Non-contacting Seals Research Articles

An Experimental Investigation of the Steady-State Response of a Noncontacting Flexibly Mounted Rotor Mechanical Face Seal

Recent theoretical work on the dynamics of the noncontacting flexibly mounted rotor (FMR) seal has shown that it is superior in every aspect of dynamic behavior compared to the flexibly mounted stator (FMS) seal. The FMR seal is inherently stable regardless of the operating speed, the maximum relative misalignment response is smaller, and the critical stator misalignment is larger. All these are measures of superior performance. This work undertakes the experimental investigation of the dynamic behavior of a noncontacting FMR seal. The steady-state response of the FMR seal was measured at various operating conditions. The results are given in terms of dynamic and static transmissibilities, i.e., amplitude ratio of responses to two forcing inputs: the initial rotor and fixed stator misalignments. These are then compared to the analytical predictions. Further, operation maps are drawn for each set of operation conditions. The maps indicate how safely (away from contact) the seal operates. It is shown that the combination of the seal parameters that maximize the fluid film stiffness is optimal for safe noncontacting operation.

Higher Harmonic Oscillations in a Noncontacting FMR Mechanical Face Seal Test Rig

In order to investigate experimentally the dynamic behavior of a noncontacting flexibly mounted rotor (FMR) mechanical face seal a test rig was designed and built. Test results showed that the FMR seal was vulnerable to higher harmonic oscillations with frequencies that are integer multiples of the shaft speed. Because system nonlinearities can cause higher harmonic oscillations, the dynamic moments acting on the rotor are derived to include the effects of imbalance and axial offset of the rotor. The analysis reveals that the nonlinear terms involved are of second order and generally can be neglected. Investigation is then directed to analyze the possibility of rubbing contact between the rotor and the stator. Rubbing contact can occur as a result of a high relative angular misalignment between the rotor and the stator. A contact kinematics model is proposed and a Fourier series analysis is performed on the resulting rotor response. The analysis shows that the proposed response contains higher harmonics. Fourier series expansion and numerical filtering of a sampled rotor response from the test rig yield resembling signals which contain higher harmonics. This suggests that rubbing contact is the source of higher harmonic oscillations. Design modification to the rotor flexible support system resulted in a virtual elimination of higher harmonic oscillations.

Rotordynamic Analysis of Multistage Centrifugal Pumps.

This paper describes the stability analysis of a multistage centrifugal pump in boiler feed. The individual contribution of hydraulic impeller forces, noncontact seals and bearings to rotordynamic behavior are evaluated according to logarithmic decrements. The influence of bearing and seal shapes on the rotor stability is investigated. Also, the vibration of actual multistage pumps is measured corresponding to the analysis. Theoretical stability threshold speeds are compared with experiments and are found to be compatible. As a result, it was confirmed that the rotor stability is strongly affected by the characteristics of the seal type.

B213 可視化による非接触シールの性能評価

This paper presents the results of performance of a noncontact seal for turbopumps. Experiments of flow visualization were performed using chemical reaction methods. Test results demonstrated that leakage flows through a non-contact seal were evaluated qualitatively by observing changes in color of a indicating dye, and quantitatively by measuring the pH of the sealing water. Therefore, it has been confirmed that this evaluation method is suitable for seal performance testing.

Viscous Friction in Small Gaps—Calculations for Non-Contacting Liquid or Gas Lubricated End Face Seals

To analyze the power consumption of non-contacting liquid or gas lubricated end face seals it is necessary to split the power consumption into single losses. Therefore the losses caused by churning and by viscous shear of the interface film have to be calculated. The design of the sliding faces influences the power loss due to viscous shear. To demonstrate interdependence of speed and power losses as well as to confirm the validity of the equations, the measured power consumption of two non-contacting seals is broken down into single losses. Despite some necessary assumptions the conformity between measured and calculated values is adequate. A controlled minimization of single losses can be conducted. Based on the power loss due to viscous shear of the interface film a thermal analysis by means of FEA is possible.

Development of ceramic turbomolecular pumps for fusion devices

In order to establish a transfer pumping system which is suitable for a fusion magnetic/nuclear environment, new turbomolecular pumps have been developed. The rotor assembly of the pumps is composed of ceramic (Si 3N 4) disks for gas compression, a ceramic shaft with turbine blades and a ceramic thrust disk. The rotor assembly is supported by gas bearings and is driven by an impulse gas turbine. A non-contact spiral-groove seal isolates the vacuum side from the high pressure side. After preliminary experiments with an 80 l/s ceramic turbomolecular pump, a medium-size ceramic pump was designed, manufactured and tested in 1988. The main specifications and a few of the vacuum characteristics of the pumps are given.

Gyroscopic and Support Effects on the Steady-State Response of a Noncontacting Flexibly Mounted Rotor Mechanical Face Seal

The dynamic behavior of a noncontacting rotary mechanical face seal is analyzed. A closed-form solution is presented for the response of a flexibly mounted rotor to forcing misalignments which normally exist due to manufacturing and assembly tolerances. The relative misalignment between the rotor and the stator, which is the most important seal parameter, has been found to be time dependent with a cyclically varying magnitude. The relative response is minimum when support stiffness and damping are minimum. The gyroscopic couple is shown to have a direct effect on the dynamic response. This effect is enhanced at high speeds, and depending on the ratio between the transverse and polar moments of inertia, it can either decrease or increase the dynamic response. Its effect is most beneficial to seal performance when the rotor is a “short disk.” A numerical example demonstrates that a flexibly-mounted rotor seal outperforms a flexibly mounted stator seal with regard to the total relative misalignment, the critical stator misalignment, and the critical speed.

Ceramic turbomolecular pumping system in reactor structure of FER

Abstract A new pumping system consisting of ceramic turbomolecular pumps and their ceramic fore-pumps has been studied as the main pumping system of FER since these pumps have been developed at JAERI recently. The turbomolecular pump has a rotor of silicon nitride, which is driven by a gas turbine, oil free gas bearings and a non-contact spiral groove seal. The effects of neutron radiation, heat loads and magnetic fields on the ceramic pumps are evaluated based on the FER design conditions.

An experimental study on the static and dynamic characteristics of pump annular seals. 1st report. The result for the static characteristics and the effect of rotor spinning speed on the dynamic characteristics.

A new test apparatus, which can simulate various working conditions, is constructed and applied to investigate the static and dynamic characteristics of non-contact seals. In this report the pressure drops and the leakages in four types of annular seals were measured, and the influence of rotor spinning speed on the dynamic characteristics was investigated. As a result, the static characteristics such as the pressure drops and the leakages relate closly to the inlet pressure, but they appear little change for the preswirl velocity and rotor whirling speed. As for the dynamic characteristics, they are largely affected by the rotor spinning speed. The increasing of rotor spinning speed gives an instabilizing effect to the rotor system.

A Face Seal With Circumferential Pumping Grooves and Rayleigh-Steps

This paper presents an analysis of a new noncontacting pumping seal. A hydrodynamic lubricating film is maintained due to Rayleigh-steps. If the low pressure side of the seal is filled with a fluid, the fluid can be pumped into the high pressure side by pumping grooves until the shaft speed reaches a limit value. The experimental results confirm the theoretical predictions for lubrication and sealing performance. Due to the high pumping ability in addition to the high stiffness of its hydrodynamic film, the seal can operate without wear and leakage for a high pressure fluid.

The Rotor Dynamic Coefficients of Coned-Face Mechanical Seals With Inward or Outward Flow

The linearized fluid film dynamic coefficients, i.e., stiffness and damping, of flexibly-mounted rotor noncontacting mechanical face seals are found. The coefficients are derived from a previous study where the flexibly mounted element was the stator. The two cases of inward and outward flows, both having converging gaps in the direction of flow, are analyzed for the two mounting configurations, and it is found that the later case possesses higher angular stiffness.

Nonlinear Dynamic Analysis of Noncontacting Coned-Face Mechanical Seals

The complete nonlinear equations of motion of a flexibly mounted stator in a noncontacting coned-face mechanical seal are solved numerically. The solution utilizes a transient dynamic analysis and takes into account rotor axial runout and assembly tolerances in the form of initial stator misalignment. Cavitation of the fluid film is also accounted for. A parametric investigation is performed and the effect of various design parameters and operation conditions on the seal dynamics is presented and discussed. A critical shaft speed is found above which the seal becomes dynamically unstable. A critical rotor runout is found which, if exceeded, will cause seal failure due to local face rubbing contact. A comparison is made between the numerical results and those of a simpler analytical solution. It is found that the analytical solution is valid for most practical applications of mechanical seals. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

The Hydrostatic Noncontact Seal Including Fluid Inertia Effect

Hydrostatic noncontact seals are becoming more widely used as shaft seals for high-pressure water pumps. This work shows that the hydrodynamic pressure head (ρ · u2/2), where ρ is fluid density) should not be ignored in such seals where leakage velocity, u, is high. This “inertia” effect causes a change of pressure profile on the seal surface and affects force equilibrium and the seal separation. Previous work has been reported in Refs. (1) and (3) where the left-hand-side of the following equations has been equated to zero. However, in this paper the fluid inertia effect is added using following equation: In the light of high-pressure seal test results presented here, the analysis including the fluid inertia effect indicates a better correlation than previous analysis neglecting this effect.

The Hydrostatic Gas Noncontact Seal

Hydrostatic gas noncontact seals are becoming more widely used as high-speed shaft seals for gas turbomachinery. Previous work for incompressible flow has been adapted to cover the gas flow dynamics of a hybrid taper/flat face seal, and the equivalent annular arrangement. The seal gap stability is discussed, along with various tests to confirm performance. The gas flow characteristics based on analysis are shown to be consistent with test data. This work has enabled gas seals to achieve successful operation in high-speed applications. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Tribology conference in Atlanta, Georgia, October 8–10, 1985

Stability Threshold and Steady-State Response of Noncontacting Coned-Face Seals

The dynamic behavior of a noncontacting coned-face seal is analyzed. Stiffness and damping properties of the fluid film and flexible support including elastomeric secondary seal are fully accounted for. Stability threshold and steady-state response in the presence of rotor axial runout and assembly misalignment are investigated. An expression is provided for the critical speed above which the seal becomes dynamically unstable. For stable operation, the relative misalignment between the mating faces is given as a function of rotor runout, assembly misalignment, design parameters, and operation conditions. An expression is provided for the critical rotor runout above which the seal will fail due to face contact. Although the analysis is based on small perturbation assumption, it is shown to be valid in many practical cases.

A New Concept of Zero-Leakage Noncontacting Mechanical Face Seal

A new concept of zero-leakage noncontacting mechanical face seal is described. Analysis of the seal performance and optimization of the seal ring geometry are presented. The main features of the new seal is a balance between pressure-induced and shear-induced flows and the simplicity of construction and performance prediction. Design charts for optimum seal configuration are given and the potential of the new concept is demonstrated by numerical examples.

Experimental Observation of the Dynamic Behavior of Noncontacting Coned-Face Mechanical Seals

The dynamic behavior of a coned-face noncontacting seal is experimentally observed by means of three proximity probes monitoring the motion of the flexibly mounted stator. The various tilts of the stator are analyzed and the relative misalignment between stator and rotor is found. The effects of both the rotor runout and the flexible support on the relative misalignment are discussed. The tests demonstrate both stable and transition to unstable modes of seal operation. Reasons for failure and other phenomenon during stable seal operation are explained.

Dynamic Analysis of Noncontacting Face Seals

The dynamic behavior of a noncontacting coned face seal is analyzed for the case of a rigidly mounted rotating seat and a flexibly mounted stationary ring taking into account various design parameters and operating conditions. The primary seal ring motion is expressed by a set of nonlinear equations for three degrees of freedom. These equations, which are solved numerically, allow identification of two dimensionless groups of parameters that affect the seal dynamic behavior. Stability maps for various seals are presented. These maps contain a stable-to-unstable transition region in which the ring wobbles at half the shaft frequency. The effect of various parameters on seal stability is discussed and an approximate expression for critical stability is offered. The theoretical model assumes frictionless flexible mounting of the seal ring such as in metal bellows. However, the results for critical stability can also be used as an upper limit for cases when friction in the secondary seal is present.

Dynamic Response to Rotating-Seat Runout in Noncontacting Face Seals

The dynamic response of a flexibly-mounted ring to runout of the rotating seat in mechanical face seal is analyzed assuming small perturbations. It is found that tracking ability of the stator depends only on its dynamic characteristics and operating conditions and is not affected by the amount of runout. Three different modes of dynamic response are shown and the condition for parallel tracking is presented. The present analysis is limited to flat-faced seals with no secondary seal damping. Nevertheless it provides a good insight into the dynamic behavior of noncontacting face seals.

Wear of seal materials used in aircraft propulsion systems

The various types of seal locations in a gas turbine engine are described and the significance of wear for each type is reviewed. Starting with positive contact shaft seals, existing material selection guidelines are reviewed and the existing PV (contact pressure × sliding velocity) criteria for selecting seal materials are discussed together with the theoretical background for these criteria. Examples of wear mechanisms observed to operate in positive contact seals are shown. Design features that can extend the operating capabilities of positive contact seals, including pressure balancing and incorporation of hydrodynamic lift, are briefly discussed. It is concluded that, despite the benefits arising from these design features, improved positive contact seal materials from the standpoint of wear, erosion and oxidation resistance will be necessary for further improvements in seal performance and durability and to meet stringent future challenges. Materials used in noncontacting gas path seal applications are described and a review of wear studies performed on these materials is presented. Factors that promote drastic changes in the structure and wear behavior of highly porous gas path seal materials are discussed. A correlation between wear characteristics and a factor that includes material strength, ductility, specific heat and hotworking temperature is proposed for low porosity metallic gas path seal materials.