COMMON ERRORS OF PUMP SELECTION

PUMP OVERSIZING  is the most often error during the pump selection.

Oversizing is the main reason for the inefficient pump operation.

 

What reasons can cause oversizing of a pump?

 

- Pump user cannot define requered pump parameters and does not know the piping system curve. In this situation, the user decides to select a pump with excessive head or flow for any case. This decision comes also because everybody keeps in mind "IF Pump is too big I will use a valve to reduce excessive head.  Nobody thinks of efficiency at this moment

- Parameters of a pumping system have changed during operation.  It happens due to different reasons: 

- Changing of a load profile, for example, the reduction of water consumption.

- Increasing the pipe resistance due to corrosion of pipes, sediments on the pipe walls.

- Replacing of system's elements by others with other characteristics.

- The pumping system was developed by taking into account possible of increasing requirements. There is a perspective of an increase of a required flow in the future but pumping station operates earlier.

- Additional flow for fire fighting systems. Sometimes the capacity of a pumping station defined as a maximum possible flow plus flow for firefighting system.

Errors made by pumping systems designers. Wrong calculations of pumping systems characteristics (static head, friction losses).

 

Reasons for pump oversizing can be different, but the result is the same - inefficient pump operation, lower reliability, increased maintenance cost. More than 60% of pumps operate beyond the allowable range defined by manufacturers.

Sometimes pump users ask an interesting question.
 
How the pump with the more powerful electric motor can be overloaded and break down while the pump with lower head works without problem?
 
Bad practice - after the pump broke down user decides that the pump is not powerful enough and decides to install the pump with the more head and more powerful electric motor but the result will be the same.
 
 
The answer to this question is below.
How does it look in a graph?
Oversizing of a pump

Initial data for a pump selection: 

- required flow is Q1

- calculated piping system curve is presented in the graph.

In accordance with the system curve for flow Q1 the pump with head H1 is necessary.  Pump #1 has appropriate parameters.

 

BUT user is not sure that the system curve is right and decide to select the pump with the required capacity but with a more head and installs Pump #2. 

 

Let's define the operating points for these two pumps.

If Pump #1 has been selected then the operating point would be #1.  It will lay within the operating range.

But Pump #2 has been installed. The duty point of this pump is #2. It lays outside the operating range in the area of an overloading. This fact means that  Pump #2 will operate with low efficiency lower reliability. If we look at the power curve we will discover that the electric motor can be overloaded what can cause motor's failure despite on the fact that it is more powerful than the motor of Pump #1.

The explanation of this fact is clear enough. Pump #2 delivers more fluid and operates with more flow and consequently consumes more power. But if a user does not have flowmeter this fact is not clear. 

 
What to do if it has become clear that the pump is oversized and operates with overload?
 
The possible  solution can be:
 

- The replacement of a pump by the pump with appropriate parameters.  This solution can be expensive

- The application of throttle control. This will allow adjusting of a system curve by using the valve so that the pump will operate within the preferred operating range with high efficiency and reliability but energy will be wasted across the valve. See it in the graph. As a negative consequence, the increased valve wear should be taken into account.  

- Application of  VFD to adapt the pump curve to the system requirements. 

- Trimming of a pump impeller. This is the cheapest way of adjusting a pump curve to system characteristic. The trimming of impeller reduces the efficiency of a pump but usually, it gives a positive effect on energy reduction.

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Values of a potential saving which every method can give are presented in the table below.