Getting connected

GPS shows how its SlimFlange solves the problems of connecting PE pipes to non-PE pipes.

Unless pipeline designers are aware of all the options for connecting PE pipes to non-PE pipes and components, it is possible to increase capital and installation costs unnecessarily, to introduce flow restrictions, or to reduce pipeline security.

The preferred means of jointing PE pipes is by fusion welding - via butt-fusion or electrofusion couplers. Such jointing methods in no way compromise the integrity of the pipe system. And, in extreme tensile tests, it is normally the pipe that finally yields, not the joint.

However, PE pipes cannot be fusion welded to pipes made from other materials and, for the larger diameters used for mains pipelines, such connections are usually made using PE flange adaptors.

Unless expensive and heavy specialist mechanical fittings are used, transition connections can incur significant operational and security penalties at diameters of 250mm and above, either because the adjoining metallic components have to be upsized or else the PE pipework has to be downsized adjacent to the joint. This is because PE stub flanges need to have relatively large bolt circle diameters, due to the thick walls of PE pipes and the substantial PE stub flange shoulders needed to provide for good pull-out resistance under end-loading.

In larger pipe sizes, there can therefore often be a discrepancy in bore size between the PE pipe and the adjoining iron pipe or fitting. For example, although a 450mm OD PE100 SDR17 pipe - which has a 400mm bore - can be jointed to a DN450 valve using a traditional 450mm x 450mm PE flange adaptor (Fig 1), this will result in the bore size changing from 400mm to 450mm between the two pipe materials.

When a PE stub flange is put into axial tension, the PE shoulder elements start to rotate in the radial-axial plane, and try to pull through the bore of the backing ring. Particularly in the larger pipe sizes, manufacturers attempt to counter this tendency by using massive steel rings in order to maximise the grip on the PE shoulder.

Although the shoulder tries to resist rotation with more vigour when clamped between robust metallic members, PE's low friction coefficient and its relative lack of stiffness in bending mean that even hugely thick PE shoulders rarely prevent pull-through in the case of the larger pipe sizes.

Extensive work by GPS has indicated that in large PE sizes, for example 355mm SDR17 and above, it is all but impossible to satisfy Industry Specifiers' legitimate desire for WRc Type 1 end-load resistance (in which the pipe fails before the joint assembly) using conventional PE stub flange adapters.

Aside from not having full end-load performance, large conventional PE flange adaptors cannot allow direct connections between PE and metal pipes of the same nominal bore. To try to overcome this deficiency, combination flanges were introduced in the 1990s as an alternative to the various specialist mechanical fittings and adaptors previously needed to make size-for-size connections. A combination flange comprises a reducer, fused to a PE flange adaptor that is one size less than the PE pipe, such that the bolting diameter matches that of the metal pipe or valve.

However, the consequent bore restriction is often unacceptable in water distribution, and the reduced end-load resistance of the downsized stub flange element reduces overall joint strength considerably.

It is possible to connect most large PE pipe sizes directly to metal pipes or fittings of the same bore with full-face PE flanges with backing-ring drillings extending right through the PE shoulder. But this creates practical difficulties in the field because the whole pipeline needs to be rotated to align the bolt holes of adjacent components.

The GPS SlimFlange solves these problems. The design incorporates a stainless-steel hoop, inserted into the PE flange face. This resists the rotational forces in the shoulder that occur under tensile loading. Instead, under such loading, the PE shoulder is put into shear, which is almost as efficient structurally as tension or compression. Consequently, given sufficient PE shoulder thickness, the shoulder height can be reduced drastically, so that bolting circles can also be greatly reduced. Thus with SlimFlange, size-for-size connection (Fig 2) becomes possible at most PE pipe diameters.

SlimFlange needs no insert sleeve in the PE pipe bore and offers no resistance to flow. And the more direct application of the jointing load means improved sealing. The smaller backing ring also makes SlimFlange lighter than other PE mechanical connections. Expenditure on the installation of pumps and valves in PE pipelines can be minimised by using GPS SlimFlange adaptors.

The design obviates the need to upsize valves or pumps to enable direct connection to PE pipework without also employing a weaker PE flange or a non-rotatable, backing ring. Compared with a conventional PE flange adapter, a SlimFlange backing ring bolting circle is one nominal iron pipe size smaller, creating a more compact adaptor that is up to 50% lighter.

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