Mechanical Joints

There are circumstances where the use of electrofusion fittings or butt-welding of Polyethylene pipes is simply not possible, typically because moisture is present - such as in emergency repair situations where the site or pipe cannot be successfully de-watered to allow dry fusion welding to occur and for temporary works where disassembly may be required.

In these circumstances, mechanical connectors are a fast and efficient joining method. Mechanical connectors for Polyethylene differ to typical socket connections used on DI and PVC pipe, mechanical connectors for PE must have two elements present:

• A socket/sealing system to contain the pressure (in the same style as a DI or PVC socket joint,
• A suitable mechanical restraint system to anchor the PE pipe into the fitting and maintain the 'end to end' restraint.

Mechanical restraint allows the forces generated in a PE network (thermal, soil, pressure forces etc) to be transferred through the mechanical fitting and along the pipeline or into a fixed point. In accordance with UK Water Industry Standard WIS 4-24-01, there are three definitions of mechanical restraint joints suitable for use with PE, each carries a different designation depending on its ability to maintain the transfer of lineal forces within a PE pipeline;

• Type 1 fittings: where "the end-load resistance of the joint is greater than the longitudinal strength of the pipe". Meaning that if the pipe and fitting are pulled apart in tension, the pipe will always fail (yield) before it separates from the mechanical joint or leaks, regardless of the system being pressurised or not. This is the equivalent of a welded joint. (Note: it is much harder to pass this test if the system is unpressurised, as internal pressure holds the pipe tight against its mechanical gripping mechanism).
• Type 2 fittings: where "the end-load resistance of the joint is greater than the maximum axial forces assumed to be acting on the joint" (See IGN 4-01-02v3) Meaning the joint will provide restraint at the maximum operating pressure and temperature under normal operating conditions, but excludes additional loads on the joint such as from ground movement/settlement. 
• Type 3 fittings: where "the end-load resistance of the joint is less than that required for Type 2 fittings". Meaning no mechanical restraint - a typical rubber ring socketed joint that does not provide any end load restraint and requires anchor blocks is a Type 3 Fitting.

IGN 4-01-02 v3

The following fittings typically meet the definition of Type 1 Fittings:

• Flanged PE joints - where the PE flange adaptor complies to ISO 9624 and has been Type Tested (TT) in accordance with BSEN 12201-3 Table 4: Hydrostatic strength test at 80º C for 1000 hours.
• Threaded Adaptors - where the PE is injection moulded around the copper alloy thread and has been Type Tested as above.
• Compression Fittings with a gripping ring assembly that have undergone testing in accordance with WIS 4-24-01 and have been certified as a Type 1 compliant fitting.  
• Mechanical Transition Fittings with a gripping ring assembly that have undergone testing in accordance with WIS 4-24-01 and have been certified as a Type 1 compliant fitting.

Note: The provision of an anchor block is not a suitable substitute for the lack of mechanical restraint in fittings connecting PE. (see here why Anchor blocks should not be used in a PE system). If restraint is not available within the mechanical fitting itself, then PE puddle flanges, anchored in close proximity (~ 1-2m) each side of the joint provides sufficient restraint.