PIPE SYSTEMS
THERMOPLASTIC PIPE MATERIAL CLASSIFICATION AND PIPE STIFFNESS Ever y thermoplastic polymer has a unique CRRC (Creep Rupture Regression Cur ve) which is plotted on a log-log scale. It is the “fingerprint” of the polymer. By Mike Smart, Pr Eng*
CRRC determination
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. 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.
The CRRC of a polymer is determined by extensive pressure testing and
Stress-Strain Curves
PVC-O pipes are well suited for bulk water supply applications
T
his 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.
Rupture Stress - MPa
CREEP RUPTURE REGRESSION CURVES - 20°C 100,0 y = -1,777ln(x) + 78,083 y = -1,038ln(x) + 38,485 10,0
1,0
y = -0,381ln(x) + 14,929
1
10
100
1000
10000
100000
1000000 10000000
Time - hours
GRAPH 1: CRRC TOM®500, PVC-U and PVC-M and PE 100
PVC-O TOM®500 PVC-U & M PE 100
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
IMIESA October 2023
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