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IDENTIFICATION OF FLUID/STRUCTURE INTERACTION FOR THE DEVELOPMENT AND DESIGN OF BOILERFEED PUMPS

Objective

The fluid/structure interaction forces of annular clearance seals and impellers are the dominating factors in the dynamic (vibration) characteristics developed by large multistage boilerfeed pumps (BFP). The fluids/structure interaction forces must clearly be defined prior to the product development of a new generation High Speed Boilerfeed pump. Furthermore, these unknown interaction forces are today the leading cause of unscheduled feed pump outages in power plants worldwide. It is now recognized that centrifugal pump rotor dynamic behaviour is influenced tremendously by fluid/structure interaction forces generated at annular seals and impellers. These forces can be divided into two groups : a) motion dependent forces, b) pure hydraulic excitation forces.
Presently 2 experimental test rigs are operational radially and axially suspended in active magnetic bearings. Both test rigs allow identification of motion dependent fluid/structure forces and moments related to 5 degrees or freedom. Excitation forces also can be measured.

A new direct identification technique is developed and built into a data acquisition system for both test rigs. Theoretical fluid flow models are developed for long grooved and smooth annular seals and for the impeller shroud based on bulk flow models. Currently, finite difference fluid flow models are developed for the impeller shroud leakage path.

Research was carried out concerning fluid structure interaction forces generated in boilerfeed pump components.

It was found that the frequency response functions (FRF) of a multistage pump rotor are completely changed just by water filling and the presence of shaft mechanical seals. Mechanical seal chambers, and wetted parts of the shaft as well as chambers with one dead end develop considerable MDI coefficients which change the dry FRFs of the rotor completely.
Long annular clearance seals with large pressure differences develop important direct moment coefficients and also radial DOF to moment and angular DOF to force coefficients. Large L/D annular seals with small pressure difference develop a negative direct stiffness which is very destabilizing. Also, the finite difference, Kepsilon 2-dimensional fluid flow model can be used for shrouds as well as geometrically complex shaped wearrings.

The MDI forces of shroud leakage flows were under estimated. It appeared in using real boiler feed pump impellers with actual annular seals show that most of MDI coefficients are generated in the leakage path of shroud and seal. It also appears that the leakage flow velocity and pressure field of the shroud as well as the MDI forces are determined by the impeller seal configuration.

KMDI coefficients have also been identified. So far the investigations revealed for the radial and angular DOF of impellers that the most important coefficients for the radial rotor FRFs are the radial DOF force coefficients. The other coefficients are much less important for the linear response of the rotor.

Axial KMDI coefficients were also generated for impellers and seals. Although no major coupling between the radial and axial DOF were discovered the direct axial coefficients are very high for both mechanical seals and impellers. Axial dynamic rotor studies of pumps can be carried out indecently from the radial bending dynamic analyses.
The fluid/structure interaction forces of annular clearance seals and impellers are the dominating factors in the dynamic (vibration) characteristics developed by large multistage boilerfeed pumps (BFP). The fluids/structure interaction forces must clearly be defined prior to the product development of a new generation High Speed Boilerfeed pump. Furthermore, these unknown interaction forces are today the leading cause of unscheduled feed pump outages in power plants worldwide. It is now recognized that centrifugal pump rotor dynamic behaviour is influenced tremendously by fluid/structure interaction forces generated at annular seals and impellers. These forces can be divided into two groups : a) motion dependent forces, b) pure hydraulic excitation forces.

Coordinator

BW/IP INTERNATIONAL BV,
Address
Parallelweg 6
4878 AH Etten-leur
Netherlands

Participants (2)

LEUVEN SYSTEMS AND MEASUREMENTS BV
Belgium
UNIVERSITÄT KAISERSLAUTERN
Germany
Address

Karlsruhe