On the Selection of the Type of Pipes for Water Distribution

Networks

Boyan BorisovUASEG - Sofia

The current situation

• Pipes are basic elements of Water Distribution Networks /WDN/ and single pipelines

• Polyethylene /PE/ and ductile cast iron /DCI/– two of the most widespread pipe materials for water supply pipelines

• The current situation with the selection of pipe material in practice

Outline of presentation

• 1. Short history and descriptions of compared pipe materials

• 2. Description of the selection methodology and quantitative and qualitative parameters

• 3. Analysis of some of the compared parameters

• 4. Discussion and conclusions

A short description of compared materials

1. Ductile cast iron /DCI/

2. Polyethylene /PE/

Comparison methodology

N ParameterWeight

coefficient

Rating for pipe mat.

j

Valuation for pipe mat. J

Rating for pipe mat.

j+1

Valuation for pipe mat. j+1

iParame

ter iKi Rij

Vi,j = Ki.Ri,j

Ri,j+1 Vi,j+1 = Ki.Ri,j+1

i+1

Parameter i+1

Ki+1 Ri+1,j

Vi+1,j = Ki+1.Ri+1,j

Ri+1,j+1

Vi+1,j+1 = Ki+1.Ri+1,j+1

nParame

ter nKn Rn,j

Vn,j = Kn.Rn,j

Rn,j+1 Vn,j+1 = Kn.Rn,j+1

The quantitative parameters

1. Initial investment1. Purchase costs2. Construction costs

2. Whole life costs1. Initial investment2. Operation and

maintenance cost

, , 1,

,

min ( ; )i j i ji j

i j

C CR

C

Ci,j – the value of “initial investment” or “WLC” for pipe type “j”

The chosen comparison parameters:qualitative ones

3. Environmental aspects4. Corrosion resistance for DCIP

and slow crack growth resistance for PE

5. Impact resistance6. Thermal stability7. Durability8. Operational experience9. External load resistance10. Resistance to water hammer

Rating “R” is between 1 and 3

Weight coefficient is between 5 and 15%.

Parameter 1 - Initial investment1. Price characteristics of both types of pipes

€/m’

PE

DCI

DN

Turning point phenomenon

Parameter 1 - Initial investment and turning points of pipes and pipelines – real data

Parameter 2 – Whole Life Cost

• After initial investment the energy consumption, i.e. head losses is decisive factor

• An analysis is made for the influence of pipe roughness and the internal diameter

Nt

tt 1

CWLC

(1 r /100)

Influence of hydraulic roughness

• In transition zone friction factor “f” is slightly influenced

• Difference in head losses for k=0.01 and k=0.3 is under 5 %

Hydraulic roughness PE pipes DCIP

Pipes;“Individual concept”

k1=0.01mm k1=0.03mm

Pipelines and networks;“Global concept”

k2=0.1mm k2=0.1mm

Influence of internal diameter

• Difference by 1 % in internal diameter causes difference in head losses by 5 %

• There are bigger differences in internal diameters of different pipe materials, incl. PE and DCI

Two common WDN cases

WLC pipe mat. X1

WLC pipe mat. X2

Install. costs X2

Install. costs X1

Operating /Energy/ costs

Diameter

Costs

Opt. DX2Opt. DX1

WLC DX2

WLC DX1

The correct approach for choosing diameter

WLC DX2 <WLC DX1

Environmental aspects• Raw materials: for DCI pipes fresh iron ore or iron scrap for PE pipes: oil • Carbon print

Type of pipe

g of CO2 eq/per kg of pipe/year

g of CO2 eq/1 m (DN 110 or DN

100) /year

g of CO2 eq/1 m (DN 630 or DN 500) /year

PE 40.1 88.23 2819DCI 20.7 246.33 1900

Note: The data is for the production and laying stage, without transportation.

Durability for DCI and PE• DCI pipes: the service life is so long that it is

even not mentioned in the product standard

• Thinner wall since 2010 and rubber gaskets

• PE pipes: the standard service life declared in EU is 50 years; in some standards – 100 years

• There are now PE pipes which are 50 years old and are in good condition

Qualitative parameters DCI (K x R) PE (K x R)

3. Environmental aspects 5% x (2-3) 5% x (1-2)

4. Corrosion resist. for DCI and slow crack growth for PE

5% x (1-2) 5% x (2-3)

5. Impact resistance 5% x (2) 5% x (1)

6. Thermal stability 5% x (3) 5% x (2)

7. Durability 10% x (2) 10% x (2)

8. Operational experience 5% x (1-3) 5% x (1-3)

9. External load resistance 5% x (2-3) 5% x (1-2)

10. Resist. to water hammer 5% x (2-3) 5% x (2)

Summary - Qualitative parameters

Discussion• In practice - tendency for exaggeration of some

disadvantages and advantages of the pipes;

• Standards, technical literature and own research should be used instead manufacturer’s catalogues and common opinions

• It has been confirmed that generally PE is appropriate for smaller diameters and DCI for larger ones

• Consensus of stakeholders is needed for ratings of the qualitative parameters and all weight coefficients

Conclusions • Qualitative parameters still could be decisive

for the selection of pipe material

• The result of the comparison methodology could vary in the course of time and for each particular case

• The comparison methodology could be used for all pipe materials

Next step• The selection methodologies have to be

included in some form in the national design legislation or gradually to be accepted as a good practice