Why PVC pipe is the most cost effective, long term solution for water reticulation and bulk water pipelines
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PVC pressure piping commenced in about 1935 and in 85 years PVC has undergone significant technical development culminating in PVC-O (Oriented Polyvinyl Chloride). PVC-O has also undergone five developmental improvements (Classifications) in its 40 years existence.
Blue PVC-U (Unplasticised Polyvinyl Chloride), PVC-M (Modified Polyvinyl Chloride) and PVC-O (Oriented Polyvinyl Chloride) pressure pipes dominate the potable water supply and reticulation market, sand coloured PVC-U pipes dominate sewerage systems and white PVC-U pipes soil, waste and vent.
Thermoplastic pipes are manufactured from polymer raw material. Every polymer has a unique characteristic, its “fingerprint”, determined by conducting thousands of tests over thousands of hours and plotting the results on a log/log graph – to produce the polymer’s CRRCs (Creep Rupture Regression Curves). The plot is configured with the time to rupture in hours the Abscissa (X-coordinate or X-axis) and the stress at rupture the Ordinate (Y-coordinate or Y-axis) – the polymer’s “family” of curves for various temperatures is plotted.
Graph 1: Creep Rupture Regression Curves – 20°C
Graph 1 shows the 20°C CRRC line, blue for PVC-O TOM 500® (Classification 500), orange for PVC-U and PVC-M and the grey for PE100 – note the CRRC line for PVC-U and PVC-M is precisely the same, they have the same MRS (Minimum Required Strength). ISO (International Standards Organisation) specifies the MRS of the 20°C CRRC at 50 years (438 000 hours) is used to design pipes. The MRS is divided by the applicable C (Design Coefficient) to obtain the Allowable Design Stress (σ). However, for PVC-U σ = 12.5 MPa (25/2) and for PVC-M σ = 18 MPa (25/1.4). The reduced C value of PVC-M is based on tough characteristics engineered by the addition of impact modifiers that enhance its impact strength – it “behaves tough”.
The 20°C CRRC for PVC-O TOM 500® gives an MRS of 55 MPa at 50 years – it must not be less than 50 MPa for Classification 500 PVC-O. Note the regression curve for TOM 500® is still above 50 MPa at 100 years (876 000 hours) – the service life now demanded by end users.
Be careful not to confuse “Class”, the pipe’s pressure class (bar), and “Classification” the polymer’s MRS. There are five PVC-O Classifications, viz., 315, 355, 400, 450 and 500, specified in the applicable South African National Standard, SANS 16422: “Pipes and joints made of oriented unplasticised poly (vinyl chloride) (PVC-O) for the conveyance of water under pressure – Specifications” and the Classification number is simply 10 x MRS of the polymer – a common nomenclature system for all polymers. Note, PVC-U is actually PVC-U 250. The strength of PVC-O has increased by nearly 75% during its existence. Other attributes have also improved as shown in Diagram 1, that compares the blue regular hexagon of PVC-U 250 with the grey irregular hexagon of PVC-O 500.
Diagram 1: Matrix of Mechanical Properties
SANS 16422 PVC-O specifies the required attributes of PVC-O pipes including inter alia the following:
Annex A “Establishment of the minimum required strength (MRS)” specifies it must be proven in accordance with ISO 9080 “Determination of the long-term hydrostatic strength of thermoplastics materials in pipe form by extrapolation” and ISO 12162 “Classification and designation – Overall service (design) coefficient”.
Annex B “Minimum depth of engagement of sockets” also ensures the adequacy of the joint considering Poisson’s contraction, temperature contraction, angular deflection, chamfer length and safety allowance. Furthermore, the standard ensures, inter alia, the pipe’s impact strength, ring stiffness and tensile strength, and the joint’s negative and positive pressure strength and end-load-bearing leak tightness.
SANS 791: Unplasticised poly (vinyl chloride) (PVC-U) sewer and drain pipes and pipe fittings, commonly known as “solid wall S&D pipes” are engineered for maximum stiffness, to withstand the external impressed backfill and superimposed loads, whilst flowing partially full in a buried condition.
Graph 2: Ring Stiffness 791 cf. TOM 500®
The two Pipe Stiffnesses available, commonly known as “Normal Duty” and “Heavy Duty” are shown in Table 1.
Table 1: Pipe Stiffness
SANS 1601: Structured wall pipes and fittings of unplasticised poly (vinyl chloride) (PVC-U) for buried drainage and sewerage systems, commonly known as “structured wall sewer pipes” contains Clause 5.2.8 as follows:
- Type 1 pipe is stiffness class 100 (Pipe stiffness 100 kPa), or stiffness class 200 (Pipe stiffness 200 kPa), or stiffness class 400 (Pipe stiffness 400 kPa).
- Type 2 pipe is stiffness class 400 (Pipe stiffness 400 kPa).
There are three (3) types of stiffness as follows:
- Pipe Stiffness (PS) = F / ΔL
- Pipe Stiffness Factor (PSF) = E I / r³
- Pipe Ring Stiffness (PRS) = E I /D³
The relationship between the three types of stiffness is as follows:
0,149 PS = PSF = 8 PRS
PS = 6,71 PSF = 53,69 PRS
Table 2: PS, PSF and PRS Relationship