Material developments in ductile iron water and sewer pipes
David Smoker, Business Development Director, Saint-Gobain PAM UK
Recent changes in the standards for ductile iron water pipes, in particular the closer alignment of BS ISO 2531: 2009 and BS EN 545: 2010, have shifted the consideration of pipeline performance from pipe thickness (K) to working pressure (C), and made it easier for pipes to satisfy both standards, opening up greater options for manufacturers and specifiers in the area of material selection.
Previously, pipes and fittings were classified in ‘K’ classes, calculated from an empirical formula based mainly on manufacturing constraints at the time that BS EN545 was first published in 1995. Especially in smaller diameters, this resulted in over-specified pipes with pressure capabilities far in excess of their intended usage. For example, a DN150 flexibly jointed pipe of K9 minimum thickness was rated at a working pressure of 79 bar which, with a safety factor of 3, meant an actual failure pressure in the region of 235 bar – many times normal network operating pressures. Even in larger diameters of say DN1200, the actual failure pressure topped 110 bar.
The new standard recognises the developments in both manufacturing techniques and computer controlled technology which have enabled the consistent production of homogeneous ductile iron pipes to more realistic thicknesses appropriate to their intended use.
BS EN 545 now details three ‘preferred’ pressure classes: Class 40 (40 bar working pressure) for diameters DN40 to DN300; Class 30 for DN350 to DN600; and Class 25 for DN700 to DN2000 diameters. These standards still retain a safety factor of 3 with the relationship with wall thickness easily calculable. Other pressure classes are available with the provision to create a class for a specific project.
A key example of the closer alignment of the two standards is in the area of water quality. BS EN 545 states that: “when used under the conditions for which they are designed, in permanent or in temporary contact with water intended for human consumption, the components shall not change the quality of that water to such an extent that it fails to comply with the requirements of national regulations”. Meanwhile ISO2531 reads: “When used under the conditions for which they are designed, in permanent or in temporary contact with water intended for human consumption, ductile iron pipes, fittings and their joints shall not have detrimental effects on the properties of that water for its intended use.”
The revision of the standards includes minimum performance requirements for couplings, flange adaptors and saddles manufactured for use with ductile iron pipes and fittings. In the area of couplings and flange adaptors, this means the joints are now subjected to the same demanding tests that have been in force for flexible socket and spigot joints since the first publication, including elevated internal pressure and vacuum under extremes of tolerance and thickness, angular deflection and shear.
Also defined is the need to ensure that components from different suppliers meet the performance requirements of the standard – giving added assurance of leak-free performance to the customer.
Evaluation of conformity is now a normative requirement within BS EN 545, which details the requirements for performance testing and factory production control by the manufacturer, including product assessment. It also recommends that the manufacturer has a quality management system in compliance with ISO 9001.
This means the standard not only guarantees quality of manufacture, but still provides the end user with the highest pressure related safety factor for any pipeline material on the market, resulting in savings on materials and energy , while delivering long-term water network reliability.
Towards a ‘composite’ solution
The new specifications also open up greater choice for specifiers and purchasers in relation to both internal coatings, where the coating can be matched more closely to the anticipated pipe contents, and external coatings, where again a coating can be specified which is optimised to the ground in which the pipe is to be located.
For example, there is now the possibility to opt for zinc aluminium, PE, PU or reinforced fibre cement as external coatings. Ductile iron pipes are themselves changing too with ongoing investment by leading manufacturers in materials research and development creating products which offer major benefits in strength, performance, reliability and longevity. What this means is that effectively ductile iron pipes are now composite pipes, offering the key performance characteristics associated with ductile iron along with the desired attributes of the coating.
With the new standards focusing on pipeline performance, rather than pipe thickness being the main criterion, a lighter pipe in terms of weight can now be safely and legally specified so that transport, handling, positioning and connection of pipes are now also much less demanding – meaning that more than ever ductile iron is the true ‘best value’ solution for all potable water pipe requirements.