pump specific speed application and its significance

In this video, I will explain the specific speed application and its significance.
the Specifics speed can be defined as the speed of an imaginary pump, geometrically similar to the given pump, which will deliver flow of one cubic meter per second and produce 1 meter of head.

It seems that, intend of this definition is to compare two geometric similar pumps. However, practically, specific speed also used to compare two different pumps with dissimilar geometry.
Let me explain this with an example, if we look at this sketch, here different optimum impeller geometries are shown with respect to specific speed.

There is an optimum impeller geometry, which only can work efficiently for a given specific speed.
For example, at specific of 1000 US units, radial flow pump will work more efficiently as compared with mixed flow pump. However, at higher specific speed like 9000 US units, axial flow pumps are more efficient.So now it is clear that, specific speed can be used as an index to identify Optimum impeller geometry. specific speed is an index of pump size and it also affects the maximum attainable efficiency of the pump.

Maximum attainable efficiency increases with increase in the specific speed. Hence, for a given pump capacity, larger the design specific speed, greater the expected pump efficiency.

The primary reason of this improvement in the efficiency is due to increase in Reynolds number and reduction in relative roughness.
Because pump with higher specific speed have bigger size, hence machine Reynolds number increase due to increase in pump characteristic dimension. Hence viscous effect become relatively less important as the pump size increases
And at the same time, because of this increase in characteristic dimension, relative roughness reduces significantly, which results in in smother flow passage.
Additionally, the gap between impeller and pump casing remain more or less same respective of pump size. Thus, the size of leakage path not increases with increase in pump size. Hence, increasing pump size also improves volumetric efficiency.
so we can say that, pump with higher specific speed have higher efficiency. This is why, larger pumps have higher efficiency as compared with small pumps.

The specific speed not only dictates the optimum impeller geometry but also governs the shape of pump characteristic curve.

If you look at pump head versus flow curve, here it is clearly shown that, pump with low specific speed have relatively flat curve, however increasing the specific speed makes pumps HQ curve steeper.

Now if we look at pumps power curve, pump with low specific speed have continuous rising power curve. Whereas, medium specific speed pumps like mixed flow pumps have a flatter power curve.
The power curve of higher specific speed pump is opposite to power curve of low specific speed pump. Here maximum power is required at low flow.
Efficiency curve become narrow with increase in pump specific speed. This is why pump with higher specific speed have small allowable operating ranges.

This video also gives answers to following pump interview questions

What are the practical applications of pump specific speed 0:02 to 4:00
how specific speed affects centrifugal pump impeller geometry 0:02 to 4:00
how to calculate specific speed of pump 0:07 to 1:14
pump specific speed formula 0:07 to 1:14
How specific speed affects pump performance 0:02 to 4:00
How specific speed affects pump efficiency 1:14 to 3:05
How specific speed affects pump performance curve 3:05 to 4:00
Why big pumps have higher efficiency 1:14 to 3:05
why pump efficiency increases with increase in the pump specific speed1:14 to 3:05
concept of specific speed 0:02 to 4:00
specific speed application 0:02 to 4:00
pump specific speed application 0:02 to 4:00
application of specific speed 0:02 to 4:00
specific speed of centrifugal pump 0:02 to 4:00
effect of specific speed on pump efficiency 1:14 to 3:05
how specific speed affects pump curves 3:05 to 4:00
practical application of specific speed
significance of specific speed 0:02 to 4:00
explanation of specific speed 0:02 to 4:00
pump specific speed 0:07 to 1:14
definition of specific speed 0:07 to 4:00
specific speed definition 0:07 to 1:14
specific speed explained 0:02 to 4:00
specific speed of pump 0:02 to 4:00
pump interview questions and answers 0:02 to 4:00



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