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 Losses due to adverse pressure gradient occur in pumps as the pressure of flow increases from the inlet to the outlet of pumps and the flow travels from a region of low pressure to high pressure. As such, it causes the formation of shear layers and flow separation. Flow oscillation may also occur which accounts for the noise and vibration of pumps. The effect of adverse pressure gradient is more significant in low flow condition. For shock losses, they are induced when the inflow into pumps is not radial and contains swirl. In an ideal situation, the flow within the pump should be parallel to the impellers such that the flow angle is very close to the impeller angle. The deviation of the above situation from design causes energy losses and vibration. Fig. A diagram showing point of best efficiency≠operating point 7. What is the minimum volume of sump volume for pumps? Maximum pumping rate = Volume of sump = V Inflow Rate = Qi Cycle Time Tc = t + t t 1 V = Q − Q p V t 2 = Qi i 1 2 Q p 378
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Tc = Minimum cycle time ⎛ ⎞ ⎜ 1 1 V = T + ⎟ c / ⎜ ⎟ ⎝ Q p − Qi Qi ⎠ For Minimum Volume, ∂V ∂Q i = ∂ ∂Q i ⎪⎧ ⎨T ⎪⎩ c ⎛ / ⎜ ⎝ Q p 1 − Q i 1 ⎞⎪⎫ + ⎟ ⎬ Qi ⎠⎪⎭ Minimum volume occurs when Q = 0. 5Q i Hence, Minimum volume of ⎛ sump = T / ⎜ c ⎜ ⎝ Q p TcQ p = 4 1 − 0. 5Q p 1 ⎞ + ⎟ 0. 5Q ⎟ p ⎠ 8. In selecting screw pumps in polder scheme projects, what are the factors that affect the design capacity of screw pumps? (P7) The commonly used angles of inclination for screw pumps are 30 o , 35 o and 38 o . For screw pumps of relatively high lifting head, like over 6.5m, angle of inclination of 38 o is normally used. However, for relatively lower head and high discharge requirement, angle of inclination of 30 o shall be selected. In general, for a given capacity and lifting head, the screw pump diameter is smaller and its length is longer for a screw pump of 30 o inclination when compared with a screw pump of 38 o inclination. To increase the discharge capacity of screw pumps, a larger number of flights should be selected. In fact, screw pumps with 2 flights are more economical that that with 3 flights in terms of efficiency and manufacturing cost. Moreover, the discharge capacity is also determined by the screw pump diameter and sizes of 300mm to 5000mm are available in current market. 9. What are the components of a waterproofing system in the roof of a typical pumping station? (PS1) In the design of a waterproofing system at the roof of a pumping station, p 379
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A Self-Learning Manual - Institution of Engineers Mauritius

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