TPI April 2011

Production process Focussing on corrugated tubes for transportation of liquids or gases, it is necessary to subdivide between continuous corrugated metal hoses and metal bellows with short length and special end preparations for connection. Both of them have to be gas or water tight construction. The main difference is the product length. For a corrugated metal hose the needed length goes up to 1,000 metres and even more. So also the manufacturing technology has to be different even for both cases metal forming techniques can be used. The corrugated shape for metal bellows has a limited length and is formed out of a tube section using a tooling from the in- and outside. The outer mould shows the final corrugated shape and from the inside the tube is bulged till there is full contact to the mould. To expand the tube elastic dies or liquids (hydroforming) are used. For the continuous production of an endless corrugated hose it is not possible to work from the inside. So a metal forming process is required, which is able to produce a corrugated tube solely by working at the outside of the smooth tube. Figure 3 gives an overview of a continuous corrugated hose production line. Base product is a coiled metal strip which is pulled from a reel and forwarded to the forming section. In several steps the flat tape is formed to a tube using bending operations. Afterwards the edges of the formed tube are bonded together using seam welding. The movement of the tube is done by pulling it through the line using a capstan after the forming unit. Afterwards the smooth tube enters the corrugation unit where the wave form is applied. Finally due to the higher flexibility the corrugated tube can be reeled up. So the forming of a smooth tube and the corrugation process is linked to one manufacturing step. That ensures a continuous production for very long length and reduces time losses due to handling operations. To do this in an economic way continuous and stable processes must be provided [2] . One possibility for this purpose is a corrugator with a free rotating corrugation disk as shown in Figure 4. The main tool of this device is a ring with a specific inner geometry which is positioned under a certain angle relative to the tube’s axis. Eccentric rotation of the ring leads to a wobbling motion of the tool. This ensures a continuous and nearly frictionless process, where a corrugator rotation speed up to 8,000 rpm can be realised. Depending on the tool geometry and on the process settings a helical or an annular corrugation can be produced with this process. It is clear from this description that a lot of input parameters have to interact to obtain a stable corrugation process with sufficient results.

Figure 4 : Ring type corrugation process [3]

Demands out of application Nowadays metal flexible hoses are very widespread in applications which can be summarised as follows: • Automotive and aircraft industries • Chimney tubes pipes • Cryotechnique • Floor wall heating

• Fuel/oil conduit • Gas supply line • Heat exchanger • Liquid natural gas transfer lines • Petrol transfer lines

• Solar tubes • Solar boilers

The needed product properties vary significantly for these applications out of different geometry, material and conditions of use. So for validation whether a manufacturing technology is applicable a robust simulation tool is required. Testing by trial and error is very time consuming and cost intensive because endless hose production is a continuous process. One first step into this direction is to develop a finite element model of the ring-type corrugation process described above and in Figure 4. The main goal is to provide a better understanding of what happens inside the tooling. For reduction of complexity the examination was focused on flexible hoses for solar application (compare Figure 5). This product with annular type of corrugation is used for connection between solar panel and the storage unit for heated water or for the heat exchanger tubing itself. In general stainless steel is used for corrosion resistant design. The main advantage of using annular flexible hoses for this field of application is the easier installation and end connection. No welding or brazing is needed. The end connection is done by clamping, so the demands for precision in geometry are very high. To ensure proper quality over a lifetime greater than 15 years product characteristics (eg bend ability, pressure resistance) are specified by several standards like DIN EN 14585-1 [7] and DIN EN ISO 10378 [8] .

Figure 3 : Corrugated hose production line [2]

Figure 5 : Flexible metal hose for solar application

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April 2011 Tube Products International