THE ENERGY CRISIS – DESIGNING WITH PVC AND HDPE PIPES: ENERGY SAVINGS AND CONSERVATION

M.A. Osry

DPI Plastics (Pty) Ltd., P.O. Box 389, Germiston. 1400.

ABSTRACT The South African plastics pipe industry assists with economic and social development through the supply of pipe systems for potable water and the provision of sanitation. At the same time, the worlds, and indeed South Africa’s energy crisis, has brought into focus the increasing need for energy and material efficient pipeline materials. The pipe industry faces a number of important issues, including environmental concerns, energy consumption and the depletion of natural resources. Locally manufactured plastics pipes must remain competitive against more energy intensive, traditional materials, (including those which are imported) and water authorities should take into account the energy content of pipeline materials as well as energy requirements associated with their use. The energy crisis is only too apparent in South Africa given increasing power costs, failure of supplies to commerce and industry and the need to provide electricity to new industrial developments and urban and rural communities. This paper discusses actions the plastics pipe industry has taken to deal with energy concerns, including the manufacture of material efficient pipes such as modified PVC (PVC-M), oriented PVC (PVC-O) and HDPE PE 100 pipes. It is necessary to examine the essential properties of these materials which have lead to their energy efficient manufacture, installation and operation. The evaluation of the properties of PVC-U pipes after 60 years in service gives a renewed appreciation of a design life well in excess of the commonly accepted 50 years. The subjects covered in the paper are intended to provide a holistic picture to assist engineers and the water industry in deciding on the most cost effective and energy efficient pipeline materials. INTRODUCTION Two infrastructural elements essential to economic growth and social development are water and energy, both of which are in short supply in South Africa and threatening to negatively affect growth and social development. The major sources of energy and electrical power in most countries are oil, natural gas and coal, the latter accounting for more than 90% of the energy in South Africa (1). Plastics pipe materials are made from hydrocarbons produced from these fossil fuels – coal in the case of South Africa.

The cost of fuels has risen dramatically in recent times, mainly as a result of the rapid growth in the economies of China and India and other developing countries, but the fact remains that the world has been finding less oil than it has been using for the past 20 years. Energy will be one of the defining issues of this century making the conservation of energy and non-renewable fossil fuels of critical importance to our future (2). For both

conservation and environmental reasons, most countries are evaluating means of reducing energy consumption and moving away from high-cost engineered materials (3,4). It is necessary to find alternative ways of doing things and, in the case of the water industry for example, the use of more energy efficient pipeline materials. A paper written in 1974 by Imperial Chemical Industries (UK) following the first oil crisis had this to say: “There is a growing awareness of the need to conserve materials and energy and to adopt thinner films and lighter weight mouldings, where suitable technically, to achieve optimum use of resources. The effect (is likely to be) the wider adoption of plastics on their technical merits” (5). In this paper we evaluate energy and material efficient plastics pipes, in particular, modified PVC (PVC-M) biaxially oriented PVC (PVC-O) and high density polyethylene, HDPE PE 100. The important properties of these materials are examined, including long-term strength and toughness and pipe design criteria.

The energy content or primary energy as consumed in the manufacture of plastics, steel and ductile iron pipes is compared, as well as energy savings during service of the pipes, an important element of the life cycle costs of pipelines. It is sufficient at this stage to note that energy required to manufacture steel and ductile iron pipes is up to 500% greater than for some plastics pipes. Environmental concerns involving the manufacture of modern plastics pipes and traditional materials are discussed. MATERIAL AND ENERGY EFFICIENT PLASTIC PIPES – DESIGN PRINCIPLES A better knowledge and understanding of PVC and HDPE pipe design principles has resulted from their superior long-term performance and the ongoing study of the science of these materials. It is incumbent on plastic pipe manufacturers to develop a clear understanding of the properties of the ‘new’ generation products among engineers and water authorities, so that advantage can be taken of the improved performance of these materials, including energy efficiency and other cost savings. The purpose of this section of the paper is to examine the properties of these materials and the design criteria which ensure long-term pipeline performance. PVC-U PIPES Unplasticised polyvinyl chloride (PVC-U) pipes have proven themselves in service for over 60 years and pipe design, especially with respect to strength properties, has been well documented. In order to evaluate the changes that have taken place with this material and the development of tougher and stronger, more material efficient PVC pipes, it is necessary first to examine in some detail the design principles appertaining to these pipes. LONG-TERM STRENGTH PROPERTIES The strength of plastics pipes can be defined as the maximum stress to cause failure in a given time, the burst strength being stress and time dependent. The strength at 50 years is determined by carrying out pressure tests to failure at various extended times according to the procedure given in ISO 9080 (6). The results are graphically illustrated by plotting the circumferential stress against time to rupture using a log scale, i.e. the creep rupture regression line, as shown in Figure 1 for PVC-U, PVC-M and PVC-O.

The ISO procedure gives the 50 year failure stress and from this the minimum required strength or MRS, at 50 years, is obtained. The MRS for PVC-U pipes is 25 MPa as per SANS 966 Part 1 (7). An overall service design coefficient (or safety factor), C, which is based on material properties, is applied and the design stress, σs, for the pipe is determined:

σs = MRS [1] C

Since the design stress is the constant stress that the pipe wall can resist for 50 years, the safety factor is also applicable at 50 years, i.e. a ’50 year safety factor’.

The safety factor for PVC-U pressure pipes is 2.0 (for diameters 110mm and above) giving a design stress of 12.5 MPa (7). The pipe wall thickness is then calculated using the Barlow’s formula:

p2σ

d x pes

e

+= [2]

Where: e = minimum wall thickness (mm) p = maximum operating pressure (kPa) de = mean external diameter (mm) σs = design stress (kPa)

OVERALL SERVICE DESIGN COEFFICIENT (50 YEAR SAFETY FACTOR)

At this stage it is necessary to examine the concept of the safety factor in more detail since it is apparent that its derivation is perhaps not always understood in the water industry as it is, for example, with traditional materials. This leads to problems when comparing the safety factors of plastics pipes with those given for materials such as steel and ductile iron pipes. In the case of plastic pipes the safety factor takes into consideration both the properties of the material and the service conditions, such as minor surface damage occurring during handling and installation, small surges or fluctuations in pressure or superimposed bending stresses and point loads on the pipe in service (7,8,9,10).

DURABILITY AND THE SAFETY FACTOR The two times safety factor as applied in the design of PVC-U pipes is accepted by the water industry after many years of excellent performance of this material in service. Numerous studies conducted on PVC-U pipes excavated at various times up to 60 years in service, have shown the exceptional durability of these ‘old’ pipes, with little or no difference in mechanical properties to recently manufactured pipes. Properties such as tensile strength, impact strength, burst pressure and elastic modulus (stiffness) show virtually no change with time in service (11,12). In a study conducted on 60 year old PVC-U pipes it is stated that: “although the plastics industry is a relatively young materials segment, the production of industrial volumes of PVC polymer and PVC-U pipes is now about 70 years old, which is close to the predicted service lifetime of 100 years for PVC pipe applications” (12). And work carried out by Schwencke (13) and Hucks (14) showed that PVC pipes get stronger with time under pressure.

These studies show that the life of PVC-U pipes is well in excess of 50 years and a minimum 100 year life is increasingly accepted for this material; the test results demonstrate very clearly that PVC-U and PVC-O pipes have a minimum lifetime of 100 years (15). Given the rapid progress in pipe extrusion technology it is now considered that the safety factor of 2.0 used for PVC-U pipes is (in general) too high. For new pipelines the safety factor could be as low as 1.3 (15). This implies, “that the safety factor of 1.6 as laid down in ISO 12162 (16) for PVC-U pipes should be adopted in the near future, taking into account realistic engineering design factors” (15). The point made here is that the safety factor of 2 times for PVC-U pipes is seen as conservative given the durability of the product and modern extrusion technology used for manufacturing these pipes.

Tests conducted on PVC-M pressure pipes by Stephenson and Höllwarth concluded that, “the fact that pipes are not loaded continuously means that the factor of safety in practice will be at least 2 at 50 years” (17). A report by Uponor Limited (UK) on PVC-O pipe exhumed after 25 years concluded that the pipe showed no substantial loss in strength or quality after 25 years of service, and that the pipe could be left in the ground for another 75 years to complete 100 years (18). TOUGHNESS PROPERTIES AND THE SAFETY FACTOR In addition to strength, toughness is an essential property of plastic pipe materials. Strong materials can be quite brittle, which may be a problem with these materials having small defects or notches and then subjected to impact or external stresses; high stresses developed at the tip of the notch can lead to unpredictable crack growth and eventual failure. Therefore, higher safety factors are accepted as a requirement for materials having greater strength but subject to brittle failure in certain circumstances. Such failures are due to the inability of the pipe wall to absorb and distribute the stresses through the matrix of the material and under these circumstances premature failure can occur at stresses well below the ductile or failure yield line and the mode of failure will inevitably be brittle (19,20). Toughness can be defined as resistance to impact and resistance to cracks, i.e. toughness prevents cracks from starting (initiation) and also prevents the transfer (propagation) of cracks through the pipe wall. Cracks or notches may be initiated during handling or installation or during service due to bending stresses and point loads on the pipe. Therefore, toughness must be considered equivalent in importance to strength as it is this property that increases the resistance of the material to the propagation of cracks. Brittle failure will not occur with tough materials having predictable failure properties; therefore the material’s toughness bears a direct relationship with the long-term safety factor. It is now accepted in pipe standards and by water authorities around the world that the 50 year safety factor depends as much on strength as it does on toughness. Tough materials fail by predictable ductile yielding and hence allow the use of lower safety factors while more brittle materials may suffer from ‘unstable’ (unpredictable) crack growth failure caused by stress concentration effects. Thus the 50 year safety factor relates to the type of material and its properties. HDPE has much lower strength than PVC-U but has higher toughness, hence a safety factor of 1.25. PVC-M has the same 50 year strength as PVC-U but is much tougher, safety factor; 1.4.

PVC-O is a rather unique material having exceptional long-term strength and high toughness; safety factor 1.6. It follows that a high safety factor does not mean that the product is safer to use or more reliable. In the case of HDPE PE 100 the long-term strength has been considerably increased (at least 25%) - the safety factor remains at 1.25 since long-term toughness has been improved by the use of better anti-oxidants as well as an enhanced molecular structure. SHORT-TERM SAFETY FACTOR The above discussion applies to the long-term, 50 or 100 year, safety factor but it is important to note that the short-term safety factor is much higher, 3-4 times (as with traditional metal pipes) and depends on the rate of loading, i.e. the rate of pressure increase. The greater the rate the greater the strength because the plastic material’s molecular structure reacts to resist the stress, for example, a pressure surge (21). This can be seen from the creep rupture regression lines for PVC-U, PVC-M and PVC-O in Figure 1. PVC-U has proven itself as probably the most successful and cost effective pipe material over the past 60 years. As with many products (e.g. lighter mass, more fuel efficient motor vehicles) pipe technology and materials science has moved on to the new generation PVC’s: PVC-M and PVC-O and the same can be said of HDPE developments, starting with PE 63, PE 80 and then PE 100.

This discussion regarding strength and toughness properties, regression data, safety factors and design stress has been necessary to provide a sound understanding of the properties and design of material and energy efficient pipes, PVC-M, PVC-O and HDPE PE 100. PVC-M PRESSURE PIPES Work on PVC-M pipes commenced following 15 years of very successful performance of impact modified PVC pipes in the extremely harsh coal and gold mining environments. PVC-M pressure pipes were developed in South Africa during the early 1990’s, taking several years to establish optimum formulations and pipe manufacturing technologies and especially the long-term strength combined with exceptional toughness properties (22). As mentioned, the excellent long-term strength properties of PVC-U have been retained while the toughness of the material has been enhanced to the extent that ductile failure modes are achieved according to the rigorous test regime detailed in SANS 966 Part 2, BSI (UK) and Australian Standards (23,24,25). The excellent strength properties are illustrated by the long-term creep rupture regression data shown in Figure 1 and the mechanism for increased toughness shown in Figure 2. The regression lines for PVC-U and PVC-O are also given in Figure 1 as are the design stresses for these materials.

Figure 1. Creep Rupture Regression Lines and Design Stresses for PVC-O, PVC-M and PVC-U

High stress concentration at tip of crack

Figure 2. Toughness Mechanism – ‘Type 1’. PVC-M pipes.

For PVC-M, a design stress of 18 MPa is used for the calculation of wall thicknesderived from the MRS of 25 MPa (as for PVC-U) and the application of a 50 yearfactor of 1.4 (23). Similar pipe design criteria are used for PVC-M pressure pipesmanufactured in the UK (24) and Australia and New Zealand (25). PVC-M pressure pipes have been successfully used in Southern Africa for 12 yover 40 000 km from just one pipe manufacturer having been installed. Howevacceptance of this excellent material and energy efficient product has been South Africa when compared to the UK, Australia and New Zealand where PVC

Area of local yielding of material, reducingstress

s; this is safety

ears, with er, overall limited in -M pipes

have now virtually replaced PVC-U pipes. There is increasing interest in the manufacture and use of PVC-M pipes in Europe, South East Asia and South America.

SOME NOTES ON PIPE DESIGN PROPERTIES AND THE STRESS REGRESSION LINE FOR PVC-U, PVC-M AND PVC-O PRESSURE PIPES There has long been a misunderstanding regarding the stress rupture regression

lines shown in Figure 1; the down-sloping line is sometimes interpreted as a loss of strength over time. As previously mentioned the line is drawn through a series of points each representing an individual pipe test stress and time to rupture. This shows that the higher the stress the shorter the time to failure, or, conversely, the lower the stress the longer the time to rupture. The line therefore does not relate to a loss of strength with age or time.

This is confirmed in practice on the many studies carried out on old pipelines (11,12,13,14,15).

After several years at the design working pressure, creep is no longer detectable

and the 2-3 year modulus (under constant stress) is approximately equal to the 50 year value.

For each new loading, e.g. water hammer or pressure surges, the pipes act

according to the short-term strength properties. The short-term strength is therefore independent of how much time has elapsed since the first loading; the pipe acts as a new pipe.

For short-term pressure surges, PVC pipes can resist at least twice the rated working pressure, i.e. stresses greater than the 50 year strength.

This also applies to live soil loads – use short-term modulus for design purposes.

PVC-O PRESSURE PIPES The molecular orientation process is used in the manufacture of plastic products where increased strength is required. A good example is polypropylene (PP) strapping tape where the PP molecules are oriented only in the length direction, giving exceptional strength to the tape and making it well suited for the strapping of heavy crates and bundles. The orientation process used for producing PP strapping tape is known as mono-axial orientation. In the case of pipes, it is necessary to provide strength properties in both the circumferential (hoop) direction and the axial (length) direction of the pipe. This is done by a special extrusion process known as biaxial orientation, where the molecules are stretched (oriented) in both circumferential and axial directions leading to substantial improvements in the physical properties, both strength and toughness (26,27). The expansion process during which the molecules are oriented in the hoop direction is shown in Figure 3.

Figure 3. PVC-O: the molecular orientation process.

Mono-axial and biaxial orientation of the long chain PVC molecules is illustrated in Figure 4 (a) and (b).

Figure 4(a) Figure 4(b).

The substantial increase in strength properties which results from molecular orientation is shown by the stress rupture regression line for PVC-O pipes in Figure 1.

PVC-O is a unique plastics pipe in that it has both exceptional strength and high toughness. The superior toughness properties arise from the biaxial orientation of the molecules which results in a layered or laminar structure as shown in Figure 5.

Figure 5. PVC-O pipes showing the laminar structure.

The importance of toughness in the design of pipes has been discussed above. In the case of PVC-O, if cracks do form during service, high stresses arise at the crack tip because of stress concentration effects, however, with the layered structure the stress is reduced at the interface of each discrete layer and the crack ceases to propagate (27). This is shown in Figure 6 where toughness mechanism Type 2 is illustrated for PVC-O pipes.

Fi

The strength and Figure 7.

Laminar or layered structure

Notch with highstress concentration

gure 6. Toughness Mechanism ‘Type 2’. P

toughness properties are further illustrated

Interface opens up to reduce stress concentration at tip of crack, preventing failure

VC-O Pipes.

in the following photographs,

Figure 7. A construction site illustration of the toughness properties of PVC-O pipes.

The minimum required strength at 50 years (MRS) for PVC-O pipes is 45 MPa as specified in SABS (28), ISO (29) and Australian (30) standards. Thus the 50 year strength is 80% greater than that of PVC-U and PVC-M (MRS 25 MPa) and over four times that of HDPE PE 100 pipes (MRS 10 MPa). Given the outstanding strength and toughness properties, a 50 year safety factor of 1.6 is applied, giving a design stress of 28 MPa. PVC-O is thus the most material efficient plastics pipe for potable water conveyance, with material savings of 50, 30 and 50%, respectively, against the equivalent PVC-U, PVC-M and HDPE PE 100 pipes. PVC-O pressure pipes have given excellent performance in several European countries and Australia for over 25 years. The pipes have many advantages including high impact strength for better handling and installation, enhanced resistance to surge and fatigue and greater hydraulic capacity and life cycle energy savings.

Applications include pumped water reticulation systems and rising sewer mains and installations where greater safety is required with respect to pressure surges, water hammer or pressure cycling. HDPE PE 100 PRESSURE PIPES Similar advances in material savings have been made over the years with high density polyethylene pipe materials, PE 100 being the newest generation. The PE ‘100’ figure relates to the 50 year MRS, i.e. 10 MPa. A safety factor of 1.25 applies, thus for PE 100 material a design stress (σs) of 10/1.25 = 8 MPa, is used for calculation of the wall thickness (31). In terms of material efficiency, PE 100 pipes have approximately 20% saving over PE 80 pipes and the latter pipes are about 20% lighter than PE 63 pipes. The improvements in the mechanical properties, i.e. toughness and long-term strength of modern PE 80 and PE 100 materials have resulted from advances in the polymerisation process (converting ethylene gas to high density polyethylene) as well as changes to the molecular structure, density and processing characteristics. Enhancements have also been made to the long-term toughness properties, especially resistance to slow crack growth, through the use of improved anti-oxidants and long-term material stability.

SOME INTERIM COMMENTS The title of this paper commences with the words, ‘The Energy Crisis’, but little has yet been said about the use of energy in the manufacture of pipes, energy conservation or the environmental issues involved! We have dealt with the properties of material efficient pipes which have resulted from many years of intensive scientific work and evaluation and significant investment by both raw material and pipe manufacturers. Unfortunately, as previously mentioned, these advances have not been fully utilized to the benefit of the water industry and it is hoped that the foregoing discussions on new generation plastics pipe technology and design will lead to a wider acceptance by the water industry of these products. ENERGY AND CONSERVATION

At the present time one thing seems certain: the times of easy oil are over with a new price level having been set for the commodity and, while new energy sources are being found, oil, coal and gas will remain the principle sources for some time to come. In South Africa the energy crisis is evidenced by daily domestic power cuts, failure of supplies to commerce and industry and the need to provide power for economic development.

In addition, South Africa has annual electricity cost increases around double that of the inflation rate, perhaps understandable, given the capital expenditure, R84bn, planned over the next five years, to 2011. Demand for electricity will grow by more than 4% driven by government’s plan to boost growth to 6% p.a. (1). In the present world crisis, the following three actions are necessary: Diversify sources of supply; Production of fuel from sources other than oil; Take precautionary measures.

Precautionary actions include, “the possibilities which flow from advances in technology” (32). The use of alternative, material and energy efficient pipelines is one such action we can take.

In the report on the effects of the 1973 oil crisis (5) two of the major conclusions were that: The making of plastic products requires, in general, less energy than traditional

materials, even allowing for the oil used for feed-stock. “There is a growing awareness of the need to conserve materials and energy and to

adopt thinner films and lighter weight mouldings, where suitable technically, to achieve optimum use of resources. The effect of this is likely to be offset by the wider adoption of plastics on their technical merits”. These issues are as applicable today as they were during the first oil crisis!

DEFINITIONS AND CONVERSION FACTORS RELATING TO ENERGY Energy can be defined as the capacity for doing work, as measured by the capability of doing work (potential energy) or the conversion of this capability to motion (kinetic energy). Most of the world’s convertible energy comes from fossil fuels that are burned to produce heat that is then converted to mechanical or other means in order to accomplish tasks. For example, coal when burnt in air can do work in driving a steam engine or turbine, thus coal possesses energy. Electrical energy is measured in watt hours (Wh) while heat energy is measured in joules or calories. Figures for electricity production, trade and consumption can also be calculated using the energy content of the electricity at a rate of 1 TWh = 0.086 Mtoe, where ‘toe’ is ‘tons of oil equivalent’. Thus energy requirements can be measured in tons of oil equivalent and 1Mtoe = 1 000 000 toe; 1Mtoe = 11 630 GWh) (33).

ENERGY CONTENT OF RAW MATERIALS USED FOR PIPES (5,34,35) As PVC and HDPE materials are made from oil, coal or gas, it may be considered that plastics pipes use more natural resources in the form of fossil fuels than traditional pipe materials. In fact, as most electrical or heat energy required for the production of the basic raw materials comes from oil, the reverse is true. It takes less energy to produce plastics materials for pipes than it does to make steel, copper, ductile iron or aluminium pipes - these metal products are very energy intensive. This also applies to zinc which is used as galvanizing to provide corrosion resistance to metal and ductile iron pipes.

The energy required to produce the basic raw materials is known as ‘primary energy’ and is shown in Table 1 for various raw materials used to make pipes. Table 2 shows the energy required to make the pipes from the basic materials. Table I. Energy requirements for the production of basic materials. (Primary energy) (5).

Material

Density (g/cm3)

Feedstock (toe/ton)

Conversion

(toe/ton)

Basic Material

Total (toe/ton)

Energy Content toe/ton

equivalent (kJ/cm3)

Aluminium

Steel billet

Copper billet

PVC

HDPE

2.70

7.80

8.90

1.40

0.96

-

-

-

0.55

1.13

5.60

1.00

1.20

1.40

1.20

5.60

1.00

1.20

1.95

2.33

661

343

469

117

100

Thus steel requires 3 times the amount of oil or energy compared to PVC or HDPE materials.

Table 2. Energy required to produce 100km of 110mm Class 16 water pipes plus fittings.

Energy, expressed in tons of oil equivalent (toe) and in GWh required for basic material and to provide energy for the manufacture of various pipes (5,35).

Material Energy Requirement (toe)

Energy Requirement (GWh)

PVC-O

PVC-M

PVC-U

HDPE PE 100

Steel

Ductile iron

331

468

653

745

1500

1970

3.85

5.44

7.59

8.66

17.44

22.91

The energy requirement for 110mm class 16 PVC-U, PVC-M, PVC-O and HDPE PE 100 pipes has been determined from the toe per ton of basic material (Table 1) plus the actual (measured) energy used in manufacturing these pipes.

The energy requirement for modern ductile iron pipes is probably greater than the figure given in Table 2 as these pipes are usually cement lined and also zinc and bitumen coated for corrosion protection. As can be seen from the tables steel and especially ductile iron pipes use far more energy and hence more natural resources (fossil fuels) than PVC or HDPE pipes in the production of the basic raw materials and their conversion into pipes. FLOW CAPACITY AND ENERGY EFFICIENCY The superior hydraulic efficiencies of PVC-O and PVC-M pipes are compared to the other plastics pipes available, PVC-U and both HDPE PE 80 and PE 100 in Table 3. (Comparisons with ductile iron and GRP pipes are not shown as the dimensions of these pipes are to a different standard).

Table 3. Head Losses and Pumping Costs for 1000m of 200mm Class 16 Pipes

Material PVC-O PVC-M PVC-U HDPE PE100

HDPE PE80

Nominal OD (mm)

Average wall thickness (mm)

Average ID (mm)

Mean velocity (m/s)

Head loss (m)

Theoretical power required (kW)

MWh consumed p.a.

200

5.9

188.2

1.4

8.9

5.1

41.1

200

9.0

181.9

1.5

10.6

6.1

48.7

200

12.7

174.6

1.7

13.0

7.5

59.7

200

19.1

161.8

1.9

15.9

10.9

87.2

200

23.5

153.0

2.2

25.0

13.6

115.4

Difference in power consumed vs. PVC-O (%)

-

+19

+45

+112

+181

Additional pumping cost p.a. vs. PVC-O @ R0.25 per kWh (Rand)

-

2 000

5 000

12 400

20 000

Assumptions:

Horizontal pumped line Water at 20Co Operation for 7 500 hours p.a. Information is based on Colebrook-

White formula and Harland friction factor calculation

Length of line: 1 000m Flow rate: 40 litres/second Roughness co-efficient for all pipes 0,03

Although steel and ductile iron are not compared in Table 3, it is well known that the smoothness of the bore deteriorates over time whilst PVC and HDPE perform the same after 50 years.

Flow rates for these pipes are shown in Table 4.

Table 4. Flow Capacity for 1 000m of 200mm class 16 Pipes (Dimensions as above).

Material PVC-O PVC-M PVC-U HDPE PE100

HDPE PE80

Mean velocity (m/s)

Head loss (m)

Flow capacity (l/sec)

1.76

13.1

49.0

1.72

13.1

44.8

1.67

13.0

40.0

1.58

12.8

32.5

1.52

12.8

28.0

(Flow capacity of pipes for similar head loss) Assumptions:

Horizontal pumped line Water at 20oC Information given is based on

Colebrook-White formula and Harland for friction factor calculation

Length of line: 1000m Roughness co-efficient for all pipes 0.03

The flow rate affects pumping costs, especially in the case of pumping lines (Table 3), but also indirectly for gravity lines since a smaller diameter pipe may be possible, with larger heads. THE ENVIRONMENT Society is increasingly concerned with sustainable economic development, without depleting natural resources or harming the environment; the environmental impact must be minimized while, at the same time, increasing economic and social value. Environmental concerns relate directly to energy conservation and the more effective use of natural resources. An important environmental consideration is the phenomenon of global warming caused by green house gases such as carbon dioxide. These gases are produced in industrial processes and combustion of fossil fuels, for example, power generation and motor vehicles. PVC comprises raw materials sourced from approximately 44% fossil hydrocarbons (oil, natural gas or coal) and 56% salt. At the end of their life PVC and HDPE pipes can be collected, recycled and used into other products.

It is clear from the data given in this paper that plastic pipe materials, especially PVC-M, PVC-O and HDPE PE 100, make the best possible use of natural resources and use far less energy during their manufacture than traditional steel or ductile iron pipes. CONCLUSIONS The ‘new’ generation PVC and HDPE materials, PVC-O, PVC-M and PE 100, have proven themselves over many years in service and continue to demonstrate cost effectiveness and energy efficiency. We have discussed the 50 year safety factor and shown that this depends on pipe material properties and that a high safety factor does not mean that the

pipe is safer to use or more reliable. The many benefits associated with their use, including long-term performance, ease of handling and installation, improved flow and reduced pumping power consumption as well as significant energy conservation and benefits to the environment during manufacture, are just some of the advantages in using these modern generation pipe materials. Low priced imported products such as ductile iron pipes are being increasingly used in South Africa, perhaps without reference to the issues discussed in this paper. In the overall context regard should be taken of the effects on natural resources and the environment. We must ensure the best possible use of resources and, at the same time, support of the local manufacturing industry.

ACKNOWLEDGEMENTS The author would like to thank Anton Tjabring of DPI Plastics for his valuable input and assistance in the writing of this paper.

REFERENCES

1. Financial Mail, Eskom power play. (February, 2006). 2. World Energy, Volume 8, No. 2. (2005). 3. J Sloan, Lots of demand, limited supply, Modern Plastics. (August, 2005). 4. A von Hassell and M Bella, Economy and Markets, Modern Plastics. (August, 2005). 5. Imperial Chemical Industries Limited, The competitiveness of low density poly-

ethylene, polypropylene and polyvinyl chloride after the 1973 oil crisis. (1974). 6. ISO 9080:2003. Plastics piping and ducting systems – determination of the long-term

hydrostatic strength of thermoplastics materials in pipe form by extrapolation. 7. SANS 966-1:2004. Components of pressure pipe systems unplasticised poly (vinyl

chloride) (PVC-U) pressure pipe systems. 8. D Pearson and D Walton, Even tougher plastics in the pipeline, Water And Waste

Treatment (UK). (1989). 9. ISO 4422-1. Pipes and fittings made of unplasticised poly (vinyl chloride) PVC-U for

water supply – Specifications – Part 1: General. 10. ISO 12162:1995. Thermoplastics materials for pipes and fittings for pressure applica-

tions – classification and designation – Overall service (design) coefficient. 11. J Gons et. al., 35 years of PVC water distribution pipes in the Netherlands, Wavin. 12. T Hülsmann and R E Nowack, 70 years of experience with PVC pipes, Int.

Conference, Plastics Pipes XII. (2004). 13. H F Schwencke, Structure/performance relationship of PVC pipe, Plastics And Rubber

Processing And Applications, Vol. 9, No. 2. (1988). 14. R T Hucks, Changes in Strength of pressurised PVC pipe with time, AWWA Journal.

(1981). 15. J Robeyns and P Vanspeybroeck, Molecular Oriented PVC (MOPVC) And PVC-U

Pipes For Pressure Applications In The Water Industry, Int. Conference, Plastics Pipes X11. (2004).

16. ISO 12162:1995. Thermoplastics materials for pipes and fittings for pressure applica-tions – Classification and designation – Overall service (design) coefficient.

17. D Stephenson and M Höllwarth, Physical Properties of MPVC (PVC-M) Over Time, Mechanics of Time-Dependent Materials 7:165-173. (2003).

18. Uponor Limited (UK), Exhumed, Grey, 25 Year Old, 4” Nominal Diameter, 9 Bar Rated Molecular Oriented PVC-U Pipe. (2000).

19. L R Holloway and A J Naaktgeboren, An Evaluation of Toughened PVC-U Pressure Pipes, Int. Conference, Plastics Pipes VIII. (1992).

20. M A Osry, PVC Pipes For Life – Modern Developments And Design Criteria: PVC-U, PVC-M And PVC-O, Int. Conference, Plastics Pipes XII. (2004).

21. UK Water Industry Information Guidance Note, IGN 4-37-02, 1999. Surge and Fatigue document.

22. M A Osry and A B King, Modern developments in PVC pipes for developing infrastructure in SA, Plastics Southern Africa. (1995).

23. SANS 966-2:2004. Components of pressure pipe systems modified poly (vinyl chloride) (PVC-M) pressure pipe systems.

24. BSI (UK), PAS 27: 1999. Unplasticised poly (vinyl chloride) alloy (PVC-A) pipes and bends for water under pressure.

25. AS/NZS 4765:2004. Modified PVC (PVC-M) pipes for pressure applications. 26. P G Chapman and L Agren, A New Technology For In-Line Manufacturing Of Biaxially

Oriented PVC Pipes, Plastics Pipes X. (1998). 27. L R Holloway, Properties And Performance of Biaxial PVC Pipe Manufactured By A

Continuous Process, Plastics Pipes X. (1998). 28. SANS 1808-85:2004. Water supply and distribution systems. Oriented polyvinyl

chloride (PVC-O) pressure pipes for underground use. 29. ISO/DIS 16422:2005. Pipes and joints made of oriented unplasticised poly (vinyl

chloride) (PVC-O) for the conveyance of Water-Specifications. 30. AS4441 (Int.) – 2003. Int. Australian Standard, Oriented PVC (PVC-O) pipes for water

applications. 31. SABS ISO 4427: 1996. Polyethylene (PE) pipes for water supply – Specifications. 32. L Brown, Three Steps to a New Energy Future, World Energy, Vol. 8, No.2. (2005). 33. Key World Energy Statistics, Int. Energy Agency. (2005). 34. R C Wilging, Plastics in Piping, Int. Conference on Underground Plastic Pipes. ASCE. (1981). 35. Pipes and Pipelines, Principles and Practice. Second Edition. (1998).