Known for more than 40 years, the interest of pipe manufacturers in PVC-O has increased. But the expansion has been rather slow, considering the performance and properties of the product. PVC-O pipes offer mechanical behaviour that can bring benefits to the players involved in the water chain such as water companies, installers, manufacturers and end users. At the same time new demands for widening the range of applications and overcoming traditional weak points in PVC-O pipes have arisen. To address these challenges, Spanish company Molecor has introduced its new air-based system.
Dolores Herran, Molecor Sales Manager, holding her presentation on “New developments in PVC-O pipes: big diameters, ISS+”at “Pipes in Infrastructure 2013”. (photos: Molecor)
Processability and high orientation degree as challenges
Step 1 – the shape of the socket as it has been produced during the orientation process
Step 2 – ISS+ system is introducing the gasket
Step 3 – the gasket is now placed inside and the socket has been re-shaped maintaining the orientation
For nearly two decades, the 400mm (16”) diameter pipe was the unofficial limit for PVC-O. One of the major challenges, therefore, was to produce PVC-O pipes above this traditional limit, without sacrificing processability and productivity, and without lowering the orientation degree or reducing material quality. The difficulties of producing bigger diameter pipes arise from a number of factors:
For instance, the extrusion of the feedstock pipe can be a problem due to its dimensions. It is quite common for the feedstock to be twice as thick as the final pipe, and half of the final diameter, resulting in an SDR absolutely out of line with how PVC pipes have been traditionally manufactured. These SDR dimensions change the normal parameters of extrusion, mainly in back pressure and the effects of friction. At the same time, it is very important to get a good gelation.
Temperature transmission is another source of difficulty. The equations that govern the heat transfer through the walls of a pipe follow a quadratic formula. Thus, increasing the thickness of the feedstock pipe causes a greater increase in the time required for conditioning the feedstock to the correct temperature. For instance, the time required for conditioning a feedstock pipe twice as thick as another will require up to four times the time for conditioning at the orientation temperature. In traditional in-batch water based systems this dramatically reduces output, making the process too slow to be profitable, and in in-line systems this will increase the length of the production line.
Last but not least, the forces to stretch the pipes have to be considered. To change the dimensions on the feedstock pipe for orientation, in-line systems require pullers to pull the pipe through a mandrel or similar expansion device. As the dimensions of the pipe increase, the equipment required becomes correspondingly bigger and more expensive. In the in-batch systems, expansion is achieved with pressurised water which again results in a loss of efficiency and productivity.
New full dry system
The new full dry systems of Molecor offer new features which reduce the complexity during the production of PVC-O pipes. These systems, designed for large PVC-O pipes, are in batch systems working continuously in-line, but their two principal stages, namely extrusion and orientation, are independent of one another.
As with the new system, the feedstock extrusion stage can be isolated from the rest of the process, the adjustment of extrusion parameters can be done independently of any orientation device. An advantage of the system is to reduce the length of the production line. As the temperature for orientation is close to 100°C, the feedstock pipe is air-cooled, which gives the necessary robustness to the external layer of the pipe to enable it to be pulled. This enables the cooling length of the feedstock pipe to be drastically reduced and the production line for pipes of up to DN800mm is now no longer than 60m.
New integral socket system for PVC-O pipes (ISS+)
There are a number of problems to take into account when socketing a PVC-O pipe. It is very important to apply the right orientation factor and direction to the socket. If the socket is created after the pipe orientation, it must retain the initial properties and must not affect them when re-heating and re-stretching, as re-heating can modify the orientation properties.
The new integral socket system of the company is able to incorporate any kind of joint automatically and has been developed and integrated with the new water free systems, without any kind of orientation loss or brittle effect on the socket. There are several advantages of this socket system: it can be applied to any kind of gasket or joining system, even to self-restraining systems. The place where the material is heated lies just outside the stressed area, and the system therefore maintains the orientation with no loss of properties.
www.molecor.com
