Another potentially fragile pipe system which can be joined by pipe fittings according to the invention is a swage-lined pipe system. Swage-lining is a system of lining existing below-ground decaying pipelines. A below-ground cast iron pipe has a trouble-free lifetime of the order of 50 years. After that time there is a risk that corrosion will extend in places through the pipe wall, causing leaks. Most of the pipeline after 50 years will retain its structural strength and integrity.

However even one corrosion hole makes the pipeline unserviceable. Systems of lining such pipelines have therefore been developed. One such lining system is swage-lining. A thin-walled lining pipe of a thermoplastic material such as polyethylene with an elastic memory, of a natural external diameter the same as or even marginally greater than the internal diameter of the pipelines is compressed by passing it through a swage, and is then fed into the pipeline to be lined. The liner pipe is pushed from one end and pulled from the other, and pipeline lengths of up to 1000 metres can be lined in a single run.

As the thermoplastic lining pipe recovers its original diameter it expands against the inner wall of the original cast iron pipeline which gives it its structural strength. Unfortunately during the feeding of the lining pipe down the original pipeline often creates longitudinal stresses in the lining pipe. If the lining pipe has sections under tension as it is laid it will stretch, but once laid its elastic memory causes it to try to shrink back to its original length. That places undesirable tension on the fittings used to connect to the lining pipe, and there is a high risk of failure of the joints created by those fittings. The fitting system of the invention is useful in reducing that failure risk.

The fitting system of the invention can thus be applied to plain end RTP pipes or to swage-lined pipes and will not detract from the strength of the pipeline system whether the fittings are used for joining pipe, or attaching valves, bends or plugs.

BACKGROUND ART It is an established practice to use pipe fittings that rely on friction rings and grippers in pipeline assemblies when the force or energy that applies pressure to the grippers and to the seals is the result of a series of bolts tightened to a predetermined torque. The resulting connections have relatively low pressure ratings typically between 10 and 20 BARS. Other known systems use hydraulic pressure acting on grippers and seals, and are rated to much higher pressures. All these systems suffer various disadvantages when used on thermoplastic pipes because of the creep and flexible characteristics of these pipes.

The disadvantages are increased when higher pressure RTP pipes and swage lining pipes require to be joined in the field rather than by fittings attached at the point of manufacture. These disadvantages can even make field installations virtually impossible.

An ideal system should allow for the connection system to be capable of joining the pipes without any loss of strength and be capable of assembly in the field accommodating off-circular as well as circular pipe.

DISCLOSURE OF THE INVENTION According to the present invention there is provided a pipe connection for attachment to a plain end of a thermoplastic pipe, comprising a housing for locating around the pipe end portion, an annular recess in the inner surface of the housing containing gripper elements for contacting the outer surface of the pipe end portion and means for moving the gripper elements radially inwardly against the pipe end portion, characterized in that the gripper elements are designed without sharp edges or corners that could contact the pipe end portion, and have radially inwardly facing surfaces which are formed as rounded peaks and/or valleys; and a preformed tubular insert is inserted into the pipe end portion and has an outer surface preformed into one or more cooperating rounded peaks and/or valleys so that on radially inward movement of the gripper elements the pipe end portion is deformed into a shape following the cooperating one or more rounded peaks and/or valleys and is clamped between the gripper elements and the insert. The number of segmented grippers surrounding the pipe will increase with the diameter of the pipe. The grippers are preferably urged against the outer periphery of the pipe end portion by hydraulic pressure as described and claimed in EP-B-727026. Thus when hydraulic pressure is applied they will force the pipe to conform to the shape of the insert.

Preferably a recess is formed in each of the gripper elements at an inner surface thereof, the recesses of all of the gripper elements being mutually aligned to form a channel, and a seal member is received in the channel and in use is compressed against an outer surface of the pipe and portion, to compress it against a cylindrical portion of the tubular insert outer surface. The seal member is thus carried by the grippers and is preferably a polymer sealing ring that is designed to make contact with the pipe prior to the grippers, and therefore prior to the reforming of the pipe end portion, to provide maximum sealing and also maximum axial restraint of the pipe against longitudinal forces.

If the pipe is an RTP pipe, then the cooperation between the gripper elements and insert will form undulations in the pipe end portion to grip it firmly without cutting into the pipe surface. More significantly, the KEVLAR (TM) or glass fibres, which provide the internal reinforcement of the RTP pipe end, are not cut or broken, so that the pipe end retains its full structural integrity at the join.

If the pipe is a swaged lining, then because of the relatively thin wall thickness of the lining conventional grippers would cut and weaken the pipe wall. A pipe connector according to the invention avoids that weakening because the gripper elements have no sharp edges or corners in contact with the pipe end portion. However it does hold the pipe end portion sufficiently firmly to resist the natural creep of a swaged lining that has been placed in the pipeline under tension, and the join is reliable and permanent.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a longitudinal section through a pipe connection according to the invention used for joining to RTP pipe ends. The pipe connection is shown as joined to one pipe end and ready to be joined to the other pipe end; Figure 2 is a staggered section taken along the line A-A of Figure 1; and Figure 3 is a longitudinal section through another pipe connection according to the invention used for joining an end portion of a swaged pipe liner which projects from an end of a pipeline section. Referring first to Figures 1 and 2, the connection comprises a hollow metal housing 5 in each end of which is formed an annular chamber 7. Each of these chambers 7 houses a circular array of segmented grippers 2 and an annular seal member 4. A circular array of shims 9 spans the spaces between adjacent grippers 2 and prevents entrapment of the seal 4 between adjacent grippers.

A recess is formed in each of the grippers 2, the various recesses being in alignment to form an annular channel, housing an annular pipe seal 8 which in its natural state projects slightly inwardly from the annular channel as shown at the right hand half of Figure 1. When the grippers 2 are pressed against the pipe wall as shown at the left hand'half of Figure 1 the annular pipe seal 8 is compressed within the annular channel and forms a fluid-tight seal around the pipe end.

The inwardly facing surfaces of the grippers 2 have no sharp corners or edges which could contact and cut the pipe end portion, but are formed as a circular array of rounded peaks and valleys facing the pipe 1. A pipe insert 3 made of metal or hard plastic is a push fit into each pipe end. The pipe inserts 3 are tubular members with an outstanding annular end flange to locate them axially in the pipe ends, and are formed with external annular undulations aligned to line up exactly with the rounded peaks and valleys of the grippers 2. The annular undulations of the inserts 3 and the rounded peaks and valleys of the grippers are totally complementary so that the four peaks and three valleys of each gripper line up with the complementary shaped four valleys and three peaks of the annular undulations of the corresponding insert 3.

The housing 5 also incorporates a pair of inlet valves 6 in the form of grease nipples each communicating with one of the annular chambers 7. Bleed valves 10 (Figure 2) are also provided for bleeding air from the chambers 7 prior to pressurization of the chambers.

When the chambers 7 are packed with grease under a hydraulic pressure of up to several thousand BARS, the hydraulic pressure will press onto the annular seal 4 which in turn presses on the segmented grippers 2 and the pipe seal 8. The grippers 2 and pipe seal 8 move radially inwards until the pipe seal 8 touches and seals the pipe which is reformed to conform to the shape of the grippers 2 and the insert 3. The pipe end is thus gripped between the cooperating undulations of the surface of the pipe insert 3 and the grippers 2. It is a feature of the invention that the pipe insert 3 is shaped to be complementary to the rounded peaks and valleys form of the grippers 2.

Referring next to Figure 3, there is shown a pipe connection 20 for attachment to an end of a section of a pipeline 21, for example a below-ground cast iron pipeline 21, that has been lined with a tubular swaged lining 22.

The connection 20 of Figure 3 is formed with a coupling flange 23 which may be used to connect to a similar connection 20 or to an end cap, reducer, T-junction, stopcock or any other item of pipeline hardware.

The tubular swaged lining 22 is typically a thermoplastic lining formed from a material such as polyethelene which has an elastic memory. The lining 22 is relatively thin-walled compared with the RTP pipe of Figure 1 and the thin-ness of the wall renders it liable to tearing if it is placed under tension. Unfortunately the swage-lining process involves pushing one end and pulling the other end of the liner through the pipeline 21, so that the leading end of the lining is generally under tension at the end of the lining process and tends to creep back into the pipeline after lining, by virtue of its elastic memory.

The connection 20 of Figure 3 securely retains and seals SUBSTITUTE SHEET (RULE 26) the end portion of the lining while minimizing the risk of tearing.

The pipe connection 20 of Figure 3 comprises a housing 24 in which is formed an annular recess 25 which receives an annular seal 26 and a circular array of gripper segments 27. Shims 28 are inserted between the gripper elements 27 and the seal 26, spanning the spaces between the gripper elements as in the embodiment of Figures 1 and 2.

Each gripper element 27 is arcuate, so that the complet circular array of gripper elements 27 provides a segmented circular jaw that can be clamped around the lining 22 by the introduction of high pressure hydraulic fluid such as grease into the annular recess 25 through a grease nipple 29.

A tubular insert 30 is a push-fit into the end of tubular swaged lining 22, and has a segmented and flanged end 31 which locates behind an internal shoulder 32 of the housing 24.

A single annular depression 33 is formed in the outer surface of the insert 30 which is inserted into the end of the swaged lining 22. The gripper elements 27 have a cooperating profile, interrupted by an annular recess containing a sealing member 34. In Figure 3 the annular recess is shown at a mid-point of the rounded inwardly directed face of each gripper element 27. It could however be formed at one end as in the case of the corresponding recess of Figure 1. The gripper elements 27 and sealing member 34 together provide the shape of a single rounded peak which cooperates with the rounded valley of the depression 33, whereby the tubular swaged lining 22 in use is trapped between the grippers 27 and the insert 30 and is reformed to the shape of the rounded valley of the insert 30.

The pipe connection 20 also includes means 40 for sealing around the external surface of the cast iron pipeline end 21. A second annular recess 41 is formed in the housing 24 and carries a sealing ring 42. The recess 41 can be packed with grease and then pressurized via a grease nipple 43, so as hydraulically to clamp the sealing ring 42 against the end portion of the pipeline.