th International Conference on Structural Mechanics in Reactor Technology (SMiRT ) Espoo, Finland, ugust 94, 9 SMiRT Division 6, Paper 658 ctive Control of Vibrations in Piping Systes Carsten Block a, Jürgen Engelhardt a, FritzOtto Henkel a a Woelfel Beratende Ingenieure, Hoechberg, Gerany, eail: block@woelfel.de Keywords: ctive vibration control, piping systes, active vibration absorber BSTRCT Dynaic loads acting on piping systes often are not considered in the planning phase of plants. Therefore, in ost cases no design at all or insufficient design against vibrations is perfored. However, vibrations in piping systes can lead to ajor probles. This paper presents a ethod to actively reduce vibrations in piping systes. INTRODUCTION Piping systes in plants can fulfil different tasks. Therefore, the diensions and the used aterials vary significantly. In addition, the loading highly depends on the intended use. Beside the coon static load cases dead weight, pressure and teperature dynaic load cases ay occur. Table presents an overview of typical dynaic load cases for piping systes in plants. In particular it is hard to predict steady state vibrations due to e. g. pressure surges or physical / cheical reactions occurring during operation. For that reason plants are not designed for such dynaic loads in ost cases. But vibrations occurring in piping syste after the initial start up of operation or after the backfitting of a plant can lead to serious probles. Beside safetyrelated probles, which can result in a downtie of a plant, the productivity or the quality of the product can be affected. Both cases lead to high econoic loss. Table : Typical dynaic load cases for piping systes by Schalk (99) Operating State Reason / Exaples Characteristics Noral Operation Vibrations of directly connected achines otors, pups, copressors ixers centrifuges Excitations fro external events achines in the vicinity construction sites, traffic Pressure surges switching of pups actuation of valving Physicalcheical reactions Wind, vortex shedding Motion of the sea for aritie constructions deterinistic steady state (haronic/periodic) or transient norally forcetiecurves deterinistic steady state or transient Tie dependent base excitation deterinistic transient forcetiecurves stochastic steady state, power spectru periodic, forcetiecurves deterinistic/stochastic steady state forcetiecurves, power spectru Upset operation / Incidents Tests Earthquake Explosion pressure waves Pipe burst Machine faults Loads fro tests dynaic excitation snap back tests stochastic/deterinistic transient tiecurves, response spectra, base excitation deterinistic transient, pressuretiecurves, response spectra deterinistic transient, forcetiecurves haronic/periodic/stochastic transient or steady state
If the resulting vibrations exceed perissible values and a sooth operation of a plant is not possible reedial easures have to be taken. VDI Guideline 384 offers a good overview on passive ethods to reduce vibrations in piping systes. Coon easures for vibration reduction in piping systes are: reduction of excitation detuning daping passive tuned ass dapers The efficiency of conventional passive ethods for vibration reduction is liited. The application of passive viscoelastic dapers, for exaple, requires a fixed support which is not always possible to realise due to craped confines. Tuned ass dapers deand a ass ratio of about % to reach a good attenuation. Therefore, tuned ass dapers often are unfeasible due to high static load. In addition, tuned ass dapers are restricted to one natural frequency and get ineffective if the dynaic properties of the piping syste change. To overcoe probles with conventional passive ethods of vibration reduction, active vibration control systes can be used. 3 CTIVE VIBRTION CONTROL In active vibration control external energy is transfored e. g. in a echanical force by eans of actuators. This echanical force is used to actively reduce vibrations in a controlled anner. (Figure ). excitation f sensor structure f actuator response q sensor energy charge aplifier power aplifier charge aplifier Feedforward D D controller D Feedback Figure : Coponents of a syste for active vibration control There are two ain control strategies: feedforward and feedback control. In the case of feedback control, the response of a syste due to excitation is easured with a suitable sensor and fed back into a controller to generate a proper control signal. This signal drives the actuator to attenuate the vibration. In feedforward control, the excitation and not the response is easured and fed forward into a controller to generate the control signal. Feedforward control is only applicable if a signal correlating to the excitation is available. For this case, the response of the syste can theoretically be forced to be zero. The choice of an adequate syste for active vibration control depends on any different factors. These are for exaple the type of the dynaic excitation, which can differ in direction, aplitude and frequency range, the requireents on the degree of vibration reduction and whether the vibrations shall be reduced globally or in a liited region (for ore details see Block (8)).
4 CTIVE VIBRTION BSORBER V In the present contribution the possibilities of active vibration reduction in piping systes using an ctive Vibration bsorber (V) are shown. For a verification of the vibration reduction potential the V shown in Figure was designed, anufactured and tested (see Engelhardt et al. (7)) unit I unit II Figure : ctive Vibration bsorber V (left: design, right: realization) The V s function is based on the principle that an accelerated inertial ass generates a reaction force in the supporting structure: F =! "&& x () The reaction ass is connected to the structure by eans of a spring. n actuator is located parallel to the spring. By accelerating the reaction ass, using the actuator in cobination with an appropriate control algorith, a resultant force for vibration reduction is obtained. straightforward control strategy consists in the Direct Velocity Feedback, by eans of which a broadband vibration reduction can be achieved. In order to achieve an observability and controllability of all vibration odes perpendicular to the pipe axis, the V is designed as a DOF syste. The two identical units of the V are oriented perpendicular to each other and are connected by eans of a rigid connection. Via this connection the sae reaction ass is obtained for both effective directions. Consequently, lower actuator strokes at the sae total weight are needed. The ajor part of the reaction ass is provided by the actuator agnets (Figure 3e). For actuation two electrodynaic voice coils are used. These custoised actuators provide a constant force of N (33 N peak force) and a stroke of. The actuator s agnets and coils are guided relative to each other by an adjustable linear bearing that is free of clearance (Figure 3d). The coil is connected to the fixed part of the linear bearing by eans of a frae (Figure 3c). This frae is connected via leaf springs (Figure 3f) to the interface (Figure 3g) that is attached to the piping. The interfaces are claped to the pipe with a tension belt (not shown). 3
aebfcgdh Figure 3: Coponents of the V The leaf springs fulfil various tasks: provide copliance of each Vunit perpendicular to the actuators effective directions guide the Vunits in the actuators effective directions transit the reaction forces to the piping carry the static load of the reaction ass By varying the thickness of the leaf springs, the V s natural frequencies can be tuned. By eans of the setting echanis (Figure 3a) the centre positions of the actuators are adjusted. The setting echanis is also used for adapting the V to different pipe diaeters. The two acceleroeters needed for the controller are ounted on the pipe interface (Figure 3h). Figure 4 shows the ode of operation. The static load is accepted carried by the leaf springs of both V units. For vertical otion both actuators are driven by an identical signal, for horizontal otion by an identical signal with opposite sign. The preconditioned sensor signals are separated into vertical and horizontal coponents and supplied to the decentralised controller. x + + +! Figure 4: Mode of operation of the V with a decentralized controller The efficiency of the V is shown in the next section on the basis of a ockup built up within the scope of the European research project SFE PIPES. 4
5 EXPERIMENTL STUDIES The ockup is shown in Figure 5. The outer diaeter of the steel pipe is with a thickness of 7.5. The pipe is welded on a quadratic base plate with a thickness of 56. The base plate itself is connected to the floor with four bolts. The lower part of the pipe (in the area of the highest stresses) is strengthened to avoid daages in the case of high vibration aplitudes. hanger acting as a vertical support is placed in the horizontal part of the pipe. MPB MP shaker MPD MPC MPE V vertical hanger Figure 5: Mockup with the positions of the acceleroeters, the shaker and the V The ockup was excited with a shaker in the frequency range fro to Hz and with snap back tests. The shaker is attached to the pipe at an angle of 45 in order to generate vibrations in the vertical as well as in the horizontal direction.for the snap back tests the pipe was deflected by 4.5 to 7 with a force of 5 to kn. When the required force is reached the pipe is suddenly released and the pipe freely oscillates priarily in the vertical direction. For all tests accelerations in all three translational directions were easured for the positions MP to MPE. The first two easured natural frequencies of the piping syste in the vertical and horizontal direction are in the range of 5 to Hz (see Table ). Table : Natural frequencies and asses natural frequency Hz ass kg vertical horizontal ockup 3 5.3 8. 6..3 V 6.5 3. 3. reaction ass: 3 kg 5
The V is ounted next to the easureent point MPD. Figure 6 shows the frequency response functions easured at the ockup with and without the V for the three easureent points MP, MPB und MPE when excited with the shaker..4 vertical.4 horizontal. MP without V with V...8.8.6.6.4.4.. 3.5 5 5 f Hz MPB vertical.6 5 5 f Hz horizontal 3 MPB.4.5..5.8.6.4.5..5 5 5 f Hz vertical MPE 3 5 5 f Hz horizontal MPE.5.5.5.5.5 5 5 f Hz 5 5 f Hz Figure 6: Frequency response function with / without V for an excitation with the shaker 6
The aplitudes are significantly reduced in the vertical as well as in the horizontal direction at all easureent points. reduction factor of up to 3 is achieved in the range of the natural frequencies. Figure 7 shows the behaviour of the piping syste in the tie and frequency doain for the vertical direction when it is excited by a snap back test..8.6.6 MP without V with V.5.4.. s /.4.3..4..6 5 5 t s 6.5 MPB (vertical) 5 5 f Hz 4 MPB.4 s /.3. 4. 6 5 5 t s MPE (vertical) 5 5 f Hz 3 MPE.5. s /.5..5 3 5 5 t s 5 5 f Hz Figure 7: Behaviour of the piping syste with / without V for snap back tests 7
For these tests as well a significant reduction of the vibrations can be achieved when using the V. In particular the long decay tie of the vibrations without the active syste is reduced. 6 CONCLUSION The present contribution shows that active vibration control is suitable to efficiently reduce vibrations in piping systes. ain advantage of the active vibration absorber V over conventional passive ethods consists in the ability to reduce vibrations over a wide frequency range including several natural frequencies. Passive tuned ass dapers, for exaple, are tuned to a certain natural frequency. Therefore, they can only reduce vibrations of single odes. nother advantage is that the control perforance is not sensitive even to distinct paraeter changes of the structure. In addition, the V does not need a fixed support as is required for viscoelastic dapers. This allows for an unprobleatic ipleentation even after a plant started operation. Currently the V is further developed within the scope of a European research project in order to ipleent the V in a real plant. 7 REFERENCES Block, C.: ctive control of huan induced vibrations of long span civil engineering structures. Dissertation TU Darstadt, FortschrittBerichte VDI, Reihe, Nr. 336, VDI Verlag, Düsseldorf, 8 Engelhardt, J. et al.: ctive Vibration bsorbers for Industrial Piping Systes. daptronic Congress 7, 34 May, Goettingen Gerany, 7 M. Schalk, F.O. Henkel: Rohrleitungsberechnungen für dynaische Lastfälle, 3rd international, Heft 9, 99, Reprint in Handbuch Rohrleitungstechnik, 5. uflage, VulkanVerlag Safety ssessent and Lifetie Manageent of Industrial Piping Systes, SFE PIPES, Sixth Fraework Prograe, Deliverable D7, NMPCT53898, 8 VDI Guideline 384: Vibrations in piping systes, 4 8