A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU environment. (ii) In the event of high water table, water would infiltrate into sewer manholes. Therefore, a higher volume of sewage would be delivered to sewage treatment plant for treatment and this essentially increases the cost for treating this additional volume of water. 13. What are possible causes of manhole explosions? (S3) It is not uncommon that manhole explosion occurs nowadays. Manhole covers are dislodged from the frames which is associated with a release of energy. Manhole explosion occurs mostly owing to the ignition and combustion of flammable gas. Sources of flammable gas include the followings: (i) Natural gas as a result of leakage of nearby gas line; (ii) In sewer manholes, it is rich in methane which tends to accumulate inside manhole; (iii) Gas generated by degradation of cable insulation. 14. In the design of dry well pumping stations, which arrangement is better, “turned-down” bellmouth or horizontal intake? (P1) Pumps can be installed as dry well or wet well. The wet well is commonly used because of its simplicity and low cost. However, this type of pump arrangement has the potential problem of maintenance. For instance, it requires the de-watering of sump and removal of pumps out of the sump, which is suitable for stormwater pumping station which does not require pumping for most of the time. For dry well, the pumps could be assessed and maintained all of the time. In general “turned-down” bellmouth of pump inlet is more popular because of the following two reasons: (i) It is less susceptible to vortex action with similar water height. (ii) It accommodates a lower water cover. 15. What is the purpose of bellmouth entry to a circular pipe for pumps? (P1) If sharp edged inlet to pipes connecting to a pump is adopted, flow separation will occur. Flow separates from sharp edges and a recirculation 154
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU zone is formed. Moreover, turbulence shall form at downstream when the flow at vena contracta subsequently expands to fill up the unfilled void. Flow separation leads to significant head loss. The design of bellmouth entry is to ensure that the flow is uniform over the entire intake section and the head loss induced at inlet section is minimized. 155
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A Self-Learning Manual - Institution of Engineers Mauritius

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