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Improved Material Performance

The alloying of PVC with modifying polymers achieves improvement in resistance to cracking. The result is the minimisation of the effect of stress concentrators such as scratches. With a consequent reduction in the factor of safety, higher wall stresses are allowable which lead to reduced wall thickness.

The increased internal diameter for a given external diameter makes Vinidex Hydro® an efficient conduit. The following graphs and tables illustrate that considerable savings in pumping costs or pipe sizing can be achieved with Vinidex Hydro®.

Hydraulic Efficiency

GRAPH 1 – DN 150 Gravity Flow Application
Pipe flow capacity for a constant hydraulic gradient of 1m/100m.

Light Weight

PVC is already recognised as the lightest and easiest of pipeline materials to handle. Vinidex Hydro® further increases this advantage. Depending on size and class, weight savings in excess of 10% over PVC is available.

System Compatibility

Whether supplied in Series 1 for the irrigation industry or Series 2 for the water industry, Vinidex Hydro® is fully compatible with existing pipeline systems with the full range of valves and fittings available.

Standards and Dimensions

Vinidex Hydro® pipes are manufactured in accordance with AS/NZS 4765:2007 with Standards Mark numbers 2560, 2561, 2562 and 2563. AS/NZS 4765:2007 provides two manufacturing series relating to the outside diameter of the pipe. Series 1 is ISO compatible outside diameters (mostly metric) and Series 2 is ductile iron compatible outside diameters. Refer to Table 2 and 3 for dimensional data. In general, the irrigation industry uses Series 1 and the potable water industry uses Series 2. Vinidex Hydro® is available in both Series 1 and 2 to suit these industries.

System Life

It is a common misconception that plastics pipes have a design life of 50 years, arising from the use of regression curves and adoption of the 50 year point for classification purposes. In relation to hydrostatic stress analysis and pipe life, AS/NZS 4765 states the following:

"The analysis adopts the 50 years extrapolation point on the regression curve as the reference for design purposes. This is consistent with long standing international practice. It should not be taken that either – (a) the pipes weaken with time; or (b) the predicted life is 50 years.
Actual system life is dependent on manufacture, transport, handling, installation, operation, protection from third party damage and other external factors. For water supply applications, the actual life can logically be expected to be well in excess of 100 years before major rehabilitation is required."

Hydraulic

The principles of closed conduit flow and behaviour of fluids is well established. Vinidex recommend the use of the Colebrook-White formula for the analysis of flow parameters for Vinidex Hydro® pipe. A roughness coefficient k = 0.003mm is recommended and all computations are based on water at 20° C. Additional background information on hydraulic design can be found in the Vinidex Water Supply Manual for PVC Pipe Systems. Computational assistance is available in the Vinidex program "Fluff", which can provide an analysis of a wide range of pipeline materials.
Flow Charts for Vinidex Hydro® Series 1 and Series 2 are shown below.

Structural

Under general pressure pipe installation conditions, including under roads, detailed calculations predicting pipe performance are not necessary. Following an extensive study of installed pipe performance, a joint project conducted by the European Plastic Pipe and Fitting Association (TEPPFA) and independent
experts concluded that final deflection of pipes was controlled by the settlement of the soil after installation. Where installation was controlled, or self-compacting granular material was used, pipe deflections were consistently low regardless of installation depth and traffic or other loads. For unusual conditions, or depths greater than 6 metres, design calculations may be performed in accordance with AS/NZS 2566.1. The structural design aspects of buried flexible pipes to be considered are vertical deflection, ring bending strain and buckling. Differential pressure conditions between the inside and outside of a pipe can cause a pipe to buckle inwards leading to collapse. Such conditions can arise from high external loading or negative internal pressure transients as a consequence of pipeline operating conditions. A pipes resistance to buckling is directly proportional to ring bending stiffness. As PVC-M pipes have reduced wall thickness compared to PVC of the same PN rating, the ring bending stiffness is significantly reduced. Consequently the resistance to buckling of PVC-M is also significantly reduced. Designers should be aware of pipeline operating criteria and consult appropriate design material including AS/NZS 2566.1:1998 to ensure the suitability of PVC-M in such applications.

Table 4 of critical collapse pressures are based on short term loading of an unsupported circular pipe and does not include a factor of safety. The choice of factor of safety depends on the certainty of operation parameters and should be nominated by the designer.
For buried pipes the soil surround provides additional support against buckling providing the minimum cover height exceeds 500mm. Such support can only be realised where the embedment is properly placed and compacted with no voids around the pipe and the embedment cannot subsequently be removed or leached away. In general where sustained negative pressures or full vacuum conditions are likely to occur, e.g. suction lines, Vinidex recommends that PN 12 pipe or higher be selected based on an appropriate factor of safety against buckling.

Flowcharts

Fatigue

Materials subject to repetitive or cyclic loads will fail at lower stress levels than materials subject to constant load. This is known as fatigue failure. For thermoplastics pipe materials, fatigue only becomes a design parameter when very high numbers of cycles are applied.
The behaviour of thermoplastic materials in cyclic operation conditions has been extensively studied. Vinidex has developed an appropriate design procedure for such applications.

The two design parameters are the magnitude of the stress range and the frequency of the application, leading to the calculation of the number of cycles applied over the design life of the pipeline. In cases where lifetime cycles exceed 26,000, a higher class of the pipe may be required than indicated by the static or maximum pressure. Extensive studies into the fatigue behaviour of thermoplastics have been used to establish a relationship between stress range, defined as the difference between maximum and minimum stress (see Figure 1) and the number of cycles to failure. This relationship yields a load factor which is applied to the operating pressure to enable the selection of the appropriate class of pipe. The approach adopted by Vinidex is conservative recognising that the experimental data demonstrates a degree of scatter. This ensures an appropriate factor of safety given potential changes in the pipeline operating conditions over the life of the pipe and other operational conditions such as installation and maintenance standards. For simplicity, the pressure range is defined as the maximum pressure minus the minimum pressure including all transients experienced by the system during normal operations as per Figure 1. The effect of accidental conditions such as power failure may be excluded. Figure 1 illustrates the definition of a cycle as a repetitive event.

In some cases the cycle pattern may be complex and it may be necessary to consider the contribution of secondary cycles.
For more detailed background information on the fatigue design of thermoplastic pipe including examples and a full list of references, consult Vinidex technical notes VX-TNTN-4J and VX-TN -4H. Note that in fatigue loading applications, the maximum pressure in a cycle should not exceed the static pressure rating of the pipe. Figure 2 and Table 5 provide graphical and numerical values of the recommended fatigue load factors.

Fatigue Design Procedure

To select the appropriate pipe class for fatigue loading, the following procedure should be adopted:

1. Estimate the likely pressure range, ΔP i.e. the maximum pressure minus the minimum pressure.
2. Estimate the frequency or the number of cycles per day, which are expected to occur.
3. Determine the required service life and calculate the total number of cycles which will occur in the pipe lifetime.
4. Using Table 5 or Figure 2, look up the fatigue load factor for the appropriate number of cycles.
5. Divide the pressure range by the fatigue load factor to obtain an equivalent operating pressure.
6. Use the equivalent operating pressure to determine the class of pipe required.

Temperature

The nominal working pressure rating of Vinidex Hydro® pipes is determined at 20°C. However, Vinidex Hydro® pipes are suitable for use at temperatures up to 50°C. Where the service temperature exceeds 20°C, use Table 6 to determine appropriate pipe class.

Vinidex recommend that Vinidex Hydro® PVC-M pipes are constructed in accordance with AS 2032 – Installation of PVC pipe systems and the Vinidex Water Supply Manual. Installation techniques for Vinidex Hydro® pipes are similar to those used for standard PVC pipes and the same degree of care and caution must be exercised. Note that the thinner wall of Vinidex Hydro® pipes means that they will experience higher lateral loads and care should be taken to ensure that the pipe is fully supported. Quality non-cohesive material should be used for pipe bedding, side support and overlay. In general 1° deflection for rubber ring joints, is available at each socket-spigot joint. The pipe side support material should be placed evenly on both sides of the pipeline to two thirds the height of the pipe diameter and compacted by hand tamping.

Side fill material should be worked under the sides of the pipe to eliminate all voids and provide maximum pipe haunching. The pipe overlay material should be levelled and compacted in layers to a minimum height of 150mm above the crown of the pipe or as specified. For above ground installations the support spacings recommended in AS 2032 can also be used for Vinidex Hydro® pipes. These support spacing result in negligible deflection in PVC pipes full of water. For PVC-M pipes supported at these intervals, the stress levels in the pipe wall are still within acceptable limits. However, for the same class of pipe, deflection between the supports will be increased by around 50%. Since deflections are very small, this will not usually be of functional significance. The field testing procedures specified in AS2032 and the Vinidex Water Supply Manual should also be followed for Vinidex Hydro® pipes.