THERMOPLASTIC PIPE MATERIAL CLASSIFICATION AND PIPE STIFFNESS

mathematical extrapolation, conforming to ISO 9080 Determination of the long-term hydrostatic strength of thermoplastics materials in pipe form by extrapolation. The Design Coefficient C (Safety Factor) is determined in conformance to ISO 12162 Classification and designation – Overall service (design) coefficient. These two values are used in a simple formula to determine the Allowable Design Stress σ for the pipe: σ = MRS/C

Thermoplastic polymers have improved over time and two of the most commonly used polymers, HDPE and PVC, have both had several iterations in their development: PE 63, PE 80 and PE 100 for the former and PVC-U, PVC-M and PVC-O for the latter. PVC-O itself has developed through five classifications over more than forty years.

The classification of a thermoplastic is ten times its MRS at 50 years and 20°C. For example, PE 100 has a MRS of 10 MPa at 50 years and 20°C. PVC-U, and PVC-M, has an MRS of 25 MPa at 50 years and 20°C and should therefore be called “PVC-U 250”, to be technically correct.

This point is well illustrated in Graph 1, where the abscissa is the logarithm of Time in Hours and the ordinate the logarithm of Rupture Stress in MPa. From this graph the MRS (Minimum Required Strength) of the polymer at 20° Celsius and 50 years (438 000 hours) is determined – the ISO (International Standards Organisation) design protocol for all thermoplastic pipes.

The product performance characteristics reflected in Graph 1 are for “TOM®500”, which is the brand name of Sizabantu Piping Systems’ technology partner, Molecor, for their Classification 500 PVC-O pipe. The MRS of TOM®500 is 55 MPa at 50 years and 53,8 MPa at 100 years. That proves the service life of their pipe is greater than 100 years because the MRS is greater than 50 MPa at 100 years.

CRRC determination

The CRRC of a polymer is determined by extensive pressure testing and

PVC-M is also a “Classification 250” material because its CRRC is precisely the same as “PVC-U 250”. Its increased Allowable Design Stress (σ) is because its Design Coefficient C has been reduced from 2.0 to 1.4, justified by the improved impact strength engineered by the addition of impact modifiers to the polymer to produce tough and ductile characteristics in the pipe.

Stress-Strain Curves

It is important to note the difference in the Stress-Strain Curves between Sizabantu Molecor’s TOM®500 and other lower PVC-O orientation classifications where the yield point has not been eliminated. The Stress-Strain Curve of PVC-O Classification 500 exhibits a fundamental change, compared to other thermoplastics, that produces: o Modulus of Elasticity increase o Yield-point elimination

Graph 2 shows the yield-point of other lower classification thermoplastic polymers and its elimination with Sizabantu Molecor’s TOM®500 PVC-O

Classification 500. It is critically important to understand that “Classification” refers to the material from which the pipe is manufactured, and “Class” (PN) refers to the pressure class of the pipe.

As the Allowable Design Stress (σ) increases, the resulting minimum wall thickness (e) of the pipe decreases. Based on the Barlow formula: e = P x OD / (2σ + P) where: e = minimum wall thickness – mm

P = pipe pressure class – MPa

OD = pipe OD – mm σ = Allowable Design Stress – MPa

The pipe’s Ring Stiffness (SR) is proportional to the wall thickness cubed:

SR = E x I / (DN – en)³ where: E = E-Modulus

I = second moment of area (B.e³ /12) e n = nominal wall thickness

DN = nominal diameter

Therefore, SR increases as the wall thickness increases. However, the pipe’s

Ring Stiffness is also proportional to E-Modulus, and this increases as the classification increases.

Graph 3 shows the increase in Ring Stiffness for PVC-O PN 16 pipes made with SANS classification materials and made with TOM®500 classification material. Each has an increased MRS and thereby an increased E-Modulus proportional to the increase in material classification, that is itself proportional to the MRS.

For Classification 450 and 500 the Design Coefficient (C) is changed from 1.5 to 1.4. The average Ring Stiffnesses of a PN 16 PVC-O pipe manufactured with various classification materials is shown in Table 1.

Ring Stiffness (SR) values are calculated based on the minimum wall thickness at any point. Because the stiffness is a function of the mean wall thickness, it is statistically impossible for these values to be obtained in practice and the actual stiffness is significantly greater. With a tolerance of 15% (Grade T) on the mean wall thickness, it will statistically be approximately 5% greater than the minimum and the resulting stiffness about 16% higher than the above values.

A similar situation exists with other pressure classes (PN) of PVC-O pipes, where the increase in the material’s E-Modulus, due to the increase in its MRS, due to the increase in the material classification, tends to compensate for the reduced wall thickness and thereby maintains, or may increase the ring stiffness of the pipe.

The South African National Standard SANS 16422 is applicable to all five PVC-O Classifications, and Clause 11.3 specifies that the minimum allowable SR shall not be less than 4 kN/m² to ensure the pipe is sufficiently stiff whilst empty, during constructing or when drained for whatever reason. The PVC-O pipe may be subjected to imposed soil and traffic loads while empty that it must withstand without assistance from internal pressure to resist these loads.

Graph 4 shows the Ring Stiffness (SR) of TOM®500 PVC-O 500 Classification 500 pressure pipes, of various pressure classes (PN), plotted against their respective Ring Stiffnesses (kN/m²). The Ring Stiffness shall not be less than 4 kN/m² for the lowest pressure class of pipe.

The two lines representing SANS 791 S&D PVC-U sewer pipes “Normal Duty 51” and “Heavy Duty 34” are included to provide a benchmark for the pressure pipe stiffnesses. SANS 791 PVC-U S&D pipes are specifically engineered to withstand superimposed loading from trench backfill and traffic while operating in a partially or completely empty condition.

TABLE 1: PN 16 Average Ring Stiffness vs. Material Classification

The TOM®500 PVC-O 500 pressure pipes have a predominantly higher Ring Stiffness than the PVC-U sewer pipes. The SANS 791 S&D pipes have a Pipe Stiffness (SP) of 100 kPa and 300 kPa respectively; equivalent to a Ring Stiffness (SR) of 1,86 kN/m² and 5,59 kN/m² respectively.

The applicable standard for PVC-O, SANS 16422, is a standard that specifies all the critically important attributes of all PVC-O pipe classifications and piping systems. It ensures the products conform strictly to all the requirements, are suitable for their intended purpose and will have a service life of not less than 50 years – TOM®500 is not less than 100 years as shown in Graph 1 in the foregoing.

Conclusions

As the number of different material classifications of thermoplastic piping systems increases, it is essential engineers and clients understand the differences between them, the implications these differences have on the performance of the pipes manufactured from them, and how these differences must be engineered for in their application.

References

SANS 16422 Pipes and joints made of oriented unplasticised poly(vinyl chloride) (PVC-O) for the conveyance of water under pressure – Specifications

A Study Assessing the Performance of PVC-O in Pressure Pipes by Catherine Michel – Shin-Etsu and, Johannes Akkerman – Wavin Plastic Pipes for Water Supply and Sewage Disposal by Prof. Lars-Eric Janson

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