# Understanding centrifugal pump performance curves

Posted by angroup on September 27, 2021

Table of Content

What is the centrifugal pump performance curve
Types of pump performance curves
Flat performance curves
Steep performance curve
Performance curve with a hump
How to read centrifugal pump performance curve

## What is the centrifugal pump performance curve?

The centrifugal pump performance curve is a graph indicating the value of the centrifugal pump basic performance parameters such as head, power, efficiency, flow rate and their interrelationship when the pump is running at a certain speed, different speed, the graph is not the same

The Centrifugal pump performance curve expresses the trend of the value change of another parameter of the pump under the condition that one parameter of the pump is determined and other working conditions are the same. The shape of the performance curve also reflects the characteristics of the pump, based on the shape of the curve, the pump can be selected to suit the characteristics of the process; the pump performance curve can also be applied to determine the pump's operating point. Flow and head Q an H curve, flow and power Q a P curve, flow, and efficiency Q-η curve, differential pressure and flow ΔP-Q curve, differential pressure and power ΔP-P curve, differential pressure, and efficiency ΔP-η curve, etc. The pump performance curves are generally obtained from the pump performance test. The expected performance curve can be obtained by calculation when the pump is designed, and the pump needs to be verified by a water test after it is made.

## Types of pump performance curves

### Flat performance curves

This performance curve is suitable for a wide range of flow adjustments, while the pressure requirements of small changes in the system. For example, the need to adjust the flow with a regulating valve, but must maintain a certain liquid level or a certain pressure in the system (such as boiler feed pump), the use of a flat performance curve of the pump can play a role in a certain range of automatic maintenance of the liquid level and pressure

### Steep performance curve

This performance curve is suitable for systems that require a large change in the head when the flow rate does not change much, or systems that require a small change in flow rate when the head fluctuates. For example, in the system of transporting fiber slurry, in order to avoid blockage of fiber slurry in the pipeline when the flow rate slows down, that is, we hope that no matter how much the resistance in the pipeline system increases, and the flow rate (flow rate) does not change much, therefore, it is more appropriate to use the pump with steep drop performance.

### Performance curve with hump

Pumps with such curves may appear unstable conditions in the process of operation. Pump operating condition point by the pump performance curve and device performance curve intersection to determine, but the pump performance curve with a hump often intersects with the pump device characteristics curve in two points, so that the pump in unstable conditions, affecting the safe operation of the pump. Therefore, the performance curve of the hump, the general provisions of the working point head must be less than the shutdown head (the export valve is closed, the flow rate is equal to zero head), so as to avoid the pump in unstable operating conditions

#### Commonly used methods for eliminating humps in performance curves

Reducing the blade exit angle, a flat declining performance curve can be obtained, thus eliminating the hump

Make the liquid into the impeller has pre-spin, so that can contribute to obtaining a fully declined performance curve. After the liquid has pre-spin, the performance curve of the pump's large flow area decreases. With a semi-spiral suction chamber pump, the liquid also has pre-spin when entering the impeller, so the pump performance curve also has the same phenomenon. Although there is a pre-spin that can contribute to obtaining a completely declining performance curve, the negative effect is that the pump head is reduced.

The area of the pump pressure chamber (including the inlet of the vortex chamber and the guide vane) not only affects the size of the shut-off head but also affects the shape of the pump performance curve. The reduction in the area of the extrusion chamber can slightly increase the shut-off head of the pump, make the performance curve steeper, and move the optimal operating point to the direction of small flow; increasing the area of the extrusion chamber can slightly reduce the shut-off head. Make the performance curve flat, and move the best operating point to the direction of large flow. However, it should be noted that excessively increasing or reducing the inlet area of the press-out chamber will cause the pump efficiency to decrease;

Eliminate the hump in the curve by using the method of beveling the impeller outer diameter so that the difference between the front and rear flow line lengths is reduced;

## How to read centrifugal pump performance curve

Centrifugal Pump curves show 4 important factors that are critical in selecting the right pump for your application

The Pump curve shows the Flowrate range on the x-axis. The first point on the curve at zero flow is referred to as “Shut Off” ( This is the head that occurs when the pump is running with the valve closed) and the last point at maximum flow is “Run Out”

The Pump curve shows the head, power, efficiency, and NPSH(r) range on the y-axis

As shown in the picture

When the flow rate and head are given, the feet of the corresponding values on the x-axis and y-axis are the rated operating points of the pump, and the other corresponding parameters can be determined successively

When choosing the pump model, we should consider all parameters to meet the working conditions, and usually choose to use the flow rate of the centrifugal pump for the best efficiency point corresponding to 70% to 120% of the flow rate, which will make the pump life longer and lower power consumption.