CPD PROGRAMME | PIPEWORK extrude adhesive and the final outer plastic layer so as to complete the main pipe construction of polymer-aluminium-polymer (PAP) pipes. The pipe is then printed with identifying information, surface finished, and either coiled or cut into straight standard lengths. The addition of the aluminium layer to what is otherwise, effectively, a flexible plastic pipe transforms the utility of the pipe material. The thin aluminium layer, with a precisely controlled thickness typically 0.2-0.7mm thick, dependent on manufacturer prevents the diffusion of oxygen through the pipe wall. (Traditional steel and copper pipes are natural oxygendiffusion barriers.) The homogeneously longitudinally welded aluminium layer maintains the pipe integrity and allows it to bend equally well in all directions. The aluminium layer also improves the pressure resistance of the pipework, as well as making the pipe more stable compared with simple flexible plastic pipes. The ductile aluminium layer offsets the inherent shape memory of the plastic layers which, otherwise, can cause recurling of uncoiled pipe while, at the same time, reducing the sagging that can take place in completely plastic pipes. The outer plastic layer protects the aluminium layer, preventing corrosion and pitting on the aluminium surface. The inner plastic layer of multilayer pipes has an absolute surface roughness of approximately 0.007mm similar to that of copper tubing and significantly lower than steel pipe. For turbulent flow, as would be normal in building services applications, surface roughness will affect the pressure drop, particularly in small pipe sizes, and a relatively smooth surface resists the accumulation of contaminants, such as limescale, magnetite and biofilms. Commonly used plastics in multilayer Figure 2: Smaller diameters may be bent by hand (Source: Geberit) Figure 3: A hand-operated pressing machine used on smaller-diameter pipework (Source: Geberit) piping are cross-linked polyethylene (PE-X), polyethylene raised temperature (PE-RT II), and high-density polyethylene (HDPE). PE-RT II was specifically developed to provide strength capabilities comparable to P-EX materials, but without the need for cross-linking. In terms of the overall performance of the complete multilayer pipe, these are all similar, with safe operating pressures in the order of 10bar for most hot- and cold-water systems. (This compares with 30+bar for copper and 60+bar for steel systems.) PE-X cannot be recycled, whereas PE-RT and HDPE are recyclable. Calculations1 undertaken by the European Plastic Pipes and Fittings Association indicate that the carbon footprint of multilayered pipework systems is substantially lower than comparative copper systems. A significant advantage is that the pipes, depending on the manufacturer, are available in continuous rolls and straight lengths up to 32mm, as well in sizes up to 75mm outside diameter in standard straight lengths. Laying the pipe from continuous coils without joints eases the installation process by allowing pipes to follow circuitous routes, and reduces the opportunities for problems by using fewer fittings. The weight of the pipes is substantially less than metal equivalents, with far easier handling, and so are commonly used in domestic and commercial applications that traditionally used copper pipe. In domestic and commercial applications, this typically includes heating, plus hot-, cold- and drinking-water systems. (Horizontal fixing centres are typically 30% closer than those for copper pipe.) Although multilayer pipe is designed not to provide a continuous electrical conductor, the aluminium layer allows electronic detection of the pipe after installation. Plastic pipework used in water down service and wholesome water applications do not need to incorporate an oxygen barrier; however, in domestic and small commercial installations it is common that, for site simplicity, the same multilayer pipe is used throughout many of the systems (although the basic material price is more expensive than simple non-barrier plastic pipe). Most manufacturers of multilayer pipe systems carry Water Regulations Advisory Scheme (WRAS) approval for use in UK drinking water installations. Smaller sizes can be formed by hand typically with a minimum bend radius of approximately five tube diameters without collapsing the tube, and larger dimensioned pipework (or tighter bends) formed with hand-operated formers or pipe-bending machines. Using multilayer press-fit systems is considerably quicker than traditional metal installations in terms of preparation and connection. This is partly because the tube preparation is less labour intensive, it has fewer steps, and the component designs and installation methods help to ensure a leak-tight system. The fittings will typically include arrangements so that pressing tools such as those shown in Figure 3 and Figure 4 may only be used correctly, so reducing the opportunity for abortive pressings. Depending on the manufacturer, the fittings are made of a variety of materials, including plastic, brass and stainless steel, with many incorporating components to ensure fast and effective connections, as well 48 May 2020 www.cibsejournal.com CIBSE May 20 pp47-50 CPD Geberit v2.indd 48 24/04/2020 16:35