GB2172081A

GB2172081A - Damper

Info

Publication number: GB2172081A
Application number: GB08602053A
Authority: GB
Inventors: Gary Macleod
Original assignee: Rheinische Armaturen und Maschinenfabrik Albert Sempell
Current assignee: Sempell GmbH
Priority date: 1985-02-01
Filing date: 1986-01-28
Publication date: 1986-09-10
Also published as: GB8602053D0; JPS61270532A; DE3601712A1; FR2576994A1

Abstract

A damper for restraining undesirable movement comprises a pair of sealed chambers (8, 9) having end wall means (2, 4, 5), and side walls (6, 7) flexible in one direction to allow movement of said end wall means (eg, 5) simultaneously with oppositely valued changes in volume of the chambers (8, 9). A flow pathway (10, 11) connects the two chambers (8, 9) to allow restricted fluid flow from one to the other. The two chambers can be side by side (Figs. 2, 3) rather than end to end. <IMAGE>

Description

SPECIFICATION Damper This invention relates to devices for controlling undesirable motion, movements or vibrations in mechanical or hydraulic devices.

The invention is particularly applicable to dampers for controlling unstable oscillatory behaviour or "chatter" in safety or control valves, although the invention is believed to be generally applicable to the control of vibration and other undesirable movements, including imposed accelerations and displacements, in mechanical or hydraulic apparatus.

Valve flutter or chatter may be caused by any one of a number of factors (see "Valve Selection Handbook") R.W. Zappe, page 177) and remains a continuing problem today. The current practice is to endeavour to reduce the possibility of chatter by altering the characteristics of the system rather than by using a valve designed to reduce the problem.

The inventor is aware of only two proposals for allegedly "chatter-proof" valves. They both rely on the theory that friction on the valve stem should reduce the likelihood of undesirable movement of the of the stem. In one valve the friction is imparted to the stem by means of an elastomeric seal loaded by the pressure of the fluid in the valve body. In the other, friction is imparted to the stem by means of a friction collar loaded by a preloaded spring. In both cases the arrangements have the major disadvantages of wear, which causes changes in the resistance characteristics and requires maintenance, and potential contamination of the valve stem, or friction element, by foreign materials, making them unreliable as devices for chatter prevention.

The inventor doubts that the theory would work in practice, and is not aware of any commercial application of the first mentioned valve. The second valve has been sold commercially, but it has not proved successful as a device for preventing chatter.

It is an object of the present invention to provide a device for reliably reducing undesired movements in mechanical or hydraulic devices and which does not suffer from the disadvantages outlined above.

The invention provides a device for controlling undesirable movements or displacements comprising a pair of sealed chambers containing fluid, each said chamber having end walls and side walls, said side walls being flexible in one direction to allow relative movement between the respective end walls of said chambers and to cause a corresponding change in the enclosed volume of each chamber, a fluid pathway between said chambers for allowing fluid to pass from one chamber to the other, and means for applying an imposed force or movement to at least one end wall to cause a change in the volume within one chamber and displacement of fluid from that chamber to the other chamber.

As the imposed force or movement causes fluid to flow from one chamber to the other, the difference in pressure between the chambers resists the imposed force or movement.

The characteristic resistance of the device may be modified by changing the dimensions of the chambers or the dimensions of the connecting fluid pathway between the chambers. Alternatively, the characteristic resistance may be modified by changing the characteristics of the fluid. For example, changing the fluid density, viscosity and compressibility would alter the characteristic resistance to imposed accelerations, velocities and displacements.

The chambers are preferably essentially cylindrical and the side walls are flexible in the axial direction and substantially inflexible in the circumferential direction. Because of the essentially circumferential inflexibility of the side walls, axial extension or compression of the chambers causes a change in the enclosed volume which in turn causes displacement of an equivalent volume of fluid from one chamber to the other.

The side walls of the chambers may be in the form of convoluted bellows made from stainless steel or some other corrosion resistant material having the necessary strength properties. The characteristics of the bellows are determined principally by the imposed force to be applied to the device, the maximum and minimum expected axial wall deflection, the maximum and minimum expected pressure in the chambers, the number of expected pressure and displacement cycles and the nature of the fluid which is to be used.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Figs. 1 to 3 are schematic representations of alternative damper configurations embodying the invention; Fig. 4 shows one preferred configuration attached to the valve stem of a typical relief valve; and Fig. 5 shows another preferred damper configuration.

Referring firstly to the damper configuration shown schematically in Fig. 1 of the drawings, the damper includes a spool-like member 1 defining an end wall 2, a central spindle 3 and an end wall 4. A common central end wall 5 is slidingly positioned on the spindle 3 and the end walls 2 and 4 and the central end wall 5 are respectively interconnected by side walls 6 and 7 to define closed chambers 8 and 9. In the embodiment shown, the side walls 6 and 7 are each in the form of stainless steel convoluted bellows or some other suitable device which is flexible in the axial direction but substantially rigid in the circumferential direction.

The common central end wall 5 is formed with two fluid pathways 10 and 11 so that fluid (for example mineral oil) contained in the chambers 8 and 9 may flow between the chambers. An imposed force or movement, indicated by the arrows, applied to the central end wall 5 causes movement of that wall 5 in either direction and the fluid within one of the chambers 8 and 9 is displaced to the opposite chamber and the resulting difference in pressure between the chambers 8 and 9 resists the imposed force or movement.

In the damper shown in Fig. 2 of the drawings, a common end wall 12 supports an upstanding post 13 which pivotally supports a lever 21 to which end walls 14 and 15 are pivotally attached. Side walls 16 and 17 are connected between the end walls 14 and 15 and the common end wall 12 to define chambers 18 and 19. The common end wall 12 is formed with a fluid passageway 20 which opens into each of the chambers 18 and 19.

An imposed force or movement indicated by the arrows applied to the pivoted lever 21 causes the fluid contained within one of the chambers 18 or 19 to be displaced to the other chamber and once again the resulting difference in pressure between the chambers 18 and 19 resists the imposed force or movement.

In the damper shown in Fig. 3 of the drawings, end walls 22 and 23 are interconnected by a conduit 24. End walls 25 and 26 are connected to the end walls 22 and 23 by side walls 27 and 28 to define chambers 29 and 30. An imposed force, indicated by the arrows, applied to one of the end walls 26 causes displacement of the fluid within one of the chambers 29 or 30 to the other chamber as in the previous embodiments.

Because of its simplicity of construction and adaptability to valve damping, the damper of Fig. 1 is preferred for this purpose and a typical installation for attaching the damper to a known relief valve is shown in Fig. 4 of the drawings. In this installation, the damper assembly is enclosed within a housing 33 to which the common end wall 5 is rigidly attached by welding or the like. The housing 31 is attached to the top of a relief valve 32 of the known construction, only part of which is shown in the drawing, and the lower end of the spindle 3 is bifurcated to enable attachment of spindle 3 to the upper end of a valve stem 33 of the relief valve 32, say by means of a split pin as shown. The end walls 2 and 4 are welded to the spindle 3 and the fluid pathway 11 in the common end wall 5 is defined by a tube having flared ends.The convoluted stainless steel bellows 6 and 7 are sealingly connected to the end walls 2 and 4 and to the common end wall 5 by welding or otherwise in accordance with the standard practice of the manufacturer of the bellows. It will be appreciated that movement of the valve stem 33 caused by operation of the relief valve 32 will cause the end walls 2 and 4 to be displaced relative to the common end wall 5 thereby displacing fluid from one of the chambers 8 or 9 to the other chamber whereby the motion of the valve stem 33 is damped.

In the embodiment shown, the bellows 6 and 7 are preferably designed in accordance with the standards of the Expansion Joint Manufacturers Association of the United States of America to suit the required pressures and deflections. As an example, damping in the order of 107 Nm-ls is required to prevent chattering of a valve with a size "J" orifice with a set pressure of 1.0 MPa and 10% blowdown situated on a vessel of 10 m3 containing air at 20"C. Such an arrangement would have a displacement of 5 mm, a pressure range of 100 KPa, the walls being designed for 106 combined pressure and displacement cycles.

The diameter of the opening 11 in the common end wall 5 should not exceed 0.10 mm greater than the diameter of the shaft C. The fluid pathway between the chambers is in this embodiment a tube approximately 3.5 mm internal diameter 50 mm long with the ends flared as shown. The fluid used is preferably a mineral oil having a dynamic viscosity of 0. 16 Ns lm. For a valve having the characteristics outlined above, the ends walls 2 and 4 should be about 80 mm in diameter and the bellows 6 and 7 should be about 93.5 mm in length.

All materials are preferably 316 stainless steel.

Referring now to Fig. 5 of the drawings, a preferred damper of the type shown in Fig. 3 of the drawings will now be described. In this arrangement, an external fluid pathway is constituted by a pipe 40 attached to fittings opening through the end walls 41 and 42. As in the previous embodiments, convoluted diaphragms 43, 44 are connected to the end walls 41 and 42 and to a common end wall 45. An attachment lug 46 is fixed to the end wall 41 and an open attachment lug 47 is attached to the common end wall 45. A spacer plate 49 maintains a predetermined spaced relationship between the end walls 41 and 42 so that the common end wall 45 is moved through the attachment lug 47 to damp movements of the element to which the attachment lug 47 is attached. A removabie fluid resistance 48 is positioned in the pipe 40 and by means of which the damping properties of the device may be varied.

The damper embodying the present invention offers the following advantages: (a) There are no mechanical seals, (b) There are no fluid ieakage paths, therefore loss of fluid is eliminated and contamination of the external environment is not possible. Conversely, the environment may not contaminate the fluid. The device will therefore act in otherwise sensitive or degrading envi ronments.

(c) There need not be any contact between moving parts. Therefore wear is minimal and maintenance is eliminated.

(d) Minimal wear ensures long life and accurate maintenance of the resistance characteristics.

(e) No external reservoir of fluid is needed.

It should be appreciated that while the damper structure shown in Fig. 2 of the drawings is presently preferred for the relief valve shown other damper constructions may be used to achieve similar results. Similarly, the chambers may be arranged in many different configurations apart from those shown in the drawings so the invention should not be regarded as being restricted to any of the configurations shown.

Claims

1. A device for controlling undesirable movements or displacements comprising a pair of sealed chambers containing fluid, each said chamber having end walls and side walls, said side walls being flexible in one direction to allow relative movement between the respective end walls of said chambers and to cause a corresponding change in the enclosed volume of each chamber, a fluid pathway between said chambers for allowing fluid to pass from one chamber to the other, and means for applying an imposed force or movement to at least one end wall to cause a change in the volume within one chamber and displacement of fluid from that chamber to the other chamber.

2. A device as claimed in claim 1, wherein said side walls are substantially cylindrical and are flexible in the axial direction and substantially inflexible in the circumferential direction.

3. A device as claimed in claim 2, wherein said side walls are convoluted bellows of corrosion resistant material having the required strength, such as stainless steel.

4. A device for controlling undesirable movements or displacements comprising spaced end walls and a common end wall, convoluted bellows attached to each end wall and to said common end wall to define a pair of sealed chambers containing fluid, said bellows being flexible in one direction and substantially inflexible in the other direction to allow relative movement between said end walls and said common end wall, a fluid pathway in said common end wall opening to each of said chambers to allow fluid to pass from one chamber to the other, and means for applying an imposed force or movement to at least one of said end walls or said common wall to axially extend or compress said chambersto change the enclosed volume of said chambers and cause displacement of an equivalent volume of fluid from one chamber to the other.

5. A device as claimed in claim 4, wherein said end walls are rigidly connected in spaced relation to a central shaft or spindle, said common wall being located between said end walls and having a central opening through which said shaft or spindle passes.

6. A device as claimed in claim 4, wherein said common end wall defines a base on which said bellows are mounted side by side, said end walls being interconnected by means of a pivoted lever to which the imposed force or movement is applied.

7. A device as claimed in claim 5, wherein said common end wall is rigidly attached to a housing enclosing said device, said imposed force or movement being applied to said central shaft or spindle.

8. A device as claimed in claim 7, wherein said fluid pathway is defined by a tube having flared ends.

9. A device as claimed in claim 1, 2 or 3, wherein said fluid pathway is constituted by an external pipe connected to openings in corresponding end walls of said chambers, said pipe including a fluid resistance which effects the control imposed by said device.

10. A device for controlling undesirable movement towards displacements substantially as hereinbefore described with reference to Figs. 1, 2, 3, 4 or 5 of the accompanying drawings.