Civil Engineering


Sep 12, 2016

Civil Engineering

Large Diameter, High Pressure, Trunk Water PVC-O Pipeline Systems now made in South Africa

Large diameter high pressure trunk water, potable and raw, pipelines have historically been steel and ductile iron pipes. Although thermoplastic pipes solved many of the problematic characteristics of these materials they were unable to achieve the large diameter and pressure capabilities required for these applications.

These limitations have been solved with the latest generation of oriented unplasticised poly (vinyl chloride) (PVC-O) Classification 500 pipes up to 630 mm OD (Outside Diameter) PN 25 bar, up to 800 mm OD PN 20 bar and soon to be produced 1000 mm OD PN 16 bar. PVC-O thermoplastic pipes are technically adequate and commercially competitive for large diameter high pressure trunk water pipeline infrastructure applications.

Historically large diameter high pressure PVC-O pipes were imported, with the associated exchange rate and logistics issues. These issues have been solved with the opening of a new “state-of-the-art” factory by Sizabantu Piping Systems (Pty) Ltd, in conjunction with their Spanish technology partner Molecor Canalisations, in the Richards Bay Industrial Development Zone to manufacture “TOM 500” PVC-O pipes.

Unplasticised poly (vinyl chloride) (PVC-U) pressure pipes have undergone significant improvements over the last sixty five years from PVC-U, to PVC-M and PVC-O, with larger diameters and higher pressures made possible with a substantially increased Allowable Design Stress (σ) as can be seen in Table 1: PVC Improvements – PVC-U, PVC-M and PVC-O.

Table 1: PVC Improvements – PVC-U, PVC-M and PVC-O 
Cast iron and ductile iron are both iron. However, ductile iron has been chemically modified to improve its mechanical strength. Likewise, PVC-M has been chemically modified with impact modifiers and exhibits tough characteristics although its strength is exactly the same as PVC-U, they have a common Creep Rupture Regression Curve seen in Graphs 1 and 2.
Graph 1: Creep Rupture Regression Curves
Graph 2: Creep Rupture Regression Curves
Note the improvement in the MRS (Minimum Required Strength) at 1 year (438 000 hours) in the PVC-O curve from Graph 1 for Classification 450 material, to the PVC-O curve from Graph 2 for Classification 500 material. Consider coal and diamond, they are both carbon, but the diamond’s structure has been modified by mechanical action of heat and pressure. Likewise, PVC-O’s structure is formed by mechanical action of heat and pressure to increase its mechanical strength characteristics – it remains PVC chemically.

In the manufacture of PVC-O, the initially extruded PVC-U “pro-forma” pipe is approximately half the diameter and twice the wall thickness of the required PVC-O pipe. The pro-forma pipe moves in-line into the molecular orientation chamber where the bi-axial orientation process is performed  in-line with temperature and pressure, using air not water, making the process environmentally friendly and causing less pollution.  Molecular orientation of PVC thermoplastic is carried out at temperatures well above the glass transition temperature (+ 75° C) and results in improvements of the physical and mechanical properties. With the in-line process, the thick-walled tube, directly after the extrusion process, is conditioned in-line at the orientation temperature, and by means incorporated to activate the orientation process in the circumferential and axial directions. After the orientation process, the pipe is cooled quickly to ambient temperature.

The orientation of the molecules creates a laminar structure with the ability to withstand brittle failure emanating from minor flaws in the material matrix or from scratches on the surface of the pipe. Therefore, PVC-O may be considered to be highly resistant to notches and there is no risk of long-line RCP (Rapid Crack Propagation). Furthermore, PVC-O has improved hoop strength and excellent impact resistance. PVC-O does not have a universal MRS (Minimum Required Strength) it has one of five possible values, 31.5, 35.5, 40.0, 45.0 and 50.0 MPa which is dependent upon the degree of orientation. This value determines which of the five classifications, 315, 355, 400, 450 and 500 respectively, as in Table 2: PVC-O Classification and Allowable Design Stress.

Table 2: PVC-O Classification and Allowable Design Stress
The MRS is dependent upon the degree of bi-axial orientation of the pipe and its magnitude is determined in accordance with ISO 9080 and ISO 12162 as stated in the applicable standard SANS 16422: “Pipes and joints made of orientated unplasticised poly (vinyl chloride) (PVC-O) for conveyance of water under pressure – Specifications”. The Design Coefficient, or Safety Factor, (C) is also determined in accordance with SANS 16422 Annex B and may be either 1.4, 1.6 or 2.0 according to the criteria set out therein. It must be remembered ISO 2045, sited therein, is applicable to PVC-U and higher strain levels are developed by the higher operating stresses in PVC-O. The higher socket strength, produced by the in-line process socket formation, and extra depth of engagement to prevent pull-out of the joint determines the Design Coefficient (C). The socket geometry, formation and resulting Design Coefficient can be seen in Diagram 1: Socket Details.
Diagram 1: Socket Details
The Forsheda 576 Anger-Lock locked-in double ring seal from Telleborg Sweden, comprising a stiff polypropylene retaining ring and a soft lipped EPDM seal, is the industry’s most reputable. All seals are rated 25 bar, irrespective of the pipe pressure rating they are inserted into, and fill the extra housing length in the socket. The seals have a negative pressure capacity greater than the ISO 13844 requirement in SANS 16422, where negative pressures up to -0.8 bar must be sustained for over 30 minutes. The seal configurations uncompressed and compressed with the associated stress distribution are shown in Diagram 2: Forsheda 576 Anger-Lock Seal Details.
Diagram 2: Forsheda 576 Anger-Lock Seal Details 
The engineer has a number of advantages with PVC-O during the design process and the resultant project feasibility. The ID (Internal Diameter) of a PVC-O pipe compared with an equivalent PVC-M pipe is about 5% larger and with a PVC-U pipe is about 10% larger giving capacities of about 12% and 23% more respectively. When engineering surge conditions, the design velocity can be higher with PVC-O due to the lower celerity value, about one third that of steel and ductile iron, that produces a lower surge pressure. The combination of these two attributes, ID and celerity, results in a greater pipeline capacity. Furthermore, according to CEN 15223 the allowable combined operating and surge pressure is twice the rated pressure of the pipe that may negate a pressure class increase. The engineer’s energy concerns when considering a higher velocity are mitigated by the superior hydraulics, lower (15%) friction factor, of PVC-O and its negligible deterioration over time, less than one third that of other materials. PVC-O pipes have a lower rugosity due to the manufacturing process. The embedded energy, defined as all the energy to manufacture the product including that of the raw material, of PVC-O is between 15% and 25% less than that of the alternative materials. And, to further reduce PVC-O’s environmental impact it may be recycled as specified by Clause 5.2 of SANS 16422, the organisation’s ISO 9001 Quality Management System, SABS Audits and SAPPMA Audits.

PVC-O does not require cathodic protection, construction in a railway or power line servitude notwithstanding, eliminating both the capital and maintenance costs. This item, where necessary, varies from about 5% to over 40% of the pipeline cost depending on the magnitude of the stray currents and the soil resistivity. PVC-O pipes are lightweight and do not require large and expensive plant to handle and lay them, even with larger diameters, making them appropriate for labour intensive construction methods. PVC-O has been in use for over 40 years and there has been significant improvement in the product since its introduction. Currently about 100,000 tonnes is produced globally in over 23 countries – this is a proven technology. Engineers have specified and installed kilometres of PVC-O pipes throughout South Africa of various diameters and pressure ratings including, inter alia, the following:

The new Sizabantu/Molecor factory will create jobs, transfer skills and import the latest technology to South Africa. To be allowed to construct the factory in the Richards Bay Industrial Development Zone the building had to be designated a “Green Building”, the production designated “Green Technology” and the company comply with the Government Target Procurement Policy.

The fittings, made of 300 μm sintered epoxy coated fabricated steel, are also manufactured local to the factory to ensure control of the process and enable a complete piping system to be offered to the market. For further information and contact details, please visit the Sizabantu Piping Systems website. www.sizabantupipingsystems.com

 

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