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 Level Two (Advanced FAQs) Part II: Pumps and Pumping Station 1. What is the difference in arranging pumps in series and in parallel?(p1) For identical pumps with similar functions, if the pumps arranged in series, the total head is increased without a change to maximum discharge. On the other hand, for pumps arranged in parallel to one another, the discharge is increased without any changes to maximum head. 2. Why are radial flow pumps suitable for small flows and high heads? (P2) In radial flow pumps, a diffuser/volute is normally designed at it outlet to convert the kinetic energy gained during the pumping process to pressure head. The diffuser is characterized by widening of outlet pipes, resulting in the decrease of velocity (by continuity equation) and an increase in pressure head (by Bernoulli’s equation). In case of large flows to be handled by the pumps, the large velocity results in formation of significant Coriolis force which tends to deviate the outlet flow from design conditions. At the inlet part of the pumps, the inlet size is smaller than the diameter of the impeller. Consequently, the velocity of flow associated with a small area is relatively large and there is less problem of separation in low flow condition. All in all, the efficiency of radial flow pumps is high when handling small flows. 3. Why are axial flow pumps suitable for large flows and low heads? (P3) It is well known that axial flow pumps are most suitable for providing large flows and low heads. The reason behind this is closely related to the configuration and design of the pumps. In axial flow pumps, the size of inlet diameter is greater than that of impeller diameter. For low flow condition the velocity is relatively small and this increases the chances of occurrence of separation which brings about additional head losses and vibration. On the contrary, if the discharge is large enough the problem of separation is minimized. 376
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 4. In terms of pumping performance, how should engineers determine the use of radial flow pumps and axial flow pumps? (P2 & P3) Specific speed is usually defined for a pump operating at its maximum efficiency. In order to minimize the cost of future operation, it is desirable to operate the pumps as close to the maximum efficiency point as possible. The specific speed for radial flow pumps is relatively small when compared with that of axial flow pumps. This implies that radial flow pumps tend to give higher head with lower discharge while axial flow pumps tend to give higher discharge with lower head. 5. What is the difference in function between backward curved vanes and forward curved vanes in pumps? (P4) The power of a pump is related to discharge as follows: 2 k2Q Power k1 Q tan A + = where k1 and k2 are constants, Q is discharge and A is the angle between the tangent of impeller at vane location and the tangent to vane. For A less than 90 o (forward curved vanes) it is unstable owing to unrestricted power growth. Large losses result from high outflow velocity. The preferred configuration is achieved when A is more than 90 o (i.e. backward curved vanes) because it has controlled power consumption and presents good fluid dynamic shape. 6. What is the difference between best efficiency point and operating point for pumps? (P5) In a pumping system, a system curve can be derived based on the static head required to lift up the fluid and variable head due to possible head losses. The pump curves which relate the performance of the pumping to head against discharge can be obtained from pump suppliers. When the system curve is superimposed on the pump curve, the intersection point is defined as the operating point (or duty point). The operating point may not be necessarily the same as the best efficiency point. The best efficiency point is a function of the pump itself and it is the point of lowest internal friction inside the pump during pumping. These losses are induced by adverse pressure, shock losses and friction. 377
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