The story of Plastics is a story of success. And the basis of that success is a human desire for a better life. Plastics – more than any other material, have sparked scientific curiosity. Designing plastics with the right technical characteristics to do the job is a never-ending story.
Plastic pipe systems fulfil a wide variety of service requirements. These requirements are precisely described in a complete set of European Product Standards for each application alongside their specific characteristics. For example:
- Conveyance of drinking water: Hygienic requirements
- Conveyance of gas: Highest Safety requirements
- Plastic pipes for radiant heating and floor heating: Temperature resistance over decades
- Sewer applications: High chemical resistance
Plastic pipes are perfectly capable of fulfilling the specific requirement for each application. They do so with a high level of performance over a long lifetime and with reliability and safety.
The key factor for success is achieved by maintaining consistently high quality levels. For plastic pipe products, these levels are closely defined by the different standards.
Two key aspects are fundamentally important for the excellent performance of plastic pipes: flexibility and long lifetime.
Flexibility:Plastic Pipes are classified by their ring stiffness. The preferred stiffness classes as described in several product standards are: SN2, SN4, SN8 and SN16. The ring stiffness of pipes is important if they are to withstand external loadings during installation. The higher the figure, the stiffer the pipe!
After correct installation , pipe deflection remains very limited but it will continue to some extent for a while. In relation to the soil in which it is embedded, the plastic pipe behaves in a ‘flexible’ way. This means that further deflection in time depends of the settlement of the soil around the pipe.
Basically, the pipe follows the soil movement or settlement of the backfill, as technicians call it. This means that good installation of pipes will result in good soil settlement. Further deflection will remain limited.
For flexible pipes, the soil loading is distributed and supported by the surrounding soil. Stresses and strains caused by the deflection of the pipe will occur within the pipe wall. However, the induced stresses will never exceed the allowed limit values.
The thermoplastic behaviour of the pipe material is such that the induced stresses are relaxing to a very low level. It has to be noted that induced strains are far below the allowable levels.
This flexible behaviour means that the pipe will not fail. It will exhibit only more deflection while keeping its function without breaking.
However, rigid pipes by their very nature are not flexible and will not follow ground movements. They will bear all the ground loadings, whatever the soil settlement. This means that when a rigid pipe is subject to excessive loading, it will reach the limit for stress values more quickly and break.
It can therefore be concluded that the flexibility of plastic pipes is such that it offers an extra dimension of safety. Buried Pipes need flexibility.
Long Life Time:Plastic pipes have been successfully used in service for over 50 years. The predicted lifetime of plastic piping systems exceeds 100 years. Several industry studies have demonstrated this prognosis.
Plastic pipe materials have always been classified on the basis of long-term pressure testing. The measured failure times as a function of the stresses in the pipe wall has been demonstrated in the so-called Regression Curves.
An extrapolation based on measured failure times has been calculated to reach 50 years. The predicted failure stress at 50 years was taken as a basis for the classification. This value is called MRS, Minimum Required Stress, at 50 years.
Safety Factor:From the qualified MRS value, a safety factor is applied to arrive at a design stress from which the nominal pressure PN has been calculated. Typical safety factors are for PE 1,25 and for PVC 2,0. The qualified nominal pressure of a plastic pipe therefore includes a safety factor, which results in a very safe design of the piping system.
It is therefore emphasised that this classification of plastic pipes has nothing to do with the real predicted lifetime of plastic pipes. This can be much longer. Several investigations have recently been published which show that PVC and PE pipes will last even longer than 100 years.
Post time: Feb-27-2018