Bolted flange connections have been a mainstay of pipeline construction for decades and are frequently the first choice joint chosen by designers connecting pipes, valves and fittings manufactured from dissimilar materials.
In recent years, the increasing popularity of high-pressure polyethylene pipe in sizes larger than DN900 has resulted in the up-scaling of the low-tech stub adaptor flanges that were originally conceived for non-demanding irrigation applications.
Often the pipeline designer has no access to reliable engineering data, and so leaves the task of determining an appropriate torque to the installer, or more frequently, the supplier of flange components.
It’s likely the supplier has no real understanding of the intended application, design pressure, bolt grade or gasket type intended.
Iplex Communication Manager Cindy Bray said “These realities, combined with contractors’ reliance on experience gained with metallic pipe flanges, may result in leaks during pressure testing.
“Attempts to stem the leakage through extra bolt tensioning may only worsen the situation, and ultimately, after several unsuccessful attempts, our sales engineers will receive a phone call claiming our flanges are faulty.
“If the designer had access to good information early in the process a lot of trouble could be avoided.”
Flanged PE pipe joints are an assembly consisting of components that are likely to have been manufactured by a variety of companies and from varying plastic and non-plastic materials.
There is currently no Australian Standard for PE flanges, leaving manufacturers with no option other than to develop their own product specifications or supply imported European components that are typically intended for use in 10 bar applications.
The assembled joint must be able to transfer long-term axial forces whilst maintaining essential gasket-sealing stress over the anticipated 50 to 100-year service life of the pipeline.
However polyethylene’s viscoelasticity will inevitably result in creep of the flange faces, which reduces the gasket sealing stress developed during initial bolt tensioning.
In the case of stub adaptor flanges, there is an additional risk due to the relatively small contact surface area between the steel backing ring and the flange’s shoulder: that excessive bolt force will cause compressive yielding of the PE flange. This situation presents as an apparent loosening of bolts and commonly, leaks will develop.
Faced with these complexities, what is a PE pipeline designer to do?
“Iplex has been working on an on-line solution to enable designers and installers to access guidance on PE flanges, including their configuration, the gasket and bolt type, torque and a tensioning procedure,” said Ms Bray.
“The tool has formed part of our PocketENGINEER suite and allows users to input actual system conditions and instantly access a flange tightening specification.”
This partner content is brought to you by Iplex. For more information, visit www.pocketengineer.com.au or call 1300 0IPLEX (1300 0 47539).