The present invention relates to a piping system for visco elastic damping, which system oscillates with high frequency, small amplitude oscillations.
In particular, the invention has been developed in connection with the growing problem of acoustic oscillations in piping structures or systems used for transport of fluids. The problem may rise to serious dimensions if the pipes and associated pipe and structural details have natural frequencies which coincide with the excitation frequency in such a way that resonance arises. Additional amplification may arise if the natural frequency of the pipes and associated pipe and structural details, such as support arrangements, attached valves and measuring devices etc., coincides with acoustic natural frequencies such that fluid-structure interaction occurs. Such types of resonance have in some cases caused ruptures in the affected piping systems with leakage and subsequent risk of explosions as a consequence.
Attempts to solve the problem by adding additional stiffening components have little effect, as such stiffening only changes the natural frequencies of the system and does not remove either the excitation or the response energy. As the excitation spectrum is relatively broad with typical values from above 50 Hz and multiples thereof, and the base excitation frequency varies with pressure and speed, it is extremely difficult to significantly improve the situation by the use of stiffening components alone.
For a suspended vibrating element there are four different physical forces acting directly on the element: gas pressure, inertial forces (mass acceleration), spring forces (stiffness-displacement) and damping forces. It is these forces which govern the response energy.
Damping forces have the characteristic that they take energy out of the system. Visco elastic damping gives forces proportional to velocity. Secondary damping may occur through friction in the supporting arrangement and air stream damping. Without damping, a free oscillation will continue to infinity and the response at resonance will increase to infinity. Installing stiffening components made of, for example, steel will only effect the stiffness and inertia. Metal piping systems have therefore little natural damping, which is partly the reason that resonance becomes a problem.
Traditional dampers, such as mass dampers, snubbers, hydraulic dampers, plasma dampers and “Gerb”-dampers, do not function well at the type of load in question, namely high frequency, broad frequency spectra and small amplitudes.
A possible method of damping oscillations is visco elastic damping, but there is hitherto no known method for applying this form of damping to piping systems which are located in an environment with temperature variations beyond the working range of the visco elastic material and in which the vibrations have very small amplitudes. The latter has the consequence that it is difficult to achieve the necessary deformation in the damping material.
Visco elastic damping of pipelines which are excited and set into oscillation by wind forces, is already public knowledge, see for example U.S. Pat. No. 5,193,644. The method of damping disclosed in this US patent includes suspension of mass elements on the pipeline by means of elastic elements which are subjected to shear forces.
Arrangements where visco elastic damping has been used to damp out vibrations in beams, for example in buildings, are also known. In such arrangements damping material has been placed between two stiff components or structures, see for example U.S. Pat. No. 4,039,050. Also in these cases the aim is to damp oscillations caused by external effects and the damper is arranged between a fixed structure and the end of an appropriate beam.
The present invention is primarily aimed at damping oscillations in fluid conducting pipes or pipe structures in which harmful and/or destructive acoustic oscillations may occur in the high frequency ranges, typically from 50 to 1000 Hz and with small amplitudes, typically less than 0.1 mm, without strictly limiting the present invention to these frequency and amplitude ranges.
One main object of the present invention is to provide a piping system for visco elastic damping which in a practical and effective way substantially counteracts the internal loads, such as flow induced pressure variations, in the piping system, in order to among other things increase the safety.
The main object of the invention is achieved by means of the piping system as initially defined, characterized in that the system comprises one or more autonomous visco elastic damper links of the type which comprises one or more layers of visco elastic damping material fixedly bonded between a first stiff member and a second stiff member.
According to a preferred embodiment of the invention the visco elastic damper or damper links is/are stiffly joined between mutually connected pipe sections (pipes) or pipe details in the piping system.
According to another preferred embodiment of the invention a visco elastic damper of the said type is arranged for damping of oscillations in the longitudinal direction of a first pipe (11), and that two visco elastic dampers of the said type are arranged for damping of oscillations across said longitudinal direction of said pipe.
According to another preferred embodiment of the invention two visco elastic dampers are arranged mutually rotated 90 degrees in the longitudinal direction and fastened between the pipe legs of a pipe bend.
According to another preferred embodiment of the invention the respective visco elastic damper is provided with a temperature controlling element and that the visco elastic damping material and the temperature controlling element are surrounded by an insulation layer which is encapsulated by a protective means, such as a protective jacket. By using temperature controlled visco elastic dampers the necessary prerequisite (temperature) conditions for each individual visco elastic damper in question may be secured.
A fundamental idea of the present invention is to damp oscillations of a pipe or pipe detail(s) in a piping system by means of one or more autonomous, i.e. self-damping, visco elastic dampers or damper links by connecting two different parts of the piping system with each other and/or between the piping system or structure and another free standing object, such as a supporting structure, via one or more of said dampers or damper links, which dampers or damper links are stiffly attached to the pipe or pipe detail(s) to be damped and the piping structure. Thus, the damping of the pipe or pipe detail(s) substantially limit the relative displacements between the two different parts of the piping system, thereby limiting the oscillating material strain which is what causes fatigue failure.
The damper or damper link has a limited stroke length in comparison with other types of dampers, e.g. visco elastic Gerb dampers and is therefore limited to damping applications with small relative deformation due to variation in static loads or temperature loads. A typical non-limiting scope of application is to arrange the damper or damper link between a support location on the main pipe structure and a pipe object to be damped.
The use of discrete damping elements makes it possible to increase, structural damping of oscillations in piping systems for liquids and gases locally to individual parts or sections of the system. The discrete dampers may be regarded as components attached between two substructures, which can be structurally connected to other neighbouring surfaces. The active parts of the visco elastic dampers will transform a substantial amount of the vibration-deformation energy into heat during the course of each vibration or oscillation cycle and thereby damping the vibrations. The present invention solves the above-mentioned types of oscillation problems in piping systems through the targeted introduction of a practical form of structural damping, instead of modifying the stiffness and/or inertia of the system as in the prior art.
The loss factor of a damping material is a measure for how much of the oscillation energy that transforms to heat in the damping material. A high loss factor means that the displacement damper will transfer a substantial part of the oscillation energy to heat for each oscillation cycle. For visco elastic damping in a piping system, this is an irreversible process. The energy loss to the surroundings is due to the hysteresis in the tension-strain curve in the damping material. The most effective hysteresis is obtained by letting a layer of damping material experience shear deformations.
Some damping materials, such as for example visco elastic damping materials, have the characteristic that the loss factor is dependent on both the surrounding temperature and vibration frequency, see
Further preferred embodiments and features of the present invention will appear from the independent claims and the subsequent description.
The invention will now be explained in more detail with reference to the appended drawings which show examples of embodiments, and where:
FIGS. 13 to 23 show different types of adapters.
The damper is provided with a surrounding thermal insulation 8, which is encapsulated by a water proof layer 9. An electric heating element comprising electrical heating cables 10 which are embedded in the insulation 8 at the damper 2,3,4 itself. Alternatively, a corresponding electric cooling element may be used. Such a damper can be maintained at a specific temperature or within a specific temperature range through the use of the temperature controlled heating element. It is within the knowledge of a person skilled in the art to modify the temperature controlling arrangement, i.e. the heating/cooling elements, the control system and the placement of possible sensors, to maintain a desired temperature range for the damper in a given working environment. For example, other kinds of self controlling heating/cooling elements or cables may be used. Further, a temperature sensor may for example be arranged within or in the immediate proximity of the visco elastic material.
A similar embodiment for a 90 degree pipe bend 39 is shown in
The method of attaching the visco elastic dampers or damper links is important. In addition to some of the above described figures showing an adapter, the FIGS. 13 to 23 show different additional types of adapters which can be used to achieve the attachments of the dampers or damper links to a pipe in a piping system. It is important that the individual visco elastic dampers or damper links are fastened to the adapters in such a way that displacement arises in the damper or damper link and not in the attachment to the pipe such that the energy taken out from the piping system is through the deformation in the damper or damper link. However, it is within the scope of the present invention that one or more visco elastic dampers could also be attached between a pipe and an adapter, for example in the form of a collar, fixed to the pipe, via the visco elastic damper in order to damped out vibrations directly associated with the pipe wall.
FIGS. 19 to 23 show different forms of welded joints 73,74,75,76,77 for the dampers 78,79 (only shown in
FIGS. 13 to 23 are only to be understood as examples of possible, preferable adapters and joints.
The invention is of course not in any way restricted to the preferred embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.
For example, two or more visco elastic damper links could be arranged in parallel or in series or any combination thereof if so desired depending on the specific oscillations to be damped out and the piping system application in question. Thus, two or more visco elastic dampers can extend in parallel and/or series between the object (pipe detail) to be damped and a support location on the piping system.
Several shear layers or sheets of visco elastic material which are fixedly bonded to the inner faces of the two plate elements (first stiff member) and the opposite faces of the end part of the strut (second stiff member) may for example also be used, instead of only one layer of visco elastic material. Further, a mixture of different visco elastic materials may be used depending on the specific oscillations to be damped out in the piping system, instead of only one kind of visco elastic material.
The above-described embodiment examples show different types of attachments of the damper or damper link to the pipe object or detail to be damped and the pipe structure. However, other types of such attachments than the one shown in these embodiment examples may alternatively well be used, for example welding, moulding, gluing, riveting or bolting, etc.
For example, the U-shaped structure comprising the two stiff plate elements may be secured together via an intermediate end element arranged between the plate elements and the end part of the strut or the like, instead of forming the U-shaped structure comprising the two plates in one single piece.
Number | Date | Country | Kind |
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20042050 | May 2004 | NO | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB05/01351 | 5/18/2005 | WO | 11/17/2006 |