Hydraulics and Engineering Applications

Summary

Drinking water was supplied by WESSEX Water to 2000 new houses in Poundbury. Pipes of 350mm diameter with dimensions 200 x 290 x 280 in the Armstrong power series arrangement were used at a cost of ВЈ7, 503, 927 for both gravitational and pumping mains. The pipe material is made up of ductile iron material largely because of its material strength, they (the pipes) have a power rating of 18.2 kW. The selected route has a span of about 6.11 km. The report analysed various options for the design of pipe mains and eventually arrived at the aforementioned option.

Introduction

Wessex Water has a task of supplying drinking water from a pumping station at Burton from her regional main to a service reservoir at Lambert’s Hill which will then distribute water by gravity to 2000 new houses in Poundbury. This report was borne out of the need to analyse and develop the most feasible option from a few alternatives. Factors considered in the comparative analysis were: route, pipe material, pipe diameter selection, pump arrangement and selection, protection from pressure surge and most importantly cost effectiveness. The report also considered all basic assumptions in design and in line with Wessex Water Design Standards before suggesting the most beneficial/appropriate option. All design calculations, tables and figures are in the appendix of the report.

Investigation of route

The project was limited to 3 routes due to certain criteria/ factors such as system hydraulics (ground elevation and slope), land use, sensitive areas and access for construction and maintenance. The three main routes selected for discussion were 3 gravity and pump mains routes 1, 2, and 3. The plan and sectional views of these routes can be found in the appendices. According to given topographical data, Burton pump station, Lambert’s Hill service reservoir, and the maximum/peak elevation at Poundbury are at elevations of 59.45 m, 155.70 m, 110 m above ordnance datum (AoD) respectively.

Ground conditions played an important role in choice of route. Site investigation revealed that the ground consists mostly of chalk soils which are favourable materials for subsurface pipe works/ pipe lining.

The inconvenience that goes with municipal water reticulation and supply was also put under considerable. Road closures and route diversions, traffic delay and congestion as it affects a reasonable population were put into proper perspective. Construction project design and management regulations were strictly adhered to. However, there were unavoidable major crossings which required painstaking approaches so that the project will be a successful one.

Cost consideration was also an important decision-making factor. Projects are generally successful if allowable starting capital is not exceeded. Long and short-term cost effectiveness are also very important aspects in costing as can be seen in the analysis. According to the pipe design, the routes of the pipes are carried out in certain areas. The choice of the routes of the pipeline will be governed by a number of factors such as system hydraulics (ground elevation and slope), land use (landowners, road/rail and river crossings), sensitive areas (AONB, CWS, SSSI, SAM) and access (construction and maintenance) accordingly. As stated by the data the Burton’s pump station has he elevation of 59.45 metres above ordnance datum (AOD) whereas the Lambert’s hill SR has 155.70 metres above ordnance datum (AOD) and Poundbury has 110 metres above ordnance datum (AOD). The main key for the route selection is to avoid areas of sensitivity. The areas such as areas of natural beauty (AONB), county wildlife sites (CWS), sites of special scientific interest (SSSI) and scheduled ancient monuments (SAM) are all indicated in the map. As for the land use, it could be another influencing factor for landowners as well. In addition, the ground conditions also need to be considered, water and soil that is constructing on providing the condition of the ground on which pipes and other infrastructures will be laid. The cost is another influencing factor due to the fact that every project has starting capital.

Options Сonsidered

Justification of routes

As the pipeline was selected for a suitable route with the length of 6.11km, some factors need to be considered such as sensitivity areas, elevation, major crossings and trench. All sensitivity areas were avoided (SAM, ISSS, CWS) excluding AOND. In particular, in order to protect environment and landscape it is possible extra cost will be earned as a result of locating this sensitive area. The route gives for the increase in the elevation to make it cheaper and easier to pump the water into the reservoir. Major crossing needs to be minimised because it will cross rivers and roads by avoiding traffic which led to disruptions and not to mention costs will be reduced. What’s more, the trench shared is to reduce costs due to no duplication of construction staff trench, lower land take within the road services, it increased efficiently and shorter timeframe, encouraging greater productivity and cost savings.

Pipe design

The pipe design selection for the pumped main, the Darcy equation will be used. As stated by the value of a daily flow, the diameters chosen for the pumped main were 350mm and 400mm. Hf is the head-loss of pipe, f is the friction factor, l is the length of the pipe, d is the diameter of the pipe and g is the acceleration due to gravity 9.81 m^2/s.The head loss due to the fittings were calculated. The chosen entry and exit from the pipe are one of the key factors. The bell mouthed entry and exit was selected for both pumped and gravity main.

To explain the calculations, 350mm diameter pipe is feasible to use, for the velocity of the water in pipe is 0.842 m/s which is smaller than maximum velocity is 2 m/s therefore the retention time is 2hr 1 min that is also smaller than 12hrs retention time. Furthermore, pressure checks for 350mm is 10.7 bar which is less than 16 bars. Thus, it can be concluded that 350mm diameter pipe is feasible for the pumped design. Meanwhile, using the hydraulics gradient from the table 16, design head loss is calculated for gravity distribution pump. Similarly, the maximum velocity is 0.955m/s which is less than actual maximum velocity 1.5m/s. Retention time is 52 mins that is smaller than 12 hrs retention time. Hence, 350mm diameter pipe is feasible to be installed for the gravity distribution pump design. On the other hand, swabbing chamber is a tool that cleans the water from removing waste such as sand, clay and slit before undertaking chlorine testing procedures prior to the activation of the watermain. The process of installing swabbing chambers is one swabbing chamber for every two kilometres. The length of the route is 6.11 km, 4 swabbing chambers were installed. 4 swabbing chambers every 4km is k= 0.96. Another method is used for the in-lined valves where it allows fluid to flow in one direction. There are 3 in-line valves. Air values are installed manually operated which is used to release air from a water pipe or fitting. 3 air-valves were installed for the pipe. The washouts were installed to allow water to flow out during the cleaning of the pipe. The junctions at which a pipe branches out to a washout point thus the number of washouts is 3. With the network of pipelines become more and more complex many pipelines have to cross by open-cut (trench), pipe-laying and directional drilling. Open-cut trenching can be less expensive method than trenchless methods in non-pavement areas. It does not also require debris to be removed from the pipe. Directional drilling is the concept of controlling direction and placing a wellbore inside the pipe at underground location. The number of crossings is 17.

Costs

After building up the pipe design it is feasible to calculate the cost of pumped main and gravity main. From the figure, the costs for the pumped main have the diameters of 350mm, 400mm and 6110 and 3000 metres were calculated. As for the pump main trunk pipe, the scheme 3 is $7,097,943 whereas scheme 1 is $7,215,186 therefore scheme 1 is the most expensive which costs $7,215,186 per 60 years. Scheme 3 and Scheme 2 costs $7,097,943 per 60 years and $7,097,943 per 60 years. The cost needs to be as cheap as possible thus scheme 1 cannot be used. Concluding that, the pipe diameter with 350mm is suitable for scheme 2. For the gravity distribution main pipe, scheme 2 is the most expensive that costs $1,688,500 hence it’s not suitable for the diameter of the pipe. In comparison, scheme 1S is the cheaper than other one which costs $1,576,945. To finalise that, the diameter of 350 from Lambert’s Hill to Poundbury is more feasible to use the 350mm pipe.

Pump

Due to the pipe selection for the trunk pipe, the diameter of 350mm is being used. Firstly, to find the static lift, elevation at Lambert’s Hill is 155.70m, Elevation at Burton PS is 59.45m and freeboard is 0.5m. As a result, О” z is (155.70-59.45+0.5) = 96.75m. The relative operational point is at the height of 108.94m where the discharge is 0.081 m^3/s. The pipes were supposed to be series or parallel continuation. The parallel for the pipes with the high flow value. Therefore, the Armstrong pump series arrangement should be used. If the efficiency rises as the number of pump rises too. Pump system with 5 or 6 were identified as pumps that require the least amount of power. That rule out the systems with 2, 3 or 4 pumps. The 87% increase in efficiency for the 6-pump system does not explain in the cost of purchasing an extra pump. Hence, the 5-pump option is best suitable for the pipe from Burton PS to Lambert’s Hill. Using the pump performance table, 5 no. 200x290x280 pipes: 75l/s, 21m, 18.5 kW, 85% were used due to lower costs.

Pressure surge

Pressure surge is a transient sudden rise or fall of pressure in a pipeline. Pipeline surges can be positive or negative and are mostly caused by the sudden closure of a block valve or emergency shutdown of a pump. For instance, if the valve is too close or opened rapidly. If the pressure surge is in excess of the rate capacity of a pipeline it can cause splits in the piping system. Negative pressure will develop suddenly downstream of the valve as a result of the fluid being in motion. It was caused by a temporary separation of the liquid column followed by a reverse flow back towards the valve. This may lead to the destruction of both the vale and the pipe. The highest pressure surges often happen when the maximum number of pumps stops as a result of a power failure at a pumping station. As the change in pressure and velocity were not limited to the point of disturbance. It may continue up and downstream at pressure wave propagation velocity. There is another type that provide pressure surge protection which is cathodic protection. It is often used to protect steel from protection. Corrosion is cause when two dissimilar metals are immersed in an electrolytic substance such as water, soil or concrete. Cathodic protection connects steel to preserve the metal by providing a highly active metal can act as an anode and provide free electrons. This is why the active metal sacrifices its ions and keeps the less active steel from corroding.

Conclusion

In conclusion, various summary of the preferred report is outlined below. This report is the most economically feasible out of the options being analysed. The report also met all of Wessex Water’s design requirements and specifications. As a result, the length of the pumping pipe is 6110 meters and the gravity main is about 3310 meters. Pipes of 350mm diameter with dimensions 200 x 290 x 280 in the Armstrong power series arrangement were used at a cost of ВЈ7, 503, 927 for both gravitational and pumping mains. Thus, it is suitable to use 5 pipes with dimensions 200 x 290 x 280 in the Armstrong power series arrangement. In the future, the work can be undertaken by supplying full pressure surge analysis.