A Self-Learning Manual - Institution of Engineers Mauritius

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.
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