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In selecting screw pumps in polder scheme projects, what are the factors that affect the design capacity of screw pumps?

The commonly used angles of inclination for screw pumps are 30o, 35o and 38o. For screw pumps of relatively high lifting head, like over 6.5m, angle of inclination of 38o is normally used. However, for relatively lower head and high discharge requirement, angle of inclination of 30o 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 30o inclination when compared with a screw pump of 38o inclination.

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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.

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 minimum volume of sump volume for pumps?

Maximum pumping rate = Qp

Volume of sump= V

Inflow Rate= Qi

Cycle Time Tc = t1+t2

t1= V/[Qp-Qi]

t2= V/Qi

Tc= Minimum cycle time

minimum volume of sump volume for pumps

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 difference between best efficiency point and operating point for pumps?

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.

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.

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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.

head vs flow

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 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.

Filed under Hydraulics | 0 Comments