1. Head vs Flow
2. System curve
3. Flow vs Efficiency
4. Flow vs Power consumption
5. Flow rate vs Low static head and High static head.

Pump control by VFD: Variable Frequency Drive.
Effect of Over size pump

1. Head vs Flow

Centrifugal pump's Head vs Flow graph is gradually down trend.

It is called Pump Characteristic Curve.

Centrifugal pump head vs flow

It means 
when flow is minimum then Head is maximum &
When flow is maximum then Head is minimum.

2. System curve [ Head vs Friction head loss ]

Flow is increased head loss also increasing
Head is increasing head loss is also increasing.

Centrifugal pump System curve
Where Head vs flow graph and System resistance curve intersecting each other at point called pump operating point.

 At this point pump is operating on most efficiently.

Pump operating point

3. Flow vs Efficiency

Initially start up time Discharge valve is in closed condition  so on this point efficiency is zero.

When discharge valve is opening gradually flow is increasing and efficiency also increasing.

After certain point power consumption is more according to flow and efficiency is reducing.

4. Flow vs Power consumption

When pump discharge flow is increasing its power consumption is also increasing.

5. Flow rate vs Low static head and High static head as seen above.

Pump control by VFD: Variable Frequency Drive.

Pump control by VFD can Shift its duty point we can see this in graph below

Pump control by VFD Variable Frequency Drive.

We can see this...

RPM reduction 1480--> 1350
Will result into 130kW --> 65kW

Small speed reduction result into drastic reduction in power consumption.

Effect of Over size pump

Pump is selected on basis of System characteristics curve ( head vs flow curve).

This curve is change when any valve reducer Heat exchanger NRV installed.


Effect of discharge throttling
System 1 : Green curve

Suppose we had designed system for 500m³/hr

 flow for 50 meter height.

So we will choose pump with BEST EFFICIENCY POINT is near 500 m³/hr.

System 2 : Red Curve

After installation we realize that 300 m³/hr flow is sufficient.

So, we will throttling discharge valve and adding resistance to flow.

Throttling valves condition has Red Curve ( Flow vs Resistance) which has 300m³/hr flow and 70 meter height.

In actual we will operate pump at 300m³/hr and 42 meter head at point C in figure.


We can see Red double arrow that "Efficiency reduced from 82% to 77%"

We need pump which has Blue curve ( Flow vs Head).

Replacement of pump, VFD installation or Impeller trimming should be initiated.

See calculation

Thanks for reading - Centrifugal pump Advanced Concepts
Naitik Patel
Industrial Guide

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