How to Choose a Suitable Pump Model and Specification after Determining the Pumping Head
Introduction
Water pump selection is not an easy task, because it involves many factors, such as head, flow rate, efficiency, cavitation condition, materials, NPSH etc. If a pump is improperly selected, it will cause abnormal system operation, abnormally wear and tear parts, affect energy consumption, increase maintenance cost and reduce plant life. In order to correctly select a pump, it is necessary to have a clear understanding of the different terms, and follow the correct criteria to get the matching of the pump, motor and accessories.
Before selecting the pump, there are several steps that need to be taken including determining the system requirement and the static and dynamic heads, head distribution diagram, NPSH, materials and service life.
1. Determining the System Requirements
The first step in selecting the right pump is to determine exactly what the system requires for operation. You must collect data about all of the components in the system to see what size and type of pump would work best. It is important to accurately determine the required head, flow, speed and power.
2. Determining the Static and Dynamic Heads
In water systems, the static head and dynamic head are vital in determining the size and type of pump needed. Static head is the vertical distance between the liquid source and the pump discharge. It includes the static head, pressure head and elevation head in the system. However, the static head cannot be used to measure the required head for the system. Instead, dynamic head needs to be taken into consideration.
Dynamic head is a combination of static head and velocity head, which is the pressure difference caused by the velocity of the liquid in the system. The dynamic head can be calculated in terms of the flow and the height difference between the suction and the discharge of the pump.
3. Head Distribution Diagram
The head distribution diagram is a graphical representation of the static head, dynamic head and their respective summation along the pumping system. Based on the static and dynamic heads, the distribution diagram helps determine the total head across the system and the total power requirement of the pump.
4. NPSH
NPSH stands for Net Positive Suction Head, which is the absolute pressure at the suction port of the pump. It is an important parameter that should be considered while selecting a pump as it helps to identify whether the pump will be experiencing cavitation or not. The Net Positive Suction Head varies between the rotating pumps and the centrifugal pumps. The NPSH of the pump should never be less than the minimum NPSH that the fluid requires in order to prevent the suction side or the delivery side of the pump from cavitating.
5. Materials
The choice of materials for the pump depends upon the chemical, physical and temperature requirements of the system. Commonly used materials include cast iron, SS, duplex steel, super duplex steel, bronze, and many others. A suitable material should be chosen as per the requirement of the system.
6. Service Life
The service life of the pump depends upon how it is operated and the kind of maintenance that is provided. The pump should be operated and maintained in a way that is likely to maximize the service life of the pump.
Conclusion
Selecting the right pump involves a great deal of analysis and considerations. The static head, dynamic head, head distribution diagrams, NPSH, materials and service life should all be taken into account while selecting the right pump model and specification. By following the correct criteria and assessing the system requirements, the right pump can be identified.
