Centrifugal Pumps | Design Aspects

Centrifugal Pumps Design Aspects

Working of Centrifugal Pumps

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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.

Frictional Loss

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

Magnitude of this is also proportional to velocity square. So head curve will come down as shown in figure.

Fig.2 Energy loss due to friction and flow area change and corresponding drop in pump head curve

This is the reason why we always try to transform dynamic part of fluid energy to static part in a centrifugal pump.

Recirculation Loss

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

Incidence Loss

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

Energy losses we have discussed so far, which reduce head of the flow is known as hydraulic losses.

Pump Performance Curve

The effective head verses flow rate curve is shown in Fig. 5(a).

Fig.5 Typical pump performance curves

The shape could be as in Fig. 5(b) depending upon pump parameters.
Such curves are known as pump performance curves. Please note that it is quite difficult to determine pump performance curve theoretically, rather they are determined experimentally.

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

Where value of h is determined from pump performance curve for corresponding flow rate.

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

One main factor is hydraulic loss as we discussed. Other factors are volumetric loss and mechanical loss. So efficiency of a pump can be defined as power gained by fluid by power supplied to the pump.

For a typical centrifugal pump, efficiency will vary as shown in figure.

Fig.8 Change in efficiency and pump shaft power input with flow rate

Corresponding shaft power variation is also shown. You can note that, there is an operating point in pump, where efficiency is maximum. It is known as best efficiency point. Corresponding point is marked on head and shaft power input curves.

Impeller Selection

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

So back to the basic question, how to select a centrifugal pump for this application. Main condition is that fluid should be pumped at a particular flow rate to a specified height.

Fig.10 The fluid pumping problem, where we have to pump fluid at a specified flow rate for a given system

Performance characteristic of the system is given in a system curve. That means how pressure drop varies in system with flow rate. Depending upon minor losses, major losses and altitude of network it would vary as shown in figure below. Please note that system curve will change drastically depending upon valve opening. Assume following is the one system curve at a particular valve opening. Required flow rate is also marked.

Fig.11 System curve of the piping network

The operating point of pump will be intersection point of system curve and pump performance curve.

Fig.12 Different pump operating points possible depending upon selectio of impeller

So depending upon selection of impeller the pump could operate anywhere at dotted points. But we have requirement, a requirement of specified flow rate. Out of these operating points the blue one is most near to the required flow rate. So we will select corresponding impeller. In the same graph we can represent iso-efficiency curves.

Fig.13 From iso-efficieny curves we can determine efficiency of pump at operating condition

So efficiency at the operating condition also can be determined. The required shaft power can be calculated, using following equation.

Knowledge of power input requirement will lead to proper electric motor selection.

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.