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What is the difference in function between backward curved vanes and forward curved vanes in pumps?

The power of a pump is related to discharge as follows:

Power=K1Q + [K2Q2]/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.

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For A less than 90o (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 90o (i.e. backward curved vanes) because it has controlled power consumption and presents good fluid dynamic shape.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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In terms of pumping performance, how should engineers determine the use of radial flow pumps and axial flow pumps?

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.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Why are axial flow pumps suitable for large flows and low heads?

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.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Why are radial flow pumps suitable for small flows and high heads?

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.

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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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the purpose of adding cooling pipes or even using cold water for concrete in concreting operation?

All these measures aim at reducing the placing temperature and reducing thermal cracks induced during concreting of massive pours. Since the final concreting temperature should be the ambient temperature, reducing the initial placing temperature will also lower the peak hydration temperature. Therefore, the temperature difference between the hydration peak and the ambient temperature is reduced accordingly and subsequently the thermal effect to concrete structure can be reduced by controlling the placing temperature.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Filed under Hydraulics | 0 Comments