A DAMPING TREATMENT FOR PIPES AND BODIES

Abstract: A damping treatment for bodies where the temperature may rise above about 150 DEG C consists of a number of stand-off cantilever, sandwich type dampers (1), consisting of a layer of visco-elastic material (3) sandwiched between two rigid metal plates (4, 5), which are attached to a support (2) that can be attached to the body (7). The support (2) is thermally isolated from the body (7) and is positioned at a position of high amplitude of a radiating frequency that is to be reduced.

Description: A DAMPING TREATMENT FOR PIPES AND BODIES The invention relates tovibrationdamping, particularly, butnot exclusively, to the damping of high temperature pipes and bodies. Vibrations in pipes, machines and otherbodies causemovementand noise which it is often desirable to reduce or eliminatebydamping ofthe vibrations. Manydampingtechniques areknown, buttheneedforalightweight, effectivemeans fordampingbodiesbecomesmoredifficulttoachieveasthe temperature of the body increases. Free layer damping, inwhich a layer ofvisco-elasticmaterial is applied to the substrate (pipe or body surface), or stave/constrained layer damping, in which the visco-elastic layer has a stiff outer layer, cannot be used if the temperature of the body rises above about 150°C as both these techniques require the application of theviscoelasticpolymer to the substrate. Polymer properties vary with temperature and/or frequency. Generally, at low temperatures orhigh frequencies apolymer will appear brittle or glassy with little visco-elastic loss. At high temperatures or low frequencies it will appear rubbery and elastic. Between these two extremes thepolymer isvisco-elastic and able to absorb energy. Apolymer for noise reduction purposes is, therefore, selected or designed to have its visco-elastic properties in the particular frequency andtemperature rangeof interest. However, it isdifficultto obtainpolymers capable ofwithstanding temperatures ofaboveabout140°- 150°C for long periods of time. Vibration is imparted to thewalls of a pipe or other body as radial or lateral motion and this has beendaπped in some applications where high temperatures are not experienced by using auxiliarymass type dampers. There are many situations, however, where temperatures of 500° - 600°Cmay be experienced and ameans todampbodies at these high temperatures is required. Theobjectofthe invention is toprovideameans fordampinganda method of applying such means for the treatmentforpipes and otherbodies which can be used even where the temperature of the body may rise above 150°C. It is a further object of the invention to provide such a means which is lightweight and can easily be applied to a pipe or body in situ. In one aspect the inventionprovides ameans for dampingpipes and other bodies comprising one or more stand-off cantilever, sandwich-type dampers adapted to be attached to the body and wherein the dampers are thermally isolated from the body. Acantilever damper is a damper in which the mode ofvibration of the damper is a cantilever type ie a bar allowed to bend either side of a central support. A cantilever, sandwich-type damper is an auxiliary mass type damper which comprises a layer of visco-elastic material sandwiched between two rigid plates, with one plate being adapted for attachment to a body. Other cantilever dampers have no visco-elastic component and work by phase interaction with the structure. Resonant peaks of vibration are set up and the dampers are tuned to work at one frequency only. The construction of a damper having a visco-elastic polymer in a sandwichof rigidplatesallowsabroaderbandperformanceand energy is actually absorbed by the damper. Preferably the dampers are attached to a support which is then attached to the surface of thepipeor body tobe damped, the supportbeing thermally insulated from the body by a layer of insulating material. This configuration facilitates the incorporationof athermalbarrier for application to high temperature bodies. The thickness of the insulating layer determines the temperature gradientbetween the body and the damper. Thehigher the temperature of thebody, thethicker the insulationlayermustbetoavoidthedamperbeing raised to a temperature which will damage the visco-elastic material. Any suitable insulating material can be used. Advantageously the insulation is rigid or nearly rigid to ensure efficient coupling between the vibrating body and the dampers. Preferably each cantilever damper support is attached to thebody at a position of high amplitude of a radiating frequency in the range of interest. These positions of high amplitude are selected to have the maximum influence on the vibrational modes which are to be reduced or eliminated. Preferably the visco-elastic material is a polymer capable of withstanding temperatures of over 130°C. Advantageously it should be capable of withstanding temperatures of 150°C, or higher if possible. The thickness of thelayer ofvisco-elasticpolymer ispreferably the m i umpossible for adequate damping performance, so that theweight of the dampers is kept as low as possible. Advantageously the damper is tuned so as to have a resonance frequency corresponding to a particular frequency to be damped. Preferablythedamper istunedtoa frequencyslightlylower thanthattobe damped. This is because addition of extra mass to a structure tends to lower the resonant frequency of the structure. Themostconvenientwayof tuning thedamper istoalterthelength of the rigid plates sandwiching the visco-elastic polymer. The damper plates can be made of any material with suitable rigidity and density, including non-metallic materials. However, metals arethemostconvenientmaterials touseforanumberof reasonssuch as cost, ease of use etc. Preferably the plates are made of stainless steel. The support is preferably rigid to enable good coupling of the vibrational energy to the damper and is adapted for easy attachment to the body to be daπped. If the body is a pipe, the support conveniently consists ofametal, preferably steel, hoop in two ormorepieceswhichcan thenbeclampedtogether aroundthepipe. Formorecomplicatedshapesof body, thesupport ispreferablyof a formwhichenables easy fittingtothe body such as straight or shaped bars which can be clamped together around the body. An alternative support for any shapedbodyconsists oflinked metal plates ina chain, with insulatingmaterial ononeface oftheplates and the dampers attached on the other face such that the plates can be fastened around the body by attaching the ends of the chain together. Preferablyeachdamper is attachedto thesupportat itsmidpoint. The dampers may be attached by any suitable means such as bolts, welding etc. Advantageously more than one damper is attached to each support. For maximum damping effect it is preferable that as many dampers as possible are attached to each support. As it is possible to attachmore dampers to a larger support, it follows that improved levels of damping canbe achieved for largerbodies having the same vibrational levels as smaller ones. In a second aspect, the invention provides a method for damping pipes and otherbodiescomprisingtheapplicationofoneormore thermally isolated, stand-off cantilever, sandwicbr ype dampers to the body. Themethodfordampingpipesandotherbodiesbyapplicationofone or more thermally isolated, stand-off cantilever dampers to the body preferably includes the steps of: 1) identifying the positions of maximum amplitude of the vibrational modes of interest; and 2) selecting oneor moretuned cantilever dampers suitable for damping the required vibrational modes. m order to assist in identifying the modes of vibration of interest, the method preferably includes the further steps of : a) identifying the frequencies to be damped; and b) identifying the modes of vibration of the body for the frequencies to be daπped. So that the appropriate insulationmaterial maybe selected, the further step of identifying the maximum temperatures that the bodywill experience ispreferablyincludedandthethicknessofinsulationmaterial needed to give the required temperature drop between the body and the damper is calculated. Conveniently the frequency response of the body is investigated using aspectrumanalyser. Amodal analysisofthemodesofvibrationfor the frequencies of interest can then be made using a suitable computer software package. This is conveniently used to find the positions of maximum amplitude of the vibrational modes of interest. Preferably the dampers are thermally isolated from the body by means of an insulationmaterial selectedtobe as efficientaspossible so that the required temperaturedrop canbe achieved using as little of the insulation material as possible. In order that the invention may be more fully understood it will now be described, by way of exaπple only, with reference to the attached drawings of which: Figure 1 shows fourpossiblemodes ofvibration of apipe; Figures 2a & b show, in side view and end on (in the direction of arrow B) respectively, a damping treatment according to the invention, fixed on to a pipe; Figure 3 illustrates the positions of high amplitude and of the nodal points of a pipe for simple bending modes of vibration; Figure 4 shows a typical frequency response for an untreated pipe (range on the vertical axis: -50.00 to +30.00dB); Figure 5 shows the frequency response of the same pipe as in Figure 4 when damped (range on the vertical axis: -80.00 to 0.00 dB) ; Figures 6a & b show the frequency response in water of the same pipe as in Figure 4, undamped and damped respectively; Figures 7a & b show the noise radiation for the respective frequency responses of Figures 6a & b. In order to be able todampvibrations in abody efficiently, some idea of theway inwhich thevibrationsoccur is required. Figure1 shows the first fourmodes ofvibrationofapipe. n= 0 is abulgetypemode, n= 1 is a bending mode and n = 2 and n = 3 are possible breathing type modes. In each case, radial motion is imparted to the pipewalls and can thus be daπped by auxiliary mass type dampers. A bending mode is one where the tube bends in a particular vibration pattern. In bulge and breathing type modes the tube expands radially in and out. Adamping treatment according to the invention is shown inFigures 2a & b. Eight of dampers 1 arecircumferenciallyarrangedaroundapipe7, each damperbeing attachedtoacircular support2. Eachdamper1consists of a layer of visco-elastic polymer 3 sandwiched between two steel plates 4,5. Any polymer with the correct visco-elastic properties can beused. though considerationmustalsobegiventothe environmental requirements egwater uptakeor degradationbyoilofthepolymer. Thepolymerlayer3 is about 3mm thick. The support 2 consists of a split steel hoop 6, thermally insulated from the pipe 7 by a5mmlayer of rigid insulation 8. This thicknesswas foundtobesufficienttokeepthevisco-elasticlayer3 to about a third of the temperature of the pipe 7 so the treatment canbe " used on pipes of temperatures of about 400°C. As many dampers 1 as is convenient arebolted to the hoop 6 - in this case, there are8. Thetwo halves of the hoop 6 are held together around the pipe 7 by clamps 9,10. Thesteelplates 4,-5 areabout6mmthick. Thedamperassembly1, in this particular case, is about 190mm long. It is tuned to damp a particular frequency by altering the length of the plates 4,5. The length of plate required to damp a particular frequency depends on the thickness and the physical properties of theplates andthevisco-elastic polymer layer. The size of the damper determines the frequency of operation. The damping in such a system is given by: z« = βYg(l+k 3 ) 1+ (2+Y) g (l+k 3 ) + (1+Y) Q+fi2 ) g 2 ( 1+ fc 3 } : where § is the loss factor of the visco-elastic layer g(l+k 3 ) is the shear parameter For constrained layer treatments for which the extensional stiffness of the damping layer is negligible: Y = 3e3h 3 (l+2h 2 +h 3 ) 2 (l÷e3h 3 ) Cl+ 3h 3 ) where B ] _ = flexural rigidity of the base plate (bottom plate) πr j _ = mass per unit area of the base plate G 2 = shear modulus of damping layer 3 = extensional stiffness of the constraining layer (topplate) H 2 = thickness of damping layer w = angular frequency k 3 = extensional stiffness ofconstraining layer relativetothat of the base plate β3 = Young's Modulus of constraining layer relative tobaseplate h 2 = thickness of damping layer relative to base plate h 3 = thickness of constraining layer relative to base plate d 2 = damping layer density d 3 = density of constraining layer. b is a function of the shear parameter and the visco-elastic loss factor ofthe layer and is taken tobe equal to1 inthiscase. The length of the plates is, however, preferably not more than about 500πm to keep the size and weight of the daπper to a minimum. A visco-elastic polymer composition which has been found to be suitable in this application is shown in Table 1, though any suitable material may be used. TABLE 1 ΓNΠREDIENΪS PROPORTION BY WEIGHT Butadiene-Acrylonitrile Rubber 100 Stearic Acid 1 Zinc Oxide 5 SRF Carbon Black 5 Mica 150 Tetra ethyl thiuram disulphide 1.4 Sulphur 0.2 Di-morpholini-disulphide 1.4 Non toxic plasticiser 10 Acetone/diphenylamine condensation product 2 For daπping of a pipe or a series ofpipes, thefrequencies tobe daπped have to be found. Each length of pipe has different resonant frequencies. Typicalbendingwave (vibrational) frequenciesforapipe are given by: assuming the pipe to be simply supported at the ends, and where: 1 = length of pipe E = Young's Modulus of the pipe material p= density of the pipe material 2 a^= internal radiusofpipe a 2 =externalradiusofpipe The 2nd and 3rd overtones are given by : f 2 = 4fχ and f 3 - 9fχ The following table (Table 2) shows the frequencies atwhichthe first four overtones may occur for different lengths of pipe: TABLE 2 aτ_ = 150mm a 2 = 140mm: length/m fη f 2 f 3 f & 0.5 3.08 kHz 12.3 kHz 27.8 kHz 1.8 238 Hz 952 Hz 2.14 kHz 2.0 192 Hz 771 Hz 1.74 kHz 3.07 kHz 3.0 85 Hz 342 Hz 769 Hz 1.37 kHz 4.0 48 Hz 192 Hz 433 Hz 770 Hz There are also other, more complicated, modes of vibration that may radiate. If the pipe is inwater, the radiation depends on thebendingwave velocity being above the wave velocity in water (typically 1480mS~l) . Typical wave velocities for bending waves are given by: f = frequency/Hz Below about 700Hz the bending wave velocity is <1480mS""l and should not radiate. Thus inwater, daπpingtreatmentsmayonlyneedtobedesignedto deal with frequencies of greater than about 700Hz. For optimum performance, one or more supports 2 carrying the dampers 1 are sited at positions of maximum displacement of the pipe 7. Figure 3 shows the points ofmaximum andminimum aπplitude (A^x and j nin respectively) for typical vibrational bending modes 1, 4 and 7 of a pipe, with mode 1 corresponding to the fundamental frequencyofthepipe. The positions of ma and j rin a e determined by the tube length, radius and wall thickness. Figure 4 shows a typical frequency response of a steel pipe measuring 2m long and having a diameter of 0.24m with 25mm thick walls. The pipe was mounted on compliant knife edges and its frequency response over the range 0 to 3.2kHz was investigated using a spectrum analyser. Although sounds are audible to the human ear in air over a wide frequency rangeofabout0.02-20 kHz, theear ismostsensitiveto soundat about 1kHz. The daπping treatment was designed to deal mainly with frequencies inthe1kHz-2kHz range. Three rings2 eachsupporting eight cantilever daπping elements 1, as shown in Figures 2a & b, were applied to the pipe 7, at positions of high amplitude, as shown in Figure 3. The three rings 2 were attached at a) 0.45m, b) lm and c) 1.55mfromone end of the pipe 7. These are positions of high aπplitude for mode 7, which is also a radiating frequencycentral totherangeofinterest. Thedampers atpositionsa) andc) alsohaveaneffectonmode4, whilethoseatposition b) have little effect on mode 4, but have a large effect on mode 1. The vibration response of the treated pipe is shown in Figure 5. There is a30dB shift on thevertical axis, whichshows noiseintensity, in comparisonwiththeuntreated case shown infigure4and itcanbeseenthat peak noise radiation levels were reduced by around 20dB above iKHz. Althoughthe treatment increases thepipedaπping substantially from1 to 1.5 kHz, there is stillsignificantdamping atfrequenciesbelow_ and above this range. Figures 6a & b, respectively, show the frequency response of the same pipe as described with respect to Figure 4, immersed in water and beforeand after thedaπping treatmentasdescribedwith respecttoFigure 5. ComparisonofFigures 4 and 6ashowsthat thewaterloadingdecreases the frequenciesofthe individualpeaks. Aftertreatment. Figure6b, the peak values have been reduced considerably. Reduction of the radiated noise is illustrated by Figures 7a and b which show noise radiation measurementsmadewithahydrophonebothbeforeandaftertreatment. The frequency of mode 1 is measured as about 300Hz in water and hence, as expected, this modedoesnotradiatemeasurably Modes above about600Hz dohoweveranditcanbeseenthattheapplicationofthedaπperstothepipe has reduced the peak noise radiation levels by around 20dB above 1kHz. The daπping treatment is easily installed and removed and can be tuned to be effective over a given range of frequencies. It is lightweight and compact and is particularly useful for damping objects whichmayreachhightemperatures. Althoughdescribedwithreferenceto a cylindrical pipe it may be applied to other bodies. Although the damper plates have been described as being made of steel, they canbemade of anymaterial of suitable rigidity and density. Metals other than steel can be used and also any non-metallic material which can fulfil the requirements. If thepipe or body is in amediumwhich acts as acoolant, such as water, the denying treatment is even more effective as the daπper temperature is relatively lower ie it is at approximately the same temperature as the surrounding medium. Although there are other techniques for daπping pipes andbodies at temperatures below about 130°C, when visco-elastic polymers can be used, this technique is easier to apply and is more weight effective (ie lighter) than most other techniques. In addition, there are other techniques known for daπping flatplates, frames, machines etcbut skilled use of the present types of absorbers gives a more effective and lighter treatment. However, for body temperatures above about 130°C no other suitable effectiveandlightweighttechniquesareknown. This technique is thus suitable for exhaust pipes, high temperature machines, pipes carrying hot liquids or gases and so on.

Claims: α____™£ 1. A means for damping pipes and other bodies comprising one or more stand-off cantilever, sandwich-type daπpers 1 adapted to be attached to thebody7, characterised inthatthedaπpers1 arethermallyisolatedfrom the body 7. 2. Ameansfor dampingbodiesaccording toclaim1 characterisedinthat the daπpers 1 are attached to a support 2 which is then attached to the surface of thebody to be damped/ the support2 being thermally insulated from the body by a layer of insulating material 8. 3. Ameans fordaπpingbodies accordingtoclaim2 characterised inthat each cantilever daπper support 2 is attached to the bodyat aposition of high amplitude of a radiating frequency in the range of interest. 4. A means for daπping bodies according to any one of the preceding claims characterised in that each damper 1 comprises a layer of visco- elastic polymer sandwiched between two steel plates. 5. Ameansfor daπpingbodies accordingtoclaim4 characterised inthat the visco-elastic material is a polymer capable of withstanding temperatures of over 130°C. 6. Ameans fordaπpingbodies according toclaim5 characterised inthat the visco-elastic material is capable of withstanding temperatures of 150°C. 7. A means for damping bodies according to any one of the preceding claimscharacterisedinthatthedampersaretunedsoastohavearesonance frequency corresponding to a particular frequency to be daπped. 8. Ameans for daπpingbodies according to claim7 characterised inthat thedaπpers are tunedto a frequencyslightlylower thanthattobedaπped. ■ 9. A method for daπping pipes and other bodies comprising the application of one or more thermally isolated, stand-off cantilever, sandwich-type dampers to the body. 10. A method according to claim 9 characterised in that the method includes the steps of: 1) identifying the positions of maximum amplitude of the vibrational modes of interest; and 2) selecting one or more tuned cantilever dampers suitable for daπping the required vibrational modes. 11. A method according to claim 10 characterised in that the method includes the further steps of : a) identifying the frequencies to be daπped; and b) identifying the modes of vibration of the body for the frequencies to be daπped. 12. Amethod according to any one of claims 9 to11 characterised in that the method further includes the step of identifying the maximum temperatures that thebodywill experience andthethickness of insulation material isselectedtogivetherequiredtemperaturedropbetweenthebody and the daπper.

Inventors: HOUSE JOHN RICHARD CHARLES, HOUSE, JOHN, RICHARD, CHARLES
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1987
Main classification: F16L55
IPC codes: F16F7, F16F15, F16L55