Water Pipe Design Project for Poundbury - Report Example

Summary This report gives the design details of a water supply system for Poundbury town. Planning permission for the town development was granted in the 1980s and construction started in 1993. The town’s population has dramatically increased over the past decade and the population is expected to reach 6,000 with 2,500 dwellings by 2025. From the design it was established that the we need 4 pumps put in series to be able to pump the desired quantity of water at a discharge of 30l/s to a height of about 100m. Also the report gives the material type and size for pipes for use in the pumped main and the gravity main. Table of contents 1.0 Introduction 4 2.0 Investigation of the Route 5 Sensitivity areas 5 Figure 2 : CWS, SSSI, SAM and Contours surrounding the site (L) and the very large unavoidable area of AOND (R). 6 Burton PS to Lamberts Hill SR 6 Lamberts Hill SR reservoir to Poundbury 6 3.0 Options considered 8 3.1 Pipeline Routes 8 Pumped main 8 Gravity Main 9 3.2 Pipe Design 10 Design Capacity 10 Pipe Diameters 10 Design Procedure 10 Pipeline Ancillaries 12 3.3 Costs 13 3.4 Pump Selection. 14 3.5 Pressure Surge 18 4.0 Conclusions 19 References 19 Appendix 1 :Pumped main design 20 Appendix 2: Gravity main design 25 1.0 Introduction In Poundbury and Dorchester there is to have an improved reliable and efficient water distribution system because there is growth in population in addition to booming economic activities. Wessex Water is inspired by heavy interest from the Price of Wales willing to make a heavy investment so as to ensure that the Poundbury population has adequate we and high quality water supply. A new water supply system will see putting in place a modern system in replacement of the old and low tech system. Figure 1 shows the historic water tower which was used in provision of water to the then small population of the town. Development of the town over time resulted to the elevation of the tower to be10m below the highest point of the town. The current water tower in Dorchester will soon be absolute and the mains and storage service will not be able to fulfil the future demands of water. It is with this background that Wessex water has been inspired to embark on construction of new water supply system that will offer adequate was for Poundbury population in foreseeable future. The new design will be such that we have the water pipeline starting from Burton rising to Lamberts Hill and then through gravity the water is to conclude in Poundbury where we have the targeted population. In these report there is presentation of the design work for the new pipeline. In coming up with the design there was adherence to specification given in Pipeline Design Standard DS643 (Wessex Water, 2008). The design of the project by looking at the following design elements Investigating and establishing the best route with possible alternatives being looked into Determining the appropriate diameters, length of diameters putting into consideration design requirements and constraints. Here there is calculation of head losses due to fipe friction and fittings This section includes calculations for the head losses, pipe and fittings. Coming up with costs that are associated with different diameters and material types Selection of the suitable pumping systems Discussion on pressure surge and protection methods Coming up with sensible conclusions and recommendations that are inline with the design work undertaken 2.0 Investigation of the Route In this design the starting point of the pipeline is expected to be at Burton pumping station , then it will rise to Lamberts Hill Service. The pipeline will start at Burton Pumping Station to a service reservoir located Lamberts Hill and then through gravity water will be directed to Poundbury. In coming up with this design it was important to put special consideration to the route taken by the pipeline so as to ensure that design specifications are adhered to with sensitive areas being avoided as much as possible. The location of the route will be influenced by the following key factors The design area topography Conformation to design standards and codes Land use and ownership Major crossing: including railways, roads and rivers Site accessibility AONB, CWS, SSSI, SAM The prevailing ground conditions Regulations governing construction The cost associated with various aspects of the design Sensitivity areas The areas connecting the three sits where the project activities are centred are surrounded by many sensitive areas where there is need for external approval. These are as follows Areas of Outstanding Natural Beauty (AONB) County Wildlife Centres (CWS) Site of Special Scientific Interest (SSSI) Scheduled Ancient Monuments (SAM) If the design will involve crossing of any of these areas then it will require permission being sought from relevant authorities and this will come with significant level of compensation. While it may be possible for some of these areas being avoided, for AONBs this is not possible due to the fact that both Lambert’s Hill AND Poundbury are in sensitive areas. Figure 2 : CWS, SSSI, SAM and Contours surrounding the site (L) and the very large unavoidable area of AOND (R). Burton PS to Lamberts Hill SR Burton PS is to be the origin of the piping where the water is sourced from two 90m deep boreholes. The water from the boreholes undergoes some preliminary treatment. Central Area Integration Main (CAIM) also supplies water to Burton Ps from Eagle lodge. This water is then pumped to Lamberts Hill Service reservoir due to the fact that we have a constant increase in elevation in the entire route. The process of coming up with appropriate pumping system will be dealt with in another part of this report. Lamberts Hill SR reservoir to Poundbury The service reservoir has appropriately been located at the point of highest elevation to facilitate flow of water to Poundbury by gravity. The flow of water by gravity by gravity reduces the cost of pumping to a great level. 3.0 Options considered 3.1 Pipeline Routes Pumped main The pumping main takes the route shown in figure 2 with the length being 6330m (6.33km). In choosing the route the following are the parameters that were put into consideration. All SAM, ISSS, CWS areas which were considered to high sensitivity areas were avoided. However, with the design being such that the reservoir was to be located on Lambert’s hill which is an area falling under AONB there was no way of avoiding this area, even though it meant incurring of extra cost . It is important to have a design which is such that in the entire length of route the elevation of the pipe be above hydraulic gradient line of the pipe. The design has fulfilled this condition. In the route that has been chosen the elevation increases steadily and this makes pumping of water to the service reservoir to be easier and cheaper. The design route chose is such that we have minimum major crossings being involved. In the chosen route we have two rivers crossings and 8 roads crossing. Avoiding road crossing was away of reducing costs and also the levels of disruptions. In the route taken there is sharing of trench below , with the sharing of trench being a strategy of bring down costs and avoiding complexities in the construction. Considering a shorter route was put into consideration but with no shared trench being involved ibn this route , it means extra costs being incurred. Figure 4 below shows the outline of the route. Gravity Main The most suitable route for location of gravity main is as shown in figure 7. With the gravity main having a length of 3070m it is considerably shorter than the pumped main route which was found to be 6330m. The area for the route of gravity main has many scheduled ancient monuments, and thus it called for careful consideration so as to avoid the areas. The route also was chosen such 2505m of the route is share with pumped main ensuring the cost is reduced significantly. 3.2 Pipe Design So as to have a design that is the viable option to Poundbury , there was need of having detailed designs involving accurate calculations. In designing for the pipe, the current water use was put into consideration and then the usage was projected into the future. The full calculations are as presented in Appendix Design Capacity Wessex water has identified in its Dorchester supply strategy (Wessex water, 2005): “For this option the ring main pumps supply to a 0.5 Ml reservoir at Lambert’s Hill. The demand off the reservoir is approximately 6.2Ml/d at peak week by year 2019, with flows expected to vary between 30 l/s to 120 l/s. Although in the first years of operation minimum flows may be as low as 15 l/s. To meet this large and varying demand on a small reservoir, the pumps at Burton will need a great deal of flexibility” With the strategy being obeyed , the conservative design capacity of the pipeline will be: Route Design Capacity Burton PS to Lamberts Hill (Pumped Main) 7Ml/day Peak Week Demand Lamberts Hill to Poundbury (Gravity Main) 120l/s Peak Hour Flow Max Day Demand Pipe Diameters In this design Ductile Iron and Plastic SDR17 are the two materials that have been considered. The choosing of these materials was in obedience of the Pipeline Design Standard DS643 (Wessex Water, 2008). Apart from resulting to increased costs, deviation from the set standards brings about increased maintenance costs. Design Procedure So as to have comparison level of accuracy and to detect errors, there was two methods that were used in establishing which was the most suitable pipe diameter and the level of head losses incurred. There can only be flow of fluid (water in this case) in a pipe line if we have enough pressure that is able to overcome the friction and elevation changes. This means that there is need to calculate the head-loss (pressure drop) and then this is to be added to the downstream head. Moody’s Diagram and Darcy’s Equation In this method there is use of design flow rate, area and diameter of the pipe in calculation of Reynolds number which is then used in the calculation of relative roughness. Using this two (Reynolds number and relative roughness) the friction factor ir established from the Moody’s diagram. With the friction factor established Darcy’s equation is used in calculation of head loss in pipe line. HR Wallingford Tables These are tables that gives a relationship between the hydraulic gradient and the corresponding discharges and velocities. After calculating discharge and establishing the diameters there can be determination of hydraulic gradient which may be directly or through interpolation. After successful calculation of head losses, there is need for calculation of velocity and retention checks so as to ensure standards in Pipeline Design Standard DS643 (Wessex Water, 2008) are met. Check Maximum Velocity 1.5 m/s (Pumped Main) 2 m/s (Gravity Main) Retention Time 12 Hours The following table summarises the findings of the calculations for the pumped main. Diameter Ductile Iron (mm) Head Losses in Pipe (m) Losses due to Fittings (m) Moody’s and Darcy’s HR Wallingford Tables 600 15.96 15.92 0.66 700 7.393 7.32 0.36 The following table summarises the findings of the calculations for the gravity main. Diameter (mm) Material Losses along pipe (m) HR Wallingford Tables Losses due to Fittings (m) 500 D1 1 12.669 0.12 500 D1 1S 13.228 0.1 600 DI 5.381 0.26 493.6 PE1 11.28 0.1 493.6 PE1S 11.91 0.14 555.2 PE 6.828 0.170 Pipeline Ancillaries The ancillaries that are installed so as to maintain the pipe are Swabbing chambers at every 2.5km for removal of slime and tuberculation in water mains. Placing of air valves at high points on pipelines to avoid air accumulation of air and interference with flow. In-line valves to be placed after every to km reduce pressure on pipe lines. Washouts are to be placed at low points in the pipeline to serve the purpose of emptying the main or removing stagnant water. Pressure Reducing Valves (PRVs) are devices that are expected to maintain a preset level of pressure the main downstream irrespective of the upstream level of pressure. PRVs are expected to be activated when pressure goes beyond 35m. Pressure Sustaining Valves (PSVs) as expected to be incorporated in the pipeline to ensure that pressure level does not drop below a certain present level , in this case 6m 3.3 Costs The costs are determined by where the pipe is to be located and the surrounding areas. The cost will be high if a pipeline is passing through an area falling under sensitive area category and also the cost will increase significantly where we have a pipe on the road. Using the standard rates cost Ductile Iron pipes was calculated and applying relevant factors the rates for PE were calculated. Overall value of the project is significantly affected by the installation techniques adopted. In this project there was use of the open-cut in majority of route which ensured costs being kept to a minimum. Trenchless technology was used for areas that involved crossing areas that could not be altered such as the railway. Pipe jacking and directional drilling are the two options used for excavation of pipe route underneath without disturbance to the railway line above. Using these technologies resulted to significant increase in costs linked to installation. The diameter also has a bearing on the cost of the project. Spreadsheet analysis of the cost has revealed that smaller pipe size require higher pump energy due to high friction in smaller pipes. In coming up with the cost of the project the initial capital and the 60yr operating expenses are put into consideration. The summary of costs is as in table Table 5: summary of the costs for the project: Net Present Value – Pumped Main Scheme £ 60 years Pumped-DI#1-500mm 21,345,939 Pumped-DI#2-600mm 20,107,142 Pumped-DI#3-700mm 22,817,552 The detailed calculations for these estimations are as in appendices. If second option is to be chosen then Wessex water will save 12% against the most expensive choice. The low cost of option two is attributable to the larger diameter which lowers the head loss significantly and this brings down the operating cost. Table 5: summary of the costs for the gravity main : Net Present Value - Gravity Main Scheme £ Cost DI#1 – 500mm 7,898,118 DI#1S – 500mm 7,858,118 DI#2 – 400mm 10,250,767 PE#1 – 393.6mm 7,223,722 PE#1S – 393.6mm 7,188,122 G-PE#2 – 555.2mm 8,774,627 From the cost analysis table it is seen that it can be seen that PE shared pipe with a diameter of 396.6mm is the best option. 3.4 Pump Selection. Series or Parallel For the first stage we have uphill path where the elevation increasing from Burton pump station (PS) to Lamberts Hill Service Reservoir (SR) and thus the only option for moving the water is by installation of pumping equipment. Pumps are needed to pump water against some resistance which may a rise when the pipe is being used to transfer the water to the same level as the source or at a higher ground. The size of the pump to be used is usually dependant on the height to which the water is to be raised and the demand to be fulfilled. The curve characteristics give the relationship head and flow rate. To have the desired discharge or head the pumps can be combined in series or in parallel as shown in figure yielding results shown in figure Series arrangement Parallel arrangement The From graph 1 it be seen that having pumps in series is applicable where high head is required while parallel arrangement is suitable where high discharge is required at lower head. In this project we have relatively high head and this means that we need to have several pumps in serried. Number of Pumps It has been identified above that one pump is not sufficient to meet the required head or discharge. This section of the report examines the number of required pumps. The graph below shows the system curve different number of pumps compared to the discharge. Graph 2 shows the relationship between the heads produced by the pumps and the discharged produced by the number of pumps. The graph shows that all pump systems can satisfy all discharge r4equirements up to 100l/s we need at least 4 pumps so as to satisfy both the head and discharge requirements. The system curve puts into consideration height difference between beginning and end points, freeboard, pipe losses and losses due to fittings. Pump Option Selection Table 1 below gives the efficiencies and power usage . from the table it can be seen that the 4 pump combination has a high efficiency of 80% Cavitation Cavitation involves entry of water particles/air into the pump resulting to reduction in efficiency of the pump or even the pump being damaged. Destruction of the pump as a result of cavitation is experienced because the pump was designed for pumping water (liquid) and not gases. Presence of the air particles may also result to reduction of the pumping capacity. Having pumps operating at a level where the head and capacity is at maximum efficiency. 3.5 Pressure Surge Surges in the system come as a result of changes in condition of the fluid inside the pipe. This may come as a result of having sudden changes in pump demand or the valves being closed. Closing a valve results to build-up of pressure in the pipeline and the rapid pressure build up may damage the equipment, lead to leakages and the water being contaminated. The damage that can be caused by the pressure surge may be controlled by installation of the following devices Valve closure control which are devices that are used for reducing ands slowing down the rate at which the valves can be closed at their final stage of their closure. This result to slow closure and thus reducing surge pressure. Surge shafts or air vessels. In positive surge pressure vessels there is alleviation of pressure by the fluid being allow to come out of the pipeline and entering a a vessel in a vertical position thus facilitating the absorption of excess energy. This was the surge protection used at Burton WTWs. 4.0 Conclusions The preferred route is outlined in the summary below. This route is the most economically feasible out of the options analysed. Route Primary Route (Figure 1) Span 8000m Material Ductile Iron Diameter 350mm Pumps 4.no 200x330x335 Pressure Surge Pressure surge analysis required Cost £20,107,142 ( 60yrs period ) Route Primary Route (Figure 2) Span 12000m Material PE – Shared Route Diameter 393.6mm Pressure Surge Pressure surge analysis required Cost £7,188,122 (capital expenditure) Further Work: Full pressure surge analysis. Further consideration for minimum velocities. Minimum acceptable velocity is dependent on the risk of dirty water and the requirement to maintain a suitable chlorine residual. Constitution on the use of PRV and PSVs. Full geological survey. References Douglas, J.F., Gasiorek, J.M., Swaffield, J.A. and Jack, L.B. (2011) Fluid Mechanics. 6th ed. Harlow: Pearson Education. H R Wallingford (1990)Tables for the Hydraulic Design of Pipes and Sewers (5th edition); Thomas Telford. Massey, B. (2006)Mechanics of Fluids (8th edition); Taylor and Francis Savić, D. A. &Banyard, J. K. (2011)Water Distribution Systems; Institution of Civil Engineers Skeat, W. O. and Dangerfield, B. J. eds. (1969)Manual of British Water Engineering Practice – volume II: Engineering Practice (4th edition); Institution of Water EngineersWessex Water (2008). Water supply pressure pipeline: Design standard DS 643. Bath: Wessex Water. Wessex Water (2005). Dorchester supply strategy [online]. Bath: Wessex water. Available from: http://www.waterprojectsonline.com/case_studies/2005/Wessex%20Dorchester%20Supply%202005.pdf Appendix 1 :Pumped main design Calculation1 Design pumped main using Moody diagram (V ≈ 1 m/s) Flow-rate: Area: Diameter: (600mm is next) Velocity: Reynolds number: Table 38: galvanised iron, normal à k=0.15mm Relative roughness: Moody diagram… Moody diagram à Substitute into Darcy equation: Losses due to fittings = SK (V2 / 2 g) = 8.65 x (1.2272 / 2 x 9.81) = 0.66 m Velocity checks: Velocity, Read More