Seal Properties
Hydraulic forces in annular seals may have a large influence on the vibrations of high speed centrifugal pumps. To have a better knowledge about such seal effects, theoretical as well as experimental studies are important in the actual research of rotordy
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SEAL PROPERTIES
R. NordiiWID
University of Kallenlautem, Kailenlautem, FRG
ABSTRACT Hydraulic forces in annular seals may have a large influence on the vibrations of high speed centrifugal pumps. To have a better knowledge about such seal
effects,theoreti~
cal as well as experimental studies are important in the actual research of rotordynamics. In this chapter a theoretical model and an identification procedure are presented to determine the dynamic coefficients of seals by calculation and measurements as well. The identified parameters confirm the assumptions in modelling and point out, that the stiffness and damping characteristics of seals are significant for the stability behavior of pumps.
5.1 IDENTIFICATION OF STIFFNESS, DAMPING AND INERTIA COEFFICIENTS OF ANNULAR TURBULENT SEALS An important assumption for the reliability of high speed centrifugal pumps 1s a good rotordynamic behavior. Connected to this problem hydraulic forces acting on the rotor are of major importance. It is well known that neck ring seals as well as interstage seals (Fig. 1) may have a large influence on the bending vibrations of a pump rotor. Besides their designed function of reducing the leakage flow between the impeller outlet and inlet or between two adjacent pump stages, the contactless seals have the potential to develop significant forces. This type of forces, created by lateral rotor vibrations can be described by a linear N. F. Rieger (ed.), Rotordynamics 2 © Springer-Verlag Wien 1988
N. Nordmann
154
TURBOPUMP
TURBINE
SEAL LOCATIONS
Fig. 1 Seals of a High-Pressure-Turbopump Rotor in Aerospace Engineering model with stiffness, damping and inertia coefficients. If contactless seal elements are used in a turbopump the fluidmechanical interactions have to be considered when predicting the vibration behavior of the pump rotor in the design process. However, there is often a uncertainty, concerning the data for the dynamic coefficients. Up to now the stiffness and damping characteristics of seals are not very well known and there is a need for additional research in this area. This is particularly the case for grooved seals, which are very common in practice. Different research projects have been started to investigate the dynamics of
~eals
by theoretical models as well as experimental
procedures. The following chapter presents a possible model, based on a bulk flow theory and describes an experimental procedure to identify the stiffness, damping and inertia coefficients. Modelling of Annular Seals with Turbulent Flow Conditions Seal Model.
To explain the seal model, we consider a very
simple geometrical form, consisting of a cylindrical shaft with circular cross section, surrounded by a cylindrical housing (Fig. 2). This annular seal seperates the two chambers with pressure p 1
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Seal Properties
and pressure p2 , respectively. The pressure difference is 6p = p 1-p2 • Caused by this pressure difference there is a leakage flow in axial direction, which is always almost a turbulent flow
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