In this lecture, we will learn design aspects of centrifugal pumps. More precisely we will learn how to select a centrifugal pump and motor for pumping fluid at a specified rate, for a given system.Summary of above lecture is given below
Before going to design part we will extent the theoretical knowledge gained in first video to more practical sense.
Energy Loss in Pump – Head Reduction
Energy head developed by backward curved pump decreases linearly with flow. But this is theoretically maximum energy head possible. Obtained assuming whole shaft power input got transformed to fluid energy. This is true, only for ideal cases. In practice, there will be lots of energy losses associated with pump flow.
One of the main energy loss is due to effect of friction in the flow. This loss increases quadratically with velocity. A similar loss occurs when there is sudden expansion or contraction.
|Fig.1 Vortices generated due to sudden contractio or sudden expansion of flow|
|Fig.2 Energy loss due to friction and flow area change and corresponding drop in pump head curve|
Next is due to recirculation effect in the flow. When flow is below the designed flow rate, recirculation losses become predominant as shown in figure. When pump operates at its designed flow rate recirculation loss is almost zero.
|Fig.3 Phenomenon of flow circulation and corresponding head drop|
If there is a difference in blade angle and flow angle, it will cause further loss. Here energy loss happens due to flow impingement and recirculation effect. This is again prominent in off design flow conditions. So it tends to have higher losses as we move away from designed flow rate point.
|Fig.4 Flow incidence and corresponding head loss|
Pump Performance Curve
The effective head verses flow rate curve is shown in Fig. 5(a).
|Fig.5 Typical pump performance curves|
Pressure Rise across the Pump
Using pump performance curve one can easily predict what is the pressure rise across the pump, by applying energy equation across it.
|Fig.6 Pressure rise across the pump due to energy addition to it|
Power Gained by Fluid
Power gained by fluid will be lower than the power supplied.
|Fig.7 Power input to pump and power gained by the fluid|
|Fig.8 Change in efficiency and pump shaft power input with flow rate|
For a particular casing we could fit different sized impellers in it. Performance curves of different sized impellers are shown on same graph. Best efficiency points are also marked.
|Fig.9 Different pump performance curves as we chnge size of impeller|
|Fig.10 The fluid pumping problem, where we have to pump fluid at a specified flow rate for a given system|
|Fig.11 System curve of the piping network|
|Fig.12 Different pump operating points possible depending upon selectio of impeller|
|Fig.13 From iso-efficieny curves we can determine efficiency of pump at operating condition|
Problem of Cavitation
This pump will operate well if it can overcome one more problem, problem of cavitation. We will learn how to design against cavitation in a separate article.