity obtained by experiment is approximately equal to the value through the simulation with sufficient magnetic fluid. The relationship between sealing life and the motion of the reciprocating shaft is obtained experimentally. The sealing life decreases rapidly with the increase of the stroke and velocity.

A HTS-Excitation Modular Flux-Switching Linear Machine With Static Seals

This paper proposes and analyzes a novel high-temperature superconducting (HTS) modular flux-switching linear machine for long-stator applications, which adopts the stator with a low-cost salient-pole iron core and accommodates both the armature windings and HTS-excitation windings on the short mover, thus facilitating static seals of cooling Dewar for the HTS windings. The key of the proposed machine is to employ a modular mover structure composed of separate phase modules to achieve a complementary pattern, such that the cogging force can be significantly offset to effectively reduce the force ripple while offering the advantages of very symmetrical back electromotive force (EMF) and high fault tolerance. The operation principle and optimization analysis of the proposed machine with concern on the HTS-excitation characteristics are carried out. Also, by using the two-dimensional finite element method, the electromagnetic performances of the proposed machine are investigated to verify its validity. The results show that it can offer symmetrical and sinusoidal back-EMF, high fault tolerance, and low force ripple.

A Comparative Study of Ferrofluid Seal and Magnetorheological Fluid Seal

Compared with a ferrofluid (FF) seal used as a sealing material, magnetorheological (MR) fluid has higher sealing pressure at the same condition. This is because MR fluid not only has magnetic property but also shows obviously elasto-plastic performance, which differs from FFs, under the magnetic field. On this basis, this paper analyzed, respectively, the static seal performance of MR fluids and FFs based on their magnetic properties through experimental and numerical studies. The results showed that an obvious pressure drop happened when the pressure is about 400 kPa in seal cavity for MR fluid seal, but the pressure was quickly down to around 390 kPa; nevertheless, the maximum pressure of the FF seal is 200 kPa, and the seal time with leak-free is about 2.5 min, and then the value quickly goes down to zero. The maximum pressure of every pole tooth of FF seal is about 11.11 kPa. The pressure of MR fluid seal is 21.67 kPa when there is a significant decrease during pressuring, and the maximum pressure of every pole tooth of MR fluid seal is 41.37 kPa. At last, the relationship of the sealing pressure of MR fluid seal with the time was derived.

Design, Analysis, and Experimental Test of a Segmented-Rotor High-Temperature Superconducting Flux-Switching Generator With Stationary Seal

This paper proposes to implement a segme-nted-rotor high-temperature superconducting (HTS) flux-switching generator with dual electrical ports for offshore wind power generation. The originality is both electrical ports, namely armature windings and field-excitation windings, are placed in the stator so that the stationary seal of the cryogenic cooling system can be achieved. The key lies in that the HTS field-excitation windings in the stator side provide high field excitation, and the sinusoidal electromotive force in armature windings is induced by the magnetic field modulation of the segmented rotor. A new kind of racetrack-shaped Dewar with vacuum-sandwich was proposed and fabricated to accommodate and cool the HTS coils. Based on this proposed racetrack-shaped Dewar, a cryogenic cooling system of the proposed generator was established to maintain the superconducting properties of HTS wires. Both finite element analysis and experiments were carried out to verify the validity of the proposed generator and its cryogenic cooling system.

Vibroacoustic Prediction of a High-Temperature Superconducting Field-Modulation Double-Stator Machine with Stationary Seal

This paper predicted the vibroacoustic regularity of a high temperature superconducting (HTS) field-modulation double-stator (HTS-FMDS) machine with stationary seal for low-speed and direct-drive applications. The originality of this paper lies in that the spatial order and angular velocity of electromagnetic-force density of the HTS-FMDS machine were derived by using the analytical method. Moreover, the validity of the analytical solutions was verified by the finite element analysis (FEA) results. Then, the modal shapes and frequencies of the outer stator were obtained by using multiphysics coupling simulation. By transferring the electromagnetic force to the stator structural model, the regularity of electromagnetic vibration and noise of the HTS-FMDS machine were revealed.

Numerical and Experimental Analyses of Static and Dynamic Characteristics for Partially Helically Grooved Liquid Annular Seals

Numerical and experimental analyses of the static and dynamic characteristics of the liquid annular seals with axially partial helical grooves were conducted to investigate the effects of the axial length gal of a helically grooved section in a seal stator. The numerical solution and experimental procedures were applied in the same manner as in previous studies on through-helically grooved seals, wherein the grooves extend across the seal length. The numerical results qualitatively agreed with the experimental results, demonstrating the validity of the numerical analysis. The leakage flow rate Q was lower in the partially helically grooved seals than that of conventional through-helically grooved seals across a small range of rotor spinning velocities. In contrast, the reduction in Q due to the pumping effect caused by the spin of the rotor diminished with the decrease in gal. For a small concentric whirling motion of the rotor, the radial dynamic reaction force Fr and magnitude of variation in the tangential dynamic reaction force Ft with the whirling angular velocity increased with the decrease in gal, and their values approached the corresponding values for the smooth-surface seal. Under the same rotor whirling velocity, the Ft for the partially helically grooved seals became lower than that for the smooth-surface seal (similar to the case for the through-helically grooved seal), although decreasing gal tended to increase Ft. These results suggest that partially helically grooved seals can improve the efficiency and stability margin of the pumps because of the reduction in leakage flow rate and suppression of the rotor forward whirling motion (with large radial and tangential dynamic reaction forces) as compared to conventional through-helically grooved seals.

Analysis and Structure Optimization of Radial Halbach Permanent Magnet Couplings for Deep Sea Robots

Permanent magnet couplings (PMCs) can convert the dynamic seal of transmission shaft into a static seal, which will significantly improve the transmission efficiency and reliability. Therefore, the radial Halbach PMC in this paper is suitable as the transmission mechanism of deep sea robots. A two-segment Halbach array is adopted in the radial PMC, and the segment arc coefficient can be adjustable. This paper presents the general analytical solutions of the distinctive Halbach PMCs based on scalar magnetic potential and Maxwell stress tensor. The analytical solutions of magnetic field are in good agreement with 2-D finite element analysis (FEA) results. In addition, an initial prototype of the radial Halbach PMC has been fabricated, and the analytical solutions of magnetic torque are compared with 3-D FEA and experiment results. This paper also establishes an optimization procedure for PMCs based on the combination of 3-D FEA, Back Propagation Neural Network (BPNN), and Genetic Algorithm (GA). 3-D FEA is performed to calculate the pull-out torque of the samples from Latin hypercube sampling, then BPNN is used to describe the relationship between the optimization variables and pull-out torque. Finally, GA is applied to solve the optimization problem, and the optimized scheme is proved to be more reasonable with the FEA method.

An Experimental Study of the Leakage Mechanism in Static Seals

This paper investigates the fluidic leak rate through sealing contact surfaces by comparison between model calculation and experiment measurement. The focus is on an experimental device designed to measure the leak rate of the static seals with a simpler structure, smaller errors, stronger stability, and more functions. Using the device, experiments were carried out to four test pieces with different surface characteristics, whose leak rates were measured separately. Compared with the calculation results obtained from the fractal surface leak rate prediction model, the correctness and the application range of the model were verified, and the effects of different surface topographies and material properties of the four test pieces on the leak rate were analyzed as well. The experimental device was also used to perform single-factor comparison experiments, which were then combined with the theoretical prediction model to analyze the effects of the sealing surface contact load, fluid pressure, and surface apparent size on the leak rate, so that theoretical support and experimental evidence for selecting the parameters of sealing device was provided.

Design and development of large radial clearance static and dynamic magnetic fluid seal

A large radial clearance (0.7 mm) static and dynamic magnetic fluid seal is designed and its performance is tested. The radial clearance of 0.7 mm is quite higher than the most of the commercial seals which have maximum radial clearance of 0.3 mm. Hydrostatic magnetic fluid “plug” is created in 5.94 mm glass tube and it is tested for various plug heights with respect to the pole piece height. Results show that as plug height increases the pressure holding capacity increases and then for optimum fluid height it becomes maximum. The increase in fluid magnetization also increases the pressure holding capacity of fluid plug. Using these results the dynamic seal having a large radial clearance (0.7 mm) is designed and simulated using FEMM. The prototype test set-up is fabricated with multiple magnetic sources for 0.7 mm radial clearance between the rotating shaft (DIA: 25 mm) and the pole piece. The seal assembly is tested for transformer oil based magnetic fluid for variable speed and the number of stages under vacuum pressure of 10−3mbar. Results show three stages are sufficient to achieve 10−2mbar pressure difference at 1500 rpm. Such seal can work as the bearing protection in low vacuum devices.

Leakage Monitoring in Static Sealing Interface Based on Three Dimensional Surface Topography Indicator

Leakage directly affects the functional behavior of a product in engineering practice, and surface topography is one of the main factors in static seal to prevent leakage. This paper aims at monitoring the leakage in static sealing interface, using three-dimensional (3D) surface topography as an indicator. The 3D surface is measured by a high definition metrology (HDM) instrument that can generate millions of data points representing the entire surface. The monitoring approach proposes a series of novel surface leakage parameters including virtual gasket, contact area percentage (CAP), void volume (VV), and relative void volume (SWvoid) as indicators. An individual control chart is adopted to monitor the leakage surface of the successive machining process. Meantime, based on the Persson contact mechanics and percolation theory, the threshold of leakage parameter is found using finite element modeling (FEM). Experimental results indicate that the proposed monitoring method is valid to precontrol the machining process and prevent leakage occurring.

A New Structural Dynamic Model for Pump Mechanical Seals Vibration Analysis Incorporating Squeeze Motion of O-Ring Seals and General Dynamic Motion of the Pump Housing and the Pump Shaft

New models are developed for flexibly mounted stator (FMS) and flexibly mounted rotor (FMR) mechanical seals that incorporate the radial reaction force components produced by supporting O-rings due to relative squeezing motion across the O-rings. Supporting data come from tests done in relation to O-ring supports for ball bearing races. The reaction-force model is linear but a nonlinear function of excitation frequency. The model accounts for the axial displacement doz of the O-ring from the mass center of the seal stator (FMS configuration) or seal rotor (FMR configuration), which couples the radial and pitch–yaw motion of the model's stiffness and damping matrices. Greens' coned-face-seal model is used to define the reaction moment arising across the seal faces via stiffness and damping matrices. The damping matrix does not coincide with Green's. His is constant; the matrix developed here contains terms that are harmonic at twice theprecession frequency. When averaged over one precession cycle, the new average damping matrix coincides with Green's result. When the averaged damping matrix is used, the resultant model is linear. However, because of the viscoelastic reaction-force and reaction-moment models used for the O-ring coefficients, most of the stiffness and damping matrices are strong functions of the assumed precession frequency. The new FMR model contains a skew-symmetric stiffness matrix due to the O-ring damping terms. In rotordynamics, skew symmetric stiffness matrices due to internal damping in the rotor can lead to rotordynamic instabilities.

Electromagnetic Characteristics Analysis of a High-Temperature Superconducting Field-Modulation Double-Stator Machine with Stationary Seal

This paper has proposed a high temperature superconducting (HTS) field-modulation double-stator (HTS-FMDS) machine with a stationary seal for low-speed and direct-drive applications. The originality of this paper lies in the HTS field-excitation windings, which were fed with a direct current in order to offer strong field excitation, were placed in the inner stator, while the armature windings were installed in the outer stator so that the stationary seal of the cryogenic cooling system could be achieved. Moreover, a ferromagnetic ring was mounted on the top of each HTS coil to prevent the quench of the HTS wires resulting from the armature-reaction magnetic-field. By using finite-element analysis (FEA), the influence of the armature-reaction magnetic-field on the critical current and the electromagnetic properties were carried out so as to verify the validity of the proposed machine.

Numerical and Experimental Analyses of Dynamic Characteristics for Liquid Annular Seals With Helical Grooves in Seal Stator

Numerical and experimental analyses were carried out to investigate the dynamic characteristics of liquid annular seals with helical grooves in the seal stator. In the numerical analysis, the governing equations were the momentum equations with turbulent coefficients and the continuity equation, all averaged across the film thickness and expressed using an oblique coordinate system in which the directions of coordinate axes coincided with the circumferential direction and the direction along the helical grooves. These governing equations were solved numerically to obtain the dynamic characteristics, such as the dynamic fluid-film forces, dynamic coefficients, and whirl-frequency ratio (WFR). The numerical analysis included the effect of both fluid inertia and energy loss at the steps between the helical groove and the land sections. In the experiments, the dynamic fluid-film pressure distributions, which were induced by a small whirling motion of the rotor about the seal center, were measured to obtain the dynamic characteristics. The equivalent numerical results reasonably agree with the experimental results, demonstrating the validity of the numerical analysis. The value of the tangential dynamic fluid force induced by the rotor whirling motion decreased with increasing the helix angle γ. Consequently, the values of the cross-coupled stiffness coefficient and WFR decreased with increasing γ and became negative for large γ. In general, pump rotors rotate with a forward whirling motion under normal operating conditions. Hence, the negative value of WFR for helically grooved seals contributes to rotor stability by suppressing the forward whirling motion of the rotor.

Investigation of Static and Dynamic Seal Performances of a Rubber O-Ring

Rubber O-rings are widely applied in the static and dynamic seals of machinery, energy, chemical, aviation, and other fields. Mechanical behavior and sealing performance of the O-ring were investigated in this paper. Effects of precompression amount, fluid pressure, friction coefficient on the static and dynamic sealing performances of the O-ring were studied. The results show that the maximum stress appears on the inside but not surface of the O-ring. The static sealing performance increases with the increasing of fluid pressure and compression amount. Reciprocating dynamic sealing performance of the rubber O-ring is different with its static sealing performance; the stress distribution and deformation are changing in reciprocating motion. Sealing performance in outward stroke is better than it in the inward stroke. Overturn of the O-ring occurs when the friction torque is greater than the torque that caused by fluid pressure in the inward stroke. Distortion, bitten, and fatigue failure are the main failure modes of the O-ring in the dynamic seal. Those results can be used in design, installation, and operation of rubber O-rings in static and dynamic seals.

Experimental Analysis on Static and Dynamic Characteristics of Liquid Annular Seals with Axially Partial Helical Grooves in Seal Stator

Experimental Analysis on Static and Dynamic Characteristics of Liquid Annular Seals with Axially Partial Helical Grooves in Seal Stator