In sewerage distribution networks, pipes made of concrete, stoneware, cast iron or plastic are generally used.

Polieco opted for the latter material. However, talking about plastic pipes is quite broad as they have different characteristics depending on the polymers used.

Therefore, let us clarify the acronyms, specifications and technical characteristics.

Polyvinyl chloride (abbreviation: PVC), polypropylene (abbreviation: PP) and polyethylene (abbreviation: PE) are the most commonly used plastics in sewage distribution networks.

In particular, HDPE pipes (abbreviated as HDPE) can be produced as solid-wall or twin-wall pipes.

The twin-wall, or structured, HDPE pipe was created in the 1980s with the idea of combining the specifications of polyethylene with the advantages of a corrugated structure.

As a consequence, high density polyethylene pipes have a number of fundamental features, including:

  • very strong resistance to aggression by wastewater;
  • lightweight;
  • high ring stiffness;
  • strong resistance to laying and working stresses;
  • easy and fast laying;
  • guaranteed quality over time;
  • little maintenance;
  • very good cost/efficiency ratio.

Flexible pipes are plastic pipes where an external stress may cause deflection of the ring section (> 3%) without damage*.

What influences the flexibility of a pipe is the ring stiffness (SN), which can be calculated through the formula of the modulus of elasticity (E) whose elements are the pipe material, the moment of inertia (I) and the mean diameter of the pipe (mD).

The more flexible a pipe, the smaller its modulus of elasticity.

In the case of plastics, in fact, the values are:

  • PVC 3.6・103 Mpa
  • PP 1.4・103 MPa
  • PEAD 1.0・103 Mpa

A high modulus of elasticity does not, however, necessarily mean that we are talking about a high quality pipe; if the pipes do not also possess a strong crushing resistance, as polyethylene pipe, they will be brittle and thus prone to cracking.

The second characteristic to take into account when assessing the flexibility of a pipe is the moment of inertia of a wall (I). In order to achieve adequate ring stiffness, plastic pipes with low modulus values must have a high moment of inertia.

To avoid high thicknesses and high weights and expensive costs, the moment of inertia can be increased using the ribs.

It should also be considered that, because of their viscoelasticity, plastic pipes which are subject to a steady load generally lose their shape over time. This is called “creep“.

The stress/creep ratio is called “creep modulus“, not to be mistaken for the modulus of elasticity of the material.

Once installed, in fact, the pipe is subject to an immediate deflection followed by a slow progressive deflection over time. Time varies depending on soil conditions and methods of installation but is not longer than two years.

Why does Polieco produce corrugated HDPE pipes? To answer this question, one needs to understand what polyethylene is and what its inherent properties are.

This is a thermoplastic material which is obtained by polymerization of ethylene monomer, a by-product of oil, characterised by high molecular weight chains (CH2=CH2). Several ethylene polymerization processes have been developed over the years in the attempt to improve the physical and chemical characteristics of polyethylene such as resistance to internal pressure on a long term and resistance to high temperatures and processability.

An important parameter through which the mechanical characteristics of the different types of polyethylene used for pipes can be identified is MRS (Minimum Required Strength). MRS values (in Mpa) multiplied by 10, determines the polyethylene type: the polyethylene which is mostly used for pipes is PE 100 (with MRS equal to 10 MPa).

The black polyethylene pipe produced by Polieco is made from a material having the following characteristics:

  • resistance to the chemical aggression of the most commonly used chemical substances in sewage wastewater;
  • electrically inert;
  • resistance to biochemical and biological aggression;
  • no dispersion of environmentally harmful substances;
  • conditions being equal, it has better abrasion resistance than other materials;
  • thermal expansion coefficient of polyethylene is 1.7・10-4°C-1.

In addition to PE pipes, Polieco also manufactures polypropylene pipes with a high modulus of elasticity (PP-HM), whose modulus of elasticity is remarkably higher than that of polyethylene but with the same characteristics of processability.

Compared with high density polyethylene, the polypropylene with a high modulus of elasticity has the following characteristics:

  • lower specific weight;
  • slightly increased resistance to high temperatures and slightly lower resistance to low temperatures;
  • similar resistance to abrasion and chemical agents;
  • short and long term higher modulus of elasticity.

The modulus of elasticity of the material directly affects the polypropylene pipe stiffness and, as a result, its resistance to vertical loads. Therefore, thickness being equal, a material with a higher modulus of elasticity has an increased resistance to loads. In the case of the polypropylene with a high modulus of elasticity used by Polieco, the increase in the class of resistance, with equal weight, can be higher than 65% compared to HDPE pipes; this allows manufacturing pipes with a class of resistance higher than SN 8.

* definition given by AWWA (American Water Works Association).