Polyethylene pipe
Polyethylene pipes are pipes made of polyethylene (PE) that have been used in pipeline systems for gas , water supply and wastewater disposal since 1957 . They are also used in various industrial areas.
properties
The most important properties are corrosion resistance , resistance to various chemicals, crack resistance, low weight and simple connection techniques . In addition, polyethylene pipes have sufficient ring stiffness for underground pipes and still have a high degree of flexibility in order to be able to use e.g. B. Adjust bumps. The high resistance to environmentally harmful and aggressive media - such as oils, acids and alkalis - has meant that it is also used as a gravity line. In addition, the roughness of the pipe wall is low compared to that of steel pipes, which results in low friction losses in the fluid on the pipe wall and hardly any incrustations occur. More recently, pipelines made of cross-linked polyethylene (PEX) or polyethylene with increased temperature resistance (PE-RT) have been offered, which are designed for continuous operation with water at 10 bar pressure and +70 ° C fluid temperature. Polypropylene (PP) pipes are also approximately comparable . Pipes made of PP have a slightly higher temperature resistance than PEX. Also multilayer pipes made of PEX and aluminum are in use.
Disadvantages of pipes made of polyethylene compared to z. B. metallic pipes are the lower temperature resistance, lower inherent stability z. B. when laying on supports and a lower resistance to solar radiation.
lifespan
Pipes made of PE / PP and similar plastic materials are mainly used for pressure pipelines . Therefore, the resistance to mechanical, but also to long-term thermal stress, is one of the most important criteria for usability. The classic test method for assessing the long-term properties of pipe materials and for predicting the service life is the so-called internal creep test . In this experiment, the service lives of pipes at different pressures and temperatures are determined, the pressurization usually being carried out with water. The ambient medium for PE is usually water with a wetting agent, but air is also used. The data from the internal creep pressure test can then be used to calculate the service life at any temperature and load, with pipe systems usually being designed for a service life of 50 years at room temperature . For a number of years, however, there has been repeated discussion about a desired lifespan of up to 100 years.
Failure behavior
If pipes made of PE are subjected to internal pressure, failure mechanisms of different degrees will occur depending on the applied equivalent stress or load time. A basic distinction is made between three types of failure, each of which is assigned to an area in the creep curve. Whether a polymer has all three curve sections or only individual areas of the creep curve depends on the material itself and the test conditions. It should also be noted that with increasing temperature and constant equivalent voltage, shorter service lives are obtained. This means that the curves shift to the left or downwards:
In area 1 of the creep curve, a ductile deformation fracture occurs at high internal pressures after relatively short times. This area is mainly influenced by the yield stress of the material and failure usually occurs on the smallest wall thickness of the pipe or on imperfections.
In area 2 at slightly lower pressures, the service life is considerably longer. Failure occurs by crack initiation followed by slow crack growth and is characterized by brittle fractures and minor deformations. The starting point for these fracture processes are usually microscopic flaws and defects near the inside of the pipe, where cracks are initiated. These cracks then grow from the inside wall of the pipe to the outside surface of the pipe until failure occurs.
In area 3 , age-related failure occurs at comparatively low pressures and after very long times. At this point, the chemical aging of the polymers is so advanced that many cracks have formed. At this point in time, even slight loads are sufficient to cause a break, and the failure time is almost independent of the internal pressure.
Pipe connection
The pipe connection is a particular advantage. In addition to the connection by means of a socket, a polyethylene pipe (only PE and PE-RT, no PEX) can also be welded. This creates a cohesive connection, which is particularly advantageous for environmentally harmful media, such as contaminated waste water.
The welding takes place, among other things, by means of a welding mirror. The pipe ends are brought into the thermoplastic state with a heated metal plate. When the pipe ends are now pressed together, the material chains tangle with one another and the welded joint is created.
Another option is to use the so-called electrofusion sockets. The treated pipe ends are inserted into this socket, then the welding sockets are connected to a universal welding machine and tightened with a voltage of max. 48 volts supplied. When the heating coil is heated, the connection between the pipe and the welding socket is brought into a thermoplastic state in a controlled manner. A permanent and non-positive connection is created.
There are connection systems especially for non-weldable PEX pipes, for the assembly of which the pipe is slightly expanded and then the connection is pushed in immediately. The expanded PEX pipe only contracts again around the connection after a delay, creating a force-fit connection.
It is also possible to use compression fittings. In this process, the pipe ends are inserted into a clamp fitting and screwed together. Tightening the union nuts creates a restrained and permanently tight connection thanks to the integrated sealing mechanism.
Individual evidence
- ↑ Data sheet Aqua-plus PP pipe ( Memento of the original from February 22, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 3.2 MB).
- ↑ Data sheet and dimensioning aid for composite pipes, contains information on pressure losses in pipes and fittings ( memento of the original from March 6, 2016 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.4 MB).
- ↑ Detailed data sheet on almost all plastic pipe variants ( Memento from December 3, 2013 in the Internet Archive ) (PDF; 3.2 MB).
- ↑ Technical article on the properties of pipes made of PE80 and PE100 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 145 kB).
