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Hydraulic dimensioning of sewage lines

skyline jakartaSkyline of Jakarta

1. Basis

The development respectively the renovation of waste water ischarge system is always associated with high investments and extensive civil engineering works. Beside the purchase costs for the pipelines itself, also high proportionate costs arise for the grounds works and the pipe installation. 

Especially in topographical flat Asian metropolitan areas like Jakarta, the pipelines can often only be installed with a low longitudinal slope. Thus, the hydraulic capacity of the pipelines is decisively limited. Increasing the longitudinal slope again means expensive, extensive, technically sophisticated excavation works. In Asia, most sewage and drainage systems are made out of concrete. Those pipes have a very rough pipe wall and have a lot of connections, due to their production process and quality. By using suitable pipe materials that guarantee smooth walls and need much less pipe connections, the longitudinal slope can be reduced while maintaining the dimension and the hydraulic capacity. Thus, also the costs for excavation and civil works can be reduced considerably. Smooth walls also optimize the discharge conditions and especially avoid depositions during dry weather flow.

2. Dimensioning of the pipeline

The discharge flow condition of open channels respectively free-flow pipelines base on the hydraulic date of Prandtl and Colebrook. The working paper DWA-A110 published by the “Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V.” forms the basis for the hydraulic dimensioning and the proof of performance of sewage lines and channels in Germany.

The discharge capacity and behaviour is depending on the hydraulic cross-section, the longitudinal slope, the inflowing water amount as well a roughness respectively the single losses of the pipe system.The dimensioning and the proof can be carried out according to working paper DWA-A110 by means of overall concept or on an individual basis.

Applying the overall concept, the operational roughness of the complete systems will be determined by fixed values of the application. The individual concept should be applied to prove the performance of sewage networks considering all loss-causing influences. In doing so, the losses caused by occurring single losses as well the roughness of the pipe wall are determined. Single losses are caused for example by inaccurate positioning, pipe connections, manholes, flow deflections etc.

Concrete pipes with many connections
Concrete pipes with many connections
Concrete pipes with high roughness and socket 
Concrete pipes with high roughness and socket 

Picture 2 and 3 show exemplarily the concrete pipes available in Jakarta. The rough surface structure of the inner wall as well as the many pipe connections are clearly visible. 

For these low-quality pipes, the wall-roughness of k = 0,4 bis 2,0 mm can be considered. Furthermore, the single losses for the many concrete sockets have a negative influence on the discharge performance and additionally reduce the hydraulic capacity.
It can be assumed that the sockets cannot be carried out water - and pressure-tight and therefore exfiltration or of sewage water into the soil respectively infiltration of foreign water during high ground water level will occur.

Plastic pipe systems made of PE-HD are an alternative to concrete pipes. Those pipe have, according to the information of the producer, an absolute roughness of k = 0,0015 to 0,01 mm. The pipe connections are hydraulically optimized and can considerably be reduced compared to concrete pipes, thanks to the longer pipe lengths. The tightness is guaranteed by the possibility to weld the pipe connection.

For both pipe systems the flow rate has been calculated for pipes in diameter DN/ID 2000 mm depending on the wall roughness and for different longitudinal respectively bottom gradient with a flow depth of 1800 mm respectively 90%. In order to simplify the calculation, the influence of the pipe connections and the material-independent single losses were not considered. However, the optimized design and the quantitative reduction of pipe connections of PE lines generally lead to improvement of the flow capacity.
For the concrete pipe, according to the available pipes (picture 2 and 3) an absolute roughness of k=1,0 mm is considered. The PE pipes will be calculated with a roughness of k = 0,01 mm. This complies with the maximum value respectively the worst value according to the producer. The calculation is carried out on the basis of the equation of Prandtl-Colebrook. This equation serves to determine the loss amount due to friction. The following diagram shows the calculation results. This shows that by choosing smooth pipes the flow rates increase, considering the same bottom slope. Conversely, this means that when using smooth pipes the bottom slope can be decreased, maintaining the same dimension and hydraulic performance.

PE pipes with smooth inner surface
PE pipes with smooth inner surface
PE pipe inside
PE pipe inside

Picture 5 reveals that smooth PE pipes with a bottom slope of 2,0 ‰ can transport a higher amount of water compared to the concrete pipes available in the Asian region with a bottom slope of 3,0 ‰. In practice, this means that the depth of the pipe trench can be reduced by around 1,0 m for a pipeline length of 1000 m. Especially in flat cities like Jakarta with a big surface area, collecting pipelines with 10 km are common. The reduction of the bottom slope by 1 ‰ thus theoretically means that the trench depth at the performance low point can be reduced by 10 m.

3. Summary

Pipes with a low wall roughness like PE pipes can considerably reduce the total civil works costs and thus the total construction costs, even though they might be more expensive.

Considering the same pipe dimensions and hydraulic performance, PE pipelines can be installed with less slope. Hereby, earth works, excavation works, excavation support and water retention measurements can be reduced, especially in topographical areas with few slope. By installing pipelines with less slope, there is potential to safe costs and reduce pumping stations, operation points and operational costs.

Also the life-cycle has to be taken into account, comparing the concrete pipes available in Jarkarta with modern PE-pipes. The high quality PE pipelines have a lot of financial advantages compared to concrete pipes, taking into account the operation life and the low operational costs, also with regard to further reinvestments and maintenance.

Also the tightness of PE lines is very advantageous. The infiltration of ground- and foreign water can lead to an overload of the system capacity. Also no exfiltration of sewage water to the ground will occur.

table comparisonOther advantages of using PE pipes are:

• High abrasion resistance
• Responding to dynamic loads (e.g. earthquakes)
• Corrosion resistance
• Chemical resistance
• Few incrustrations and depositions
• Low weight and therefore simplified installation
• Variable pipe lengths
• Welded pipe connections
• Resistant to exfiltration by roots etc.

Author:
Dipl.-Ing. Ulf Heinemann

engineer heinemann ad