The sewer lines are designed according to the workload that the inhabited centers place on the drainage system and, based on other technical-design parameters, the characteristics of the pipes for transporting waste liquids.
Non-pressure pipes composed of plastic material, especially those made of high-density polyethylene and polypropylene, are widely used for decades due to the advantages that are inherent in the raw materials that make up the pipes themselves.
Recycled or virgin HDPE granules can be used that have an MFI 0.4-0.7 at 190 ° / 5 Kg., Or in PP with MFI 1.5-2 at 230 ° / 2.16 Kg. With a correct thermal stability and the right amount of carbon black.
The characteristics normally required in the design phase are:
• Resistance to external loads
• Resistance to chemical and electrochemical aggressions
• Bidirectional sealing of the joints
• Hydraulic characteristics constant over time
• Reduced adhesion to encrustations
• Easy assembly and installation
• Reduced cost of installation and maintenance
HDPE and PP pipes can be corrugated, ie have a reinforcement corrugation on the outside of the structure and a smooth finish on the inside.
In the presence of large pipes it is possible to interpose a metal structure in the intrados of the corrugation in order to increase the resistance of the product to the phenomenon called "creep" , which is configured in a visco-elastic behavior of the material, with the consequent temporary deformability of the tube.
We mentioned the presence of the two layers of the tube, the outer corrugated one and the smooth inner one, elements that therefore have two distinct functions.
The internal smooth layer, in direct contact with the transported fluids, must have a correct chemical and mechanical resistance to the transported liquids and a low flow resistance .
The external corrugated layer has the function of counteracting the compression forces acting on the laid pipe, ensuring its durability and the absence of breakages.
In the case of reinforced pipes, produced in Japan in the 90s of the last century and subsequently widely used also in the United States, the characteristics of plastics such as resistance to abrasion, lightness, minimum roughness coefficient, inertia to chemical substances and ease of installation, to the characteristics of steel which, for example, has a much higher modulus of elasticity than polyethylene.
The use of PP instead of HDPE occurs due to small differences in materials:
• Elastic modulus slightly higher
• Better behavior at high temperatures (lower, however, at low temperatures)
• Lower density and specific weight
Among the three characteristics listed, surely the difference of the elastic modulus is that more important, as the modulus influences the stiffness of the pipe and therefore the resistance to compressive loads.
Therefore, with the same thickness, a higher elastic modulus corresponds to a greater resistance to loads and, in the case of HDPE, the instantaneous elastic modulus is normally> 800 MPa, while in PP it is> 1250 MPa.
As we said, the pipes made of HDPE and PP have excellent hydraulic characteristics both as regards the roughness of the walls in contact with the fluids, but also as regards it concerns the resistance to abrasion, guaranteeing a constant hydraulic flow and a great durability of the sewer line.
Among the competitors of PP and HDPE pipes, such as concrete, coated concrete, fiberglass, stoneware and PVC pipes, it occurred, through laboratory tests, that the resistance to internal abrasion is lower among the competitors, therefore, these are subject to greater mechanical wear. Among these products, those composed with PVC, gave results close to PP and HDPE pipes.
To get an idea of what is meant by large-diameter sewer pipes, we can say that on the market there are pipes with an external diameter of up to 2500 mm. and internal of 2400 mm. about.
When laying HDPE, PP and PVC pipes in trenches, the role of the substrate on which the line will be laid is fundamental, as these artifacts are subject to the behavior mechanical of visco-elastic type, therefore subject to constant deformation over time defined as "creep".
In the case of substrates that are not extremely compact or subject to small movements, the use of corrugated pipes with metal reinforcement in the intrados of the corrugation can help to contain this phenomenon .
As regards the chemical characteristics of HDPE and PP pipes, we can say that the constituent materials have in themselves characteristics of resistance to electrochemical corrosion or to galvanic coupling, as they are not in themselves electrically conductive.
Automatic translation. We apologize for any inaccuracies. Original article in Italian.