GB2318623A - Joint assembly in a pressurized system - Google Patents

Abstract

The assembly comprises first and second couplings hollow, flexible, soft rubber pipe (3); fluid under pressure flows through the couplings applying a compressive force to the rubber, so that there is no change in length of the rubber pipe when a tensile force is applied to the two couplings. Typically, the assembly is used for joining rigid pipes (1,2) and the couplings can be an integral part of the pipes (1,2) or attached to them (7). The ratio of wall thickness to inner radius of the rubber pipe is generally 1:1 and the Poisson's ratio of the rubber used is approximately 0.5.

Description

JOINT ASSEMBLY This invention relates to a joint assembly.

Rigid pipes or conduits are commonly joined by inserting a flexible section between them. The flexible pipe is usually made of a resilient elastomer, often rubber. This has the disadvantage that when the pipes or conduits are pressurised, both the rigid pipes and the flexible pipe connecting them are subjected to both a circumferential and a tensile stress.

Where the flexible pipe is made from a resilient elastomer having a high Young's modulus, the resulting strain in the flexible section may be undesirably large in both the axial and radial directions.

Thus, in pipes or conduits where the levels of pressure and, as a consequence, tensile and circumferential stress are high, it is necessary to reinforce the resilient elastomer with a stiffer, stronger material, typically steel or fibre braid. However, the addition of such reinforcing material degrades the flexibility, shock absorption and vibration attenuating properties of the connection.

In accordance with the present invention, ajoint assembly for use in a pressurised system comprises first and second coupling means; and a hollow flexible member connected between the coupling means; wherein the flexible member comprises soft rubber; wherein fluid flows through the flexible member under pressure; wherein a tensile force is applied to the coupling means; and wherein a compressive force applied to a surface of the flexible member due to the fluid pressure relative to the tensile force is such that there is substantially no change in length of the flexible member.

The present invention avoids the problems of prior art systems by arranging that a tensile force applied to the coupling means is substantially equivalent to a compressive circumferential force on the flexible member due to the pressure of the fluid flowing through the flexible member so that no reinforcement is required in the flexible member to maintain its shock absorption and vibration attenuation properties.

Preferably, the joint assembly further comprises a pair of hollow rigid members coupled to the flexible member via the coupling means.

Preferably, the coupling means are integral with the rigid members.

Preferably, the joint assembly further comprises a shield positioned radially outward of the flexible member.

This is particularly useful for an arrangement where the fluid pressure is exerted on the inner wall of the flexible member, to contain the flexible member if the pressure exceeds that which the member can withstand.

Preferably, the coupling means are adapted such that a tensile force applied to the rigid members, causes the coupling means to apply a compressive force to each end of the flexible member.

Preferably, the flexible member has a uniform cross-section.

Preferably, the flexible member has an annular cross-section.

Preferably, the flexible member has a ratio of wall thickness to inner radius of approximately 1:1.

Preferably, the flexible member is substantially coaxial with at least part of a rigid member.

Preferably, the soft rubber has a Poisson's ratio of approximately 0.5, although values between 0 and 0.5 could be used.

Specific examples ofjoint assemblies in accordance with the present invention will now be described and contrasted with a conventional joint with reference to the accompanying drawings in which: Figure 1 is a section through a first joint assembly according to the invention; Figure 2 is a section through a second joint assembly according to the invention; Figure 3 is a plan view of a third joint assembly according to the invention; and, Figure 4 is a section through a fourth joint assembly according to the invention.

An example of a joint assembly according to the invention is shown in Figure 1, where first and second rigid pipes 1, 2 are connected by a flexible pipe 3. The flexible pipe 3 has a geometry which does not alter due to tensile stress applied to it, because the fluid flowing through the flexible pipe causes a compressive force pushing the walls outward, in effect shortening the pipe's length which counteracts the tensile force trying to stretch the flexible pipe. Thus the flexible pipe 3 continues to absorb shock and attenuates vibrations travelling along the rigid pipes. The ratio of the wall thickness to inner radius of the flexible pipe 3 is approximately 1:1. The rigid pipe has flanges by which it is coupled to the flexible pipe and optionally, a cylindrical metal jacket may be provided around the flexible pipe so that if the fluid pressure builds up above that which the flexible pipe can withstand, then it is contained.

Another example of the invention is shown in Fig. 2, where first and second pipes 4, 5 of the same, substantially circular, cross-section are connected to one another by means of a flexible pipe section 6. A couple 7 couples the pipe 4 to the flexible pipe section via a flange 8 on the pipe. The other pipe 5 is coupled to the flexible pipe 4 by a flange 9. The flexible pipe section 6 has a larger diameter bore than the pipes themselves and is positioned outside the pipes 4, 5 so that fluid flow between the pipes 4, 5 is substantially unimpeded.

The couple 7 is made from metal and the shape of the couple is arranged to enable both pipes 4,5 to be joined together in alignment.

Usually, the connection between the rigid pipes and the flexible pipe is via flanges on the couple or the rigid pipes. An alternative is that sockets are formed in the couples of the same size and thickness as the walls of the pipe and flexible pipe section to receive them.

Although most pressurised pipes have a circular section, to increase the maximum compressive force that the flexible member can withstand prior to buckling and simplify manufacture, the invention is not limited to this and the joint assembly can be adapted to other, generally uniform, shapes e.g. square, rectangular, triangular or oval section pipes.

A preferred material for the flexible pipe section is soft rubber which has a Poisson's ratio approaching 0.5, but any other resilient elastomer which has a Poisson's ratio in the range 0 to 0.5 may be used instead. The thickness of the flexible pipe section may be varied to suit the pipes being joined, but the preferred ration of inner radius to thickness is 1:1 which enables the flexible pipe section to withstand a greater pressure in the system before buckling.

In a pressurised system the flexible pipe section softens under pressure, enhancing vibration attenuation both in structure-borne and fluid-borne waves.

A further example of the invention, shown in Fig. 3, joins conduits 10, 11 having different sized cross-sections. A flexible section 12 has a cross section of a size which permits it to be positioned inside the larger conduit 10 and outside and coaxial with the smaller conduit 11. By arranging that the flexible section overlies one of the rigid conduits, the flexible conduit will collapse safely onto the rigid conduit, rather than cause a failure of the joint when a certain pressure dependent on the nature of the flexible conduit is reached.

The smaller conduit 11 is coupled to the flexible section 12 by a flange 13. This can be integral with the conduit 11 or a separate couple which adheres to the conduit and flexible section e.g. by welding or adhesive. The same construction on a larger scale may be used for coupling the other conduit 10 to the flexible section 12.

In the example shown in Fig. 4, a pair of rigid pipes 14, 15 are connected by a flexible pipe 16. One of the rigid pipes 14 is enlarged at one end so that it can fit over the flexible pipe 16. The bore of both pipes 14,15 is the same and less than the bore ofthe flexible pipe.

The flexible pipe 16 is positioned outside one pipe 14 and this pipe is coupled to the flexible pipe by a couple 17. The other rigid pipe is coupled to the flexible pipe via a flange 18.

The flexible pipe section has a uniform cross-section and may be adapted to join to the rigid pipes 14, 15 by various means such as a push fit or screw thread connection, adhesive or fusion. The end of the pipe and flexible section would need to be constructed so that they were able to receive the couple if a push fit or screw thread type were used. A similar arrangement of a couple between the flexible pipe and the rigid pipe 14 could be instead of it having the enlarged end piece.

The assembly of the present invention is particularly well suited to joining pressurised pipes. Pressurisation of rigid pipes or conduits causes an axial tensile stress to be applied to them as a result of which the flexible section joining them is subjected to a compressive force.

The flexible section would normally strain under this compressive force, however, the internal pressure within the assembly applies a tensile force to the flexible section which counteracts the compressive forces. This prevents significant strain in the flexible section when connecting pressurised rigid pipes or conduits. For a resilient elastomer with a Poisson's ratio of 0.5, there is no strain. However, 0.5 is a theoretical maximum and any Poisson's ratio above 0 usefully limits the strain. The invention is suitable both for systems which are positively pressurised such as gas or liquid filled pipes and for use where fluid such as water flows under gravity.

Claims (10)

1. A joint assembly for use in a pressurised fluid system, the assembly comprising first and second coupling means; and a hollow flexible member connected between the coupling means; wherein the flexible member comprises soft rubber; wherein fluid flows through the flexible member under pressure; wherein a tensile force is applied to the coupling means; and wherein a compressive force applied to a surface of the flexible member due to the fluid pressure relative to the tensile force is such that there is substantially no change in length of the flexible member.

2. Ajoint assembly according to claim 1, further comprising a pair of hollow rigid members coupled to the flexible member via the coupling means.

3. A joint assembly according to claim 1 or claim 2, wherein the coupling means are integral with the rigid members,

4. Ajoint assembly according to any preceding claim, further comprising a shield positioned radially outward of the flexible member.

5. A joint assembly according to claim 2 or claim 3, wherein the coupling means are adapted such that a tensile force applied to the rigid members, causes the coupling means to apply a compressive force to each end of the flexible member.

6. A joint assembly according to any preceding claim, wherein the flexible member has a uniform cross-section.

7. Ajoint assembly according to any preceding claim, wherein the flexible member has an annular cross-section.

8. A joint assembly according to any preceding claim, wherein the flexible member has a ratio of wall thickness to inner radius of approximately 1:1.

9. A joint assembly according to any of claims 2 to 8, wherein the flexible member is substantially coaxial with at least part of a rigid member.

10. Ajoint assembly according to any preceding claim, wherein the soft rubber has a Poisson ratio of approximately 0.5.